Atomic Lifecycle for Writer Refresh + Hardswap

src/api/runtime_min.rs

@@ -1,4 +1,3 @@
-use std::collections::BTreeSet;
 use std::time::{SystemTime, UNIX_EPOCH};
 
 use serde::Serialize;
@@ -17,82 +16,6 @@ pub(super) struct SecurityWhitelistData {
     pub(super) entries: Vec<String>,
 }
 
-#[derive(Serialize)]
-pub(super) struct RuntimeMePoolStateGenerationData {
-    pub(super) active_generation: u64,
-    pub(super) warm_generation: u64,
-    pub(super) warm_generations: Vec<u64>,
-    pub(super) pending_hardswap_generation: u64,
-    pub(super) pending_hardswap_age_secs: Option<u64>,
-    pub(super) reinit_inflight: usize,
-    pub(super) reinit_max_concurrency_effective: usize,
-    pub(super) draining_generations: Vec<u64>,
-}
-
-#[derive(Serialize)]
-pub(super) struct RuntimeMePoolStateHardswapData {
-    pub(super) enabled: bool,
-    pub(super) pending: bool,
-}
-
-#[derive(Serialize)]
-pub(super) struct RuntimeMePoolStateWriterContourData {
-    pub(super) warm: usize,
-    pub(super) active: usize,
-    pub(super) draining: usize,
-}
-
-#[derive(Serialize)]
-pub(super) struct RuntimeMePoolStateWriterHealthData {
-    pub(super) healthy: usize,
-    pub(super) degraded: usize,
-    pub(super) draining: usize,
-}
-
-#[derive(Serialize)]
-pub(super) struct RuntimeMePoolStateWriterData {
-    pub(super) total: usize,
-    pub(super) alive_non_draining: usize,
-    pub(super) draining: usize,
-    pub(super) degraded: usize,
-    pub(super) contour: RuntimeMePoolStateWriterContourData,
-    pub(super) health: RuntimeMePoolStateWriterHealthData,
-}
-
-#[derive(Serialize)]
-pub(super) struct RuntimeMePoolStateRefillDcData {
-    pub(super) dc: i16,
-    pub(super) family: &'static str,
-    pub(super) inflight: usize,
-}
-
-#[derive(Serialize)]
-pub(super) struct RuntimeMePoolStateRefillData {
-    pub(super) inflight_endpoints_total: usize,
-    pub(super) inflight_dc_total: usize,
-    pub(super) running_dc_total: usize,
-    pub(super) pending_dc_total: usize,
-    pub(super) by_dc: Vec<RuntimeMePoolStateRefillDcData>,
-}
-
-#[derive(Serialize)]
-pub(super) struct RuntimeMePoolStatePayload {
-    pub(super) generations: RuntimeMePoolStateGenerationData,
-    pub(super) hardswap: RuntimeMePoolStateHardswapData,
-    pub(super) writers: RuntimeMePoolStateWriterData,
-    pub(super) refill: RuntimeMePoolStateRefillData,
-}
-
-#[derive(Serialize)]
-pub(super) struct RuntimeMePoolStateData {
-    pub(super) enabled: bool,
-    #[serde(skip_serializing_if = "Option::is_none")]
-    pub(super) reason: Option<&'static str>,
-    pub(super) generated_at_epoch_secs: u64,
-    #[serde(skip_serializing_if = "Option::is_none")]
-    pub(super) data: Option<RuntimeMePoolStatePayload>,
-}
-
 #[derive(Serialize)]
 pub(super) struct RuntimeMeQualityCountersData {
     pub(super) idle_close_by_peer_total: u64,
@@ -285,100 +208,6 @@ pub(super) fn build_security_whitelist_data(cfg: &ProxyConfig) -> SecurityWhitel
     }
 }
 
-pub(super) async fn build_runtime_me_pool_state_data(shared: &ApiShared) -> RuntimeMePoolStateData {
-    let now_epoch_secs = now_epoch_secs();
-    let Some(pool) = shared.me_pool.read().await.clone() else {
-        return RuntimeMePoolStateData {
-            enabled: false,
-            reason: Some(SOURCE_UNAVAILABLE_REASON),
-            generated_at_epoch_secs: now_epoch_secs,
-            data: None,
-        };
-    };
-
-    let (status, runtime) = pool.api_coherent_snapshots().await;
-    let refill = pool.api_refill_snapshot().await;
-
-    let mut draining_generations = BTreeSet::<u64>::new();
-    let mut contour_warm = 0usize;
-    let mut contour_active = 0usize;
-    let mut contour_draining = 0usize;
-    let mut draining = 0usize;
-    let mut degraded = 0usize;
-    let mut healthy = 0usize;
-
-    for writer in &status.writers {
-        if writer.draining {
-            draining_generations.insert(writer.generation);
-            draining += 1;
-        }
-        if writer.degraded && !writer.draining {
-            degraded += 1;
-        }
-        if !writer.degraded && !writer.draining {
-            healthy += 1;
-        }
-        match writer.state {
-            "warm" => contour_warm += 1,
-            "active" => contour_active += 1,
-            _ => contour_draining += 1,
-        }
-    }
-
-    RuntimeMePoolStateData {
-        enabled: true,
-        reason: None,
-        generated_at_epoch_secs: status.generated_at_epoch_secs,
-        data: Some(RuntimeMePoolStatePayload {
-            generations: RuntimeMePoolStateGenerationData {
-                active_generation: runtime.active_generation,
-                warm_generation: runtime.warm_generation,
-                warm_generations: runtime.warm_generations,
-                pending_hardswap_generation: runtime.pending_hardswap_generation,
-                pending_hardswap_age_secs: runtime.pending_hardswap_age_secs,
-                reinit_inflight: runtime.reinit_inflight,
-                reinit_max_concurrency_effective: runtime.reinit_max_concurrency_effective,
-                draining_generations: draining_generations.into_iter().collect(),
-            },
-            hardswap: RuntimeMePoolStateHardswapData {
-                enabled: runtime.hardswap_enabled,
-                pending: runtime.pending_hardswap_generation != 0,
-            },
-            writers: RuntimeMePoolStateWriterData {
-                total: status.writers.len(),
-                alive_non_draining: status.writers.len().saturating_sub(draining),
-                draining,
-                degraded,
-                contour: RuntimeMePoolStateWriterContourData {
-                    warm: contour_warm,
-                    active: contour_active,
-                    draining: contour_draining,
-                },
-                health: RuntimeMePoolStateWriterHealthData {
-                    healthy,
-                    degraded,
-                    draining,
-                },
-            },
-            refill: RuntimeMePoolStateRefillData {
-                inflight_endpoints_total: refill.inflight_endpoints_total,
-                inflight_dc_total: refill.inflight_dc_total,
-                running_dc_total: refill.running_dc_total,
-                pending_dc_total: refill.pending_dc_total,
-                by_dc: refill
-                    .by_dc
-                    .into_iter()
-                    .map(|entry| RuntimeMePoolStateRefillDcData {
-                        dc: entry.dc,
-                        family: entry.family,
-                        inflight: entry.inflight,
-                    })
-                    .collect(),
-            },
-        }),
-    }
-}
-
 pub(super) async fn build_runtime_me_quality_data(shared: &ApiShared) -> RuntimeMeQualityData {
     let now_epoch_secs = now_epoch_secs();
     let Some(pool) = shared.me_pool.read().await.clone() else {
@@ -541,7 +370,10 @@ pub(super) async fn build_runtime_upstream_quality_data(
     }
 }
 
+// ME pool runtime-state projection.
+mod me_pool;
 // NAT/STUN runtime projection and timestamping.
 mod nat;
+pub(super) use me_pool::build_runtime_me_pool_state_data;
 pub(super) use nat::build_runtime_nat_stun_data;
 use nat::now_epoch_secs;

src/api/runtime_min/me_pool.rs

@@ -0,0 +1,194 @@
+//! ME pool runtime-state projection.
+
+use std::collections::BTreeSet;
+
+use serde::Serialize;
+
+use super::{now_epoch_secs, ApiShared, SOURCE_UNAVAILABLE_REASON};
+
+#[derive(Serialize)]
+struct RuntimeMePoolStateGenerationData {
+    active_generation: u64,
+    warm_generation: u64,
+    warm_generations: Vec<u64>,
+    pending_hardswap_generation: u64,
+    pending_hardswap_age_secs: Option<u64>,
+    reinit_inflight: usize,
+    reinit_max_concurrency_effective: usize,
+    draining_generations: Vec<u64>,
+}
+
+#[derive(Serialize)]
+struct RuntimeMePoolStateHardswapData {
+    enabled: bool,
+    pending: bool,
+    pending_writers_current: usize,
+    pending_writer_deficit: usize,
+    pending_missing_dc_groups: usize,
+    pending_map_current: Option<bool>,
+    orphan_warm_writers_current: usize,
+    replacement_preparing_current: usize,
+    replacement_retiring_current: usize,
+}
+
+#[derive(Serialize)]
+struct RuntimeMePoolStateWriterContourData {
+    warm: usize,
+    active: usize,
+    draining: usize,
+}
+
+#[derive(Serialize)]
+struct RuntimeMePoolStateWriterHealthData {
+    healthy: usize,
+    degraded: usize,
+    draining: usize,
+}
+
+#[derive(Serialize)]
+struct RuntimeMePoolStateWriterData {
+    total: usize,
+    alive_non_draining: usize,
+    draining: usize,
+    degraded: usize,
+    contour: RuntimeMePoolStateWriterContourData,
+    health: RuntimeMePoolStateWriterHealthData,
+}
+
+#[derive(Serialize)]
+struct RuntimeMePoolStateRefillDcData {
+    dc: i16,
+    family: &'static str,
+    inflight: usize,
+}
+
+#[derive(Serialize)]
+struct RuntimeMePoolStateRefillData {
+    inflight_endpoints_total: usize,
+    inflight_dc_total: usize,
+    running_dc_total: usize,
+    pending_dc_total: usize,
+    by_dc: Vec<RuntimeMePoolStateRefillDcData>,
+}
+
+#[derive(Serialize)]
+struct RuntimeMePoolStatePayload {
+    generations: RuntimeMePoolStateGenerationData,
+    hardswap: RuntimeMePoolStateHardswapData,
+    writers: RuntimeMePoolStateWriterData,
+    refill: RuntimeMePoolStateRefillData,
+}
+
+#[derive(Serialize)]
+struct RuntimeMePoolStateData {
+    enabled: bool,
+    #[serde(skip_serializing_if = "Option::is_none")]
+    reason: Option<&'static str>,
+    generated_at_epoch_secs: u64,
+    #[serde(skip_serializing_if = "Option::is_none")]
+    data: Option<RuntimeMePoolStatePayload>,
+}
+
+/// Builds the bounded runtime ME pool response projection.
+pub(in crate::api) async fn build_runtime_me_pool_state_data(
+    shared: &ApiShared,
+) -> impl Serialize {
+    let now_epoch_secs = now_epoch_secs();
+    let Some(pool) = shared.me_pool.read().await.clone() else {
+        return RuntimeMePoolStateData {
+            enabled: false,
+            reason: Some(SOURCE_UNAVAILABLE_REASON),
+            generated_at_epoch_secs: now_epoch_secs,
+            data: None,
+        };
+    };
+
+    let (status, runtime) = pool.api_coherent_snapshots().await;
+    let refill = pool.api_refill_snapshot().await;
+
+    let mut draining_generations = BTreeSet::<u64>::new();
+    let mut contour_warm = 0usize;
+    let mut contour_active = 0usize;
+    let mut contour_draining = 0usize;
+    let mut draining = 0usize;
+    let mut degraded = 0usize;
+    let mut healthy = 0usize;
+
+    for writer in &status.writers {
+        if writer.draining {
+            draining_generations.insert(writer.generation);
+            draining += 1;
+        }
+        if writer.degraded && !writer.draining {
+            degraded += 1;
+        }
+        if !writer.degraded && !writer.draining {
+            healthy += 1;
+        }
+        match writer.state {
+            "warm" => contour_warm += 1,
+            "active" => contour_active += 1,
+            _ => contour_draining += 1,
+        }
+    }
+
+    RuntimeMePoolStateData {
+        enabled: true,
+        reason: None,
+        generated_at_epoch_secs: status.generated_at_epoch_secs,
+        data: Some(RuntimeMePoolStatePayload {
+            generations: RuntimeMePoolStateGenerationData {
+                active_generation: runtime.active_generation,
+                warm_generation: runtime.warm_generation,
+                warm_generations: runtime.warm_generations,
+                pending_hardswap_generation: runtime.pending_hardswap_generation,
+                pending_hardswap_age_secs: runtime.pending_hardswap_age_secs,
+                reinit_inflight: runtime.reinit_inflight,
+                reinit_max_concurrency_effective: runtime.reinit_max_concurrency_effective,
+                draining_generations: draining_generations.into_iter().collect(),
+            },
+            hardswap: RuntimeMePoolStateHardswapData {
+                enabled: runtime.hardswap_enabled,
+                pending: runtime.pending_hardswap_generation != 0,
+                pending_writers_current: runtime.pending_writers_current,
+                pending_writer_deficit: runtime.pending_writer_deficit,
+                pending_missing_dc_groups: runtime.pending_missing_dc_groups,
+                pending_map_current: runtime.pending_map_current,
+                orphan_warm_writers_current: runtime.orphan_warm_writers_current,
+                replacement_preparing_current: runtime.replacement_preparing_current,
+                replacement_retiring_current: runtime.replacement_retiring_current,
+            },
+            writers: RuntimeMePoolStateWriterData {
+                total: status.writers.len(),
+                alive_non_draining: status.writers.len().saturating_sub(draining),
+                draining,
+                degraded,
+                contour: RuntimeMePoolStateWriterContourData {
+                    warm: contour_warm,
+                    active: contour_active,
+                    draining: contour_draining,
+                },
+                health: RuntimeMePoolStateWriterHealthData {
+                    healthy,
+                    degraded,
+                    draining,
+                },
+            },
+            refill: RuntimeMePoolStateRefillData {
+                inflight_endpoints_total: refill.inflight_endpoints_total,
+                inflight_dc_total: refill.inflight_dc_total,
+                running_dc_total: refill.running_dc_total,
+                pending_dc_total: refill.pending_dc_total,
+                by_dc: refill
+                    .by_dc
+                    .into_iter()
+                    .map(|entry| RuntimeMePoolStateRefillDcData {
+                        dc: entry.dc,
+                        family: entry.family,
+                        inflight: entry.inflight,
+                    })
+                    .collect(),
+            },
+        }),
+    }
+}

src/metrics.rs

@@ -236,6 +236,10 @@ async fn handle<B>(
     let config = runtime.config();
 
     if req.uri().path() == "/metrics" {
+        let me_hardswap = match runtime.current_me_pool().await {
+            Some(pool) => Some(pool.api_hardswap_snapshot().await),
+            None => None,
+        };
         let body = render_metrics(
             stats,
             shared_state,
@@ -244,6 +248,7 @@ async fn handle<B>(
             tls_cache,
             tls_full_cert_budget,
             web_publication,
+            me_hardswap.as_ref(),
         )
         .await;
         let resp = Response::builder()

src/metrics/render.rs

@@ -12,6 +12,8 @@ mod me_lifecycle;
 mod me_buffers;
 // ME writer selection, KDF, and hardswap metrics.
 mod me_policy;
+// Live hardswap ownership and replacement progress metrics.
+mod me_hardswap;
 // Adaptive-floor and writer-cap metrics.
 mod me_floor;
 // Desync, pool recovery, and refill metrics.
@@ -27,6 +29,7 @@ pub(super) async fn render_metrics(
     tls_cache: Option<&TlsFrontCache>,
     tls_full_cert_budget: &TlsFullCertBudget,
     web_publication: &crate::web::control::WebRuntimePublication,
+    me_hardswap: Option<&crate::transport::middle_proxy::MeApiHardswapSnapshot>,
 ) -> String {
     let mut out = String::with_capacity(4096);
     let telemetry = stats.telemetry_policy();
@@ -62,6 +65,7 @@ pub(super) async fn render_metrics(
         me_allows_debug,
     );
     me_policy::render(&mut out, stats, me_allows_normal, me_allows_debug);
+    me_hardswap::render(&mut out, me_hardswap, me_allows_normal);
     me_floor::render(&mut out, stats, config, me_allows_normal);
     me_recovery::render(&mut out, stats, me_allows_normal, me_allows_debug);
     users::render(

src/metrics/render/me_hardswap.rs

@@ -0,0 +1,156 @@
+use std::fmt::Write;
+
+use crate::transport::middle_proxy::MeApiHardswapSnapshot;
+
+pub(super) fn render(
+    out: &mut String,
+    snapshot: Option<&MeApiHardswapSnapshot>,
+    enabled: bool,
+) {
+    let snapshot = enabled.then_some(snapshot).flatten();
+    let pending = snapshot.is_some_and(|value| value.pending);
+    let pending_age_secs = snapshot
+        .and_then(|value| value.pending_age_secs)
+        .unwrap_or(0);
+    let pending_writers_current = snapshot
+        .map(|value| value.pending_writers_current)
+        .unwrap_or(0);
+    let pending_writer_deficit = snapshot
+        .map(|value| value.pending_writer_deficit)
+        .unwrap_or(0);
+    let pending_missing_dc_groups = snapshot
+        .map(|value| value.pending_missing_dc_groups)
+        .unwrap_or(0);
+    let pending_map_current = snapshot
+        .and_then(|value| value.pending_map_current)
+        .is_some_and(|value| value);
+    let orphan_warm_writers_current = snapshot
+        .map(|value| value.orphan_warm_writers_current)
+        .unwrap_or(0);
+    let replacement_preparing_current = snapshot
+        .map(|value| value.replacement_preparing_current)
+        .unwrap_or(0);
+    let replacement_retiring_current = snapshot
+        .map(|value| value.replacement_retiring_current)
+        .unwrap_or(0);
+
+    let _ = writeln!(
+        out,
+        "# HELP telemt_me_hardswap_pending Whether an ME hardswap generation is pending"
+    );
+    let _ = writeln!(out, "# TYPE telemt_me_hardswap_pending gauge");
+    let _ = writeln!(out, "telemt_me_hardswap_pending {}", usize::from(pending));
+    let _ = writeln!(
+        out,
+        "# HELP telemt_me_hardswap_pending_age_seconds Age of the pending ME hardswap generation"
+    );
+    let _ = writeln!(out, "# TYPE telemt_me_hardswap_pending_age_seconds gauge");
+    let _ = writeln!(
+        out,
+        "telemt_me_hardswap_pending_age_seconds {pending_age_secs}"
+    );
+    let _ = writeln!(
+        out,
+        "# HELP telemt_me_hardswap_pending_writers_current Authoritative warm writers in the pending generation"
+    );
+    let _ = writeln!(
+        out,
+        "# TYPE telemt_me_hardswap_pending_writers_current gauge"
+    );
+    let _ = writeln!(
+        out,
+        "telemt_me_hardswap_pending_writers_current {pending_writers_current}"
+    );
+    let _ = writeln!(
+        out,
+        "# HELP telemt_me_hardswap_pending_writer_deficit Writers missing from the pending generation floor"
+    );
+    let _ = writeln!(
+        out,
+        "# TYPE telemt_me_hardswap_pending_writer_deficit gauge"
+    );
+    let _ = writeln!(
+        out,
+        "telemt_me_hardswap_pending_writer_deficit {pending_writer_deficit}"
+    );
+    let _ = writeln!(
+        out,
+        "# HELP telemt_me_hardswap_pending_missing_dc_groups Desired DC groups missing pending-generation coverage"
+    );
+    let _ = writeln!(
+        out,
+        "# TYPE telemt_me_hardswap_pending_missing_dc_groups gauge"
+    );
+    let _ = writeln!(
+        out,
+        "telemt_me_hardswap_pending_missing_dc_groups {pending_missing_dc_groups}"
+    );
+    let _ = writeln!(
+        out,
+        "# HELP telemt_me_hardswap_pending_map_current Whether the pending generation targets the current endpoint map"
+    );
+    let _ = writeln!(out, "# TYPE telemt_me_hardswap_pending_map_current gauge");
+    let _ = writeln!(
+        out,
+        "telemt_me_hardswap_pending_map_current {}",
+        usize::from(pending_map_current)
+    );
+    let _ = writeln!(
+        out,
+        "# HELP telemt_me_hardswap_orphan_warm_writers_current Warm writers not owned by the pending hardswap generation"
+    );
+    let _ = writeln!(
+        out,
+        "# TYPE telemt_me_hardswap_orphan_warm_writers_current gauge"
+    );
+    let _ = writeln!(
+        out,
+        "telemt_me_hardswap_orphan_warm_writers_current {orphan_warm_writers_current}"
+    );
+    let _ = writeln!(
+        out,
+        "# HELP telemt_me_writer_replacement_current ME writer replacements by transaction phase"
+    );
+    let _ = writeln!(out, "# TYPE telemt_me_writer_replacement_current gauge");
+    let _ = writeln!(
+        out,
+        "telemt_me_writer_replacement_current{{state=\"preparing\"}} {replacement_preparing_current}"
+    );
+    let _ = writeln!(
+        out,
+        "telemt_me_writer_replacement_current{{state=\"retiring\"}} {replacement_retiring_current}"
+    );
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn renders_bounded_hardswap_and_replacement_state() {
+        let snapshot = MeApiHardswapSnapshot {
+            pending: true,
+            pending_age_secs: Some(42),
+            pending_writers_current: 3,
+            pending_writer_deficit: 4,
+            pending_missing_dc_groups: 2,
+            pending_map_current: Some(true),
+            orphan_warm_writers_current: 1,
+            replacement_preparing_current: 5,
+            replacement_retiring_current: 6,
+        };
+        let mut out = String::new();
+
+        render(&mut out, Some(&snapshot), true);
+
+        assert!(out.contains("telemt_me_hardswap_pending 1"));
+        assert!(out.contains("telemt_me_hardswap_pending_age_seconds 42"));
+        assert!(out.contains("telemt_me_hardswap_pending_writer_deficit 4"));
+        assert!(out.contains(
+            "telemt_me_writer_replacement_current{state=\"preparing\"} 5"
+        ));
+        assert!(out.contains(
+            "telemt_me_writer_replacement_current{state=\"retiring\"} 6"
+        ));
+    }
+}

src/metrics/tests.rs

@@ -87,6 +87,7 @@ async fn test_render_metrics_format() {
         None,
         &TlsFullCertBudget::new(),
         &test_web_publication(),
+        None,
     )
     .await;
 
@@ -223,6 +224,7 @@ async fn test_render_tls_front_profile_health() {
         Some(&cache),
         &TlsFullCertBudget::new(),
         &test_web_publication(),
+        None,
     )
     .await;
 
@@ -293,6 +295,7 @@ async fn process_tls_budget_metrics_survive_a_generation_without_tls_cache() {
         None,
         budget.as_ref(),
         &test_web_publication(),
+        None,
     )
     .await;
 
@@ -313,6 +316,7 @@ async fn test_render_empty_stats() {
         None,
         &TlsFullCertBudget::new(),
         &test_web_publication(),
+        None,
     )
     .await;
     assert!(output.contains("telemt_connections_total 0"));
@@ -346,6 +350,7 @@ async fn test_render_uses_global_each_unique_ip_limit() {
         None,
         &TlsFullCertBudget::new(),
         &test_web_publication(),
+        None,
     )
     .await;
 
@@ -367,6 +372,7 @@ async fn test_render_has_type_annotations() {
         None,
         &TlsFullCertBudget::new(),
         &test_web_publication(),
+        None,
     )
     .await;
     assert!(output.contains("# TYPE telemt_uptime_seconds gauge"));

src/transport/middle_proxy/health.rs

@@ -16,9 +16,13 @@ use crate::crypto::SecureRandom;
 use crate::network::IpFamily;
 
 use super::MePool;
-use super::pool::MeFamilyRuntimeState;
+use super::pool::{
+    MeFamilyRuntimeState, RefillTargetKey, WriterContour, WriterOpenIntent, WriterRole,
+};
+use super::pool_writer::WriterReplacementPurpose;
 
-const JITTER_FRAC_NUM: u64 = 2; // jitter up to 50% of backoff
+// Bounds reconnect jitter to at most half of the current backoff.
+const JITTER_FRAC_NUM: u64 = 2;
 #[allow(dead_code)]
 const MAX_CONCURRENT_PER_DC_DEFAULT: usize = 1;
 const SHADOW_ROTATE_RETRY_SECS: u64 = 30;

src/transport/middle_proxy/health/drain.rs

@@ -31,7 +31,7 @@ pub(in crate::transport::middle_proxy) async fn reap_draining_writers(
     let mut force_close_writer_ids = Vec::<u64>::new();
     let writers = pool.writers.read().await;
     for writer in writers.iter() {
-        if !writer.draining.load(std::sync::atomic::Ordering::Relaxed) {
+        if !writer.draining.load(std::sync::atomic::Ordering::Acquire) {
             continue;
         }
         if activity

src/transport/middle_proxy/health/family.rs

@@ -47,6 +47,7 @@ pub(super) async fn check_family(
 
     let mut live_addr_counts = HashMap::<(i32, SocketAddr), usize>::new();
     let mut live_writer_ids_by_addr = HashMap::<(i32, SocketAddr), Vec<u64>>::new();
+    let active_generation = pool.current_generation();
     for writer in pool
         .writers
         .read()
@@ -62,6 +63,15 @@ pub(super) async fn check_family(
         ) {
             continue;
         }
+        if writer.generation != active_generation {
+            continue;
+        }
+        if !dc_endpoints
+            .get(&writer.writer_dc)
+            .is_some_and(|endpoints| endpoints.contains(&writer.addr))
+        {
+            continue;
+        }
         let key = (writer.writer_dc, writer.addr);
         *live_addr_counts.entry(key).or_insert(0) += 1;
         live_writer_ids_by_addr
@@ -93,6 +103,13 @@ pub(super) async fn check_family(
         floor_plan.active_writers_current,
         floor_plan.warm_writers_current,
     );
+    let floor_targets_by_dc = Arc::new(
+        floor_plan
+            .by_dc
+            .iter()
+            .map(|(dc, entry)| (*dc, entry.target_required))
+            .collect::<HashMap<_, _>>(),
+    );
     let live_writer_ids_by_addr = Arc::new(live_writer_ids_by_addr);
     let writer_idle_since = Arc::new(writer_idle_since);
     let bound_clients_by_writer = Arc::new(bound_clients_by_writer);
@@ -162,7 +179,6 @@ pub(super) async fn check_family(
             );
         }
 
-        if alive >= required {
         maybe_refresh_idle_writer_for_dc(
             pool,
             rng,
@@ -178,6 +194,8 @@ pub(super) async fn check_family(
             idle_refresh_next_attempt,
         )
         .await;
+
+        if alive >= required {
             maybe_rotate_single_endpoint_shadow(
                 pool,
                 rng,
@@ -230,9 +248,11 @@ pub(super) async fn check_family(
             continue;
         }
         if pool
-            .has_refill_inflight_for_dc_key(crate::transport::middle_proxy::pool::RefillDcKey {
+            .has_refill_inflight_for_target(RefillTargetKey {
                 dc,
                 family,
+                generation: pool.current_generation(),
+                contour: WriterContour::Active,
             })
             .await
         {
@@ -254,6 +274,7 @@ pub(super) async fn check_family(
         let live_writer_ids_by_addr_for_dc = live_writer_ids_by_addr.clone();
         let writer_idle_since_for_dc = writer_idle_since.clone();
         let bound_clients_by_writer_for_dc = bound_clients_by_writer.clone();
+        let floor_targets_by_dc_for_reconnect = floor_targets_by_dc.clone();
         let active_cap_effective_total = floor_plan.active_cap_effective_total;
         reconnect_set.spawn(async move {
             let mut restored = 0usize;
@@ -261,6 +282,8 @@ pub(super) async fn check_family(
                 let Ok(reconnect_permit) = reconnect_sem_for_dc.clone().try_acquire_owned() else {
                     break;
                 };
+                let base_req = pool_for_reconnect
+                    .required_writers_for_dc_with_floor_mode(endpoints_for_dc.len(), false);
                 if pool_for_reconnect.active_contour_writer_count_total().await
                     >= active_cap_effective_total
                 {
@@ -270,6 +293,8 @@ pub(super) async fn check_family(
                         dc,
                         family,
                         &endpoints_for_dc,
+                        required,
+                        floor_targets_by_dc_for_reconnect.as_ref(),
                         live_writer_ids_by_addr_for_dc.as_ref(),
                         writer_idle_since_for_dc.as_ref(),
                         bound_clients_by_writer_for_dc.as_ref(),
@@ -283,8 +308,6 @@ pub(super) async fn check_family(
                         continue;
                     }
 
-                    let base_req = pool_for_reconnect
-                        .required_writers_for_dc_with_floor_mode(endpoints_for_dc.len(), false);
                     if alive + restored >= base_req {
                         pool_for_reconnect
                             .stats
@@ -304,12 +327,20 @@ pub(super) async fn check_family(
                     }
                 }
                 pool_for_reconnect.stats.increment_me_reconnect_attempt();
+                let intent = if alive + restored < base_req {
+                    WriterOpenIntent::Coverage
+                } else {
+                    WriterOpenIntent::Normal
+                };
                 let res = tokio::time::timeout(
                     pool_for_reconnect.reconnect_runtime.me_one_timeout,
-                    pool_for_reconnect.connect_endpoints_round_robin(
+                    pool_for_reconnect.connect_endpoints_round_robin_with_generation_contour(
                         dc,
                         &endpoints_for_dc,
                         rng_for_reconnect.as_ref(),
+                        pool_for_reconnect.current_generation(),
+                        WriterContour::Active,
+                        intent,
                     ),
                 )
                 .await;

src/transport/middle_proxy/health/floor_plan.rs

@@ -22,6 +22,7 @@ pub(super) async fn live_active_writers_for_dc_family(
     dc: i32,
     family: IpFamily,
 ) -> usize {
+    let active_generation = pool.current_generation();
     let writers = pool.writers.read().await;
     writers
         .iter()
@@ -32,6 +33,9 @@ pub(super) async fn live_active_writers_for_dc_family(
             if writer.writer_dc != dc {
                 return false;
             }
+            if writer.generation != active_generation {
+                return false;
+            }
             if !matches!(
                 crate::transport::middle_proxy::pool::WriterContour::from_u8(
                     writer.contour.load(std::sync::atomic::Ordering::Relaxed),

src/transport/middle_proxy/health/idle_refresh.rs

@@ -6,16 +6,43 @@ pub(super) async fn maybe_swap_idle_writer_for_cap(
     dc: i32,
     family: IpFamily,
     endpoints: &[SocketAddr],
+    required: usize,
+    floor_targets_by_dc: &HashMap<i32, usize>,
     live_writer_ids_by_addr: &HashMap<(i32, SocketAddr), Vec<u64>>,
     writer_idle_since: &HashMap<u64, u64>,
     bound_clients_by_writer: &HashMap<u64, usize>,
 ) -> bool {
+    let Some(replacement_endpoint) = endpoints
+        .iter()
+        .min_by_key(|endpoint| {
+            live_writer_ids_by_addr
+                .get(&(dc, **endpoint))
+                .map_or(0, Vec::len)
+        })
+        .copied()
+    else {
+        return false;
+    };
+
+    let mut alive_by_dc = HashMap::<i32, usize>::new();
+    for ((writer_dc, endpoint), writer_ids) in live_writer_ids_by_addr {
+        if endpoint.is_ipv4() == matches!(family, IpFamily::V4) {
+            *alive_by_dc.entry(*writer_dc).or_insert(0) += writer_ids.len();
+        }
+    }
+
     let now_epoch_secs = MePool::now_epoch_secs();
-    let mut candidate: Option<(u64, SocketAddr, u64)> = None;
+    let mut candidates = Vec::<(u64, SocketAddr, i32, u64, usize)>::new();
-    for endpoint in endpoints {
+    for ((writer_dc, endpoint), writer_ids) in live_writer_ids_by_addr {
-        let Some(writer_ids) = live_writer_ids_by_addr.get(&(dc, *endpoint)) else {
+        if endpoint.is_ipv4() != matches!(family, IpFamily::V4) {
+            continue;
+        }
+        let Some(donor_floor) = floor_targets_by_dc.get(writer_dc).copied() else {
             continue;
         };
+        if alive_by_dc.get(writer_dc).copied().unwrap_or(0) <= donor_floor {
+            continue;
+        }
         for writer_id in writer_ids {
             if bound_clients_by_writer.get(writer_id).copied().unwrap_or(0) > 0 {
                 continue;
@@ -23,67 +50,96 @@ pub(super) async fn maybe_swap_idle_writer_for_cap(
             let Some(idle_since_epoch_secs) = writer_idle_since.get(writer_id).copied() else {
                 continue;
             };
-            let idle_age_secs = now_epoch_secs.saturating_sub(idle_since_epoch_secs);
+            candidates.push((
-            if candidate
+                *writer_id,
-                .as_ref()
+                *endpoint,
-                .map(|(_, _, age)| idle_age_secs > *age)
+                *writer_dc,
-                .unwrap_or(true)
+                now_epoch_secs.saturating_sub(idle_since_epoch_secs),
-            {
+                donor_floor,
-                candidate = Some((*writer_id, *endpoint, idle_age_secs));
+            ));
-            }
         }
     }
+    candidates.sort_unstable_by(|left, right| right.3.cmp(&left.3));
 
-    let Some((old_writer_id, endpoint, idle_age_secs)) = candidate else {
+    for (old_writer_id, donor_endpoint, donor_dc, idle_age_secs, donor_floor) in candidates {
-        return false;
+        let expected_role = {
+            let writers = pool.writers.read().await;
+            writers
+                .iter()
+                .find(|writer| writer.id == old_writer_id)
+                .map(WriterRole::from_writer)
         };
-
+        let Some(expected_role) = expected_role else {
-    let connected = match tokio::time::timeout(
+            continue;
-        pool.reconnect_runtime.me_one_timeout,
+        };
-        pool.connect_one_for_dc(endpoint, dc, rng.as_ref()),
+        if expected_role.dc != donor_dc
-    )
+            || expected_role.family != family
-    .await
+            || expected_role.contour != WriterContour::Active
         {
-        Ok(Ok(())) => true,
+            continue;
+        }
+        let Some(mut reservation) = pool
+            .registry
+            .try_reserve_writer_replacement(old_writer_id)
+            .await
+        else {
+            continue;
+        };
+        let replace = pool.replace_writer_with_generation_contour_for_dc(
+            replacement_endpoint,
+            rng.as_ref(),
+            pool.current_generation(),
+            WriterContour::Active,
+            dc,
+            expected_role,
+            WriterReplacementPurpose::FloorRebalance {
+                donor_floor,
+                receiver_floor: required,
+            },
+            &mut reservation,
+        );
+        match tokio::time::timeout(pool.reconnect_runtime.me_one_timeout, replace).await {
+            Ok(Ok(())) => {
+                info!(
+                    dc = %dc,
+                    ?family,
+                    %replacement_endpoint,
+                    donor_dc = %donor_dc,
+                    %donor_endpoint,
+                    old_writer_id,
+                    idle_age_secs,
+                    "Adaptive floor cap rebalance committed"
+                );
+                return true;
+            }
             Ok(Err(error)) => {
                 debug!(
                     dc = %dc,
                     ?family,
-                %endpoint,
+                    %replacement_endpoint,
+                    donor_dc = %donor_dc,
                     old_writer_id,
                     idle_age_secs,
                     %error,
-                "Adaptive floor cap swap connect failed"
+                    "Adaptive floor cap rebalance failed"
                 );
-            false
+                return false;
             }
             Err(_) => {
                 debug!(
                     dc = %dc,
                     ?family,
-                %endpoint,
+                    %replacement_endpoint,
+                    donor_dc = %donor_dc,
                     old_writer_id,
                     idle_age_secs,
-                "Adaptive floor cap swap connect timed out"
+                    "Adaptive floor cap rebalance timed out"
                 );
-            false
-        }
-    };
-    if !connected {
                 return false;
             }
-
+        }
-    pool.mark_writer_draining_with_timeout(old_writer_id, pool.force_close_timeout(), false)
+    }
-        .await;
+    false
-    info!(
-        dc = %dc,
-        ?family,
-        %endpoint,
-        old_writer_id,
-        idle_age_secs,
-        "Adaptive floor cap swap: idle writer rotated"
-    );
-    true
 }
 
 pub(super) async fn maybe_refresh_idle_writer_for_dc(
@@ -100,10 +156,6 @@ pub(super) async fn maybe_refresh_idle_writer_for_dc(
     bound_clients_by_writer: &HashMap<u64, usize>,
     idle_refresh_next_attempt: &mut HashMap<(i32, IpFamily), Instant>,
 ) {
-    if alive < required {
-        return;
-    }
-
     let now = Instant::now();
     if let Some(next) = idle_refresh_next_attempt.get(&key)
         && now < *next
@@ -143,13 +195,46 @@ pub(super) async fn maybe_refresh_idle_writer_for_dc(
     let Some((old_writer_id, endpoint, idle_age_secs, threshold_secs)) = candidate else {
         return;
     };
-
+    let expected_role = {
-    let rotate_ok = match tokio::time::timeout(
+        let writers = pool.writers.read().await;
-        pool.reconnect_runtime.me_one_timeout,
+        writers
-        pool.connect_one_for_dc(endpoint, dc, rng.as_ref()),
+            .iter()
-    )
+            .find(|writer| writer.id == old_writer_id)
-    .await
+            .map(WriterRole::from_writer)
+    };
+    let Some(expected_role) = expected_role else {
+        return;
+    };
+    if expected_role.dc != dc
+        || expected_role.family != family
+        || expected_role.contour != WriterContour::Active
     {
+        return;
+    }
+    let Some(mut reservation) = pool
+        .registry
+        .try_reserve_writer_replacement(old_writer_id)
+        .await
+    else {
+        return;
+    };
+    let generation = pool.current_generation();
+    let purpose = if expected_role.generation == generation {
+        WriterReplacementPurpose::IdleRefresh
+    } else {
+        WriterReplacementPurpose::GenerationConvergence
+    };
+    let replace = pool.replace_writer_with_generation_contour_for_dc(
+        endpoint,
+        rng.as_ref(),
+        generation,
+        WriterContour::Active,
+        dc,
+        expected_role,
+        purpose,
+        &mut reservation,
+    );
+    let rotate_ok = match tokio::time::timeout(pool.reconnect_runtime.me_one_timeout, replace).await {
         Ok(Ok(())) => true,
         Ok(Err(error)) => {
             debug!(
@@ -160,7 +245,7 @@ pub(super) async fn maybe_refresh_idle_writer_for_dc(
                 idle_age_secs,
                 threshold_secs,
                 %error,
-                "Idle writer pre-refresh connect failed"
+                "Idle writer pre-refresh replacement failed"
             );
             false
         }
@@ -172,7 +257,7 @@ pub(super) async fn maybe_refresh_idle_writer_for_dc(
                 old_writer_id,
                 idle_age_secs,
                 threshold_secs,
-                "Idle writer pre-refresh connect timed out"
+                "Idle writer pre-refresh replacement timed out"
             );
             false
         }
@@ -183,8 +268,6 @@ pub(super) async fn maybe_refresh_idle_writer_for_dc(
         return;
     }
 
-    pool.mark_writer_draining_with_timeout(old_writer_id, pool.force_close_timeout(), false)
-        .await;
     idle_refresh_next_attempt.insert(
         key,
         now + Duration::from_secs(IDLE_REFRESH_SUCCESS_GUARD_SECS),
@@ -198,6 +281,7 @@ pub(super) async fn maybe_refresh_idle_writer_for_dc(
         threshold_secs,
         alive,
         required,
+        generation_convergence = expected_role.generation != generation,
         "Idle writer refreshed before upstream idle timeout"
     );
 }

src/transport/middle_proxy/health/recovery.rs

@@ -209,10 +209,38 @@ pub(super) async fn maybe_rotate_single_endpoint_shadow(
         );
         return;
     };
+    let expected_role = {
+        let writers = pool.writers.read().await;
+        writers
+            .iter()
+            .find(|writer| writer.id == old_writer_id)
+            .map(WriterRole::from_writer)
+    };
+    let Some(expected_role) = expected_role else {
+        shadow_rotate_deadline.insert(key, now + Duration::from_secs(SHADOW_ROTATE_RETRY_SECS));
+        return;
+    };
+    let Some(mut reservation) = pool
+        .registry
+        .try_reserve_writer_replacement(old_writer_id)
+        .await
+    else {
+        shadow_rotate_deadline.insert(key, now + Duration::from_secs(SHADOW_ROTATE_RETRY_SECS));
+        return;
+    };
 
     let rotate_ok = match tokio::time::timeout(
         pool.reconnect_runtime.me_one_timeout,
-        pool.connect_one_for_dc(endpoint, dc, rng.as_ref()),
+        pool.replace_writer_with_generation_contour_for_dc(
+            endpoint,
+            rng.as_ref(),
+            pool.current_generation(),
+            WriterContour::Active,
+            dc,
+            expected_role,
+            WriterReplacementPurpose::ShadowRotation,
+            &mut reservation,
+        ),
     )
     .await
     {
@@ -246,8 +274,6 @@ pub(super) async fn maybe_rotate_single_endpoint_shadow(
         return;
     }
 
-    pool.mark_writer_draining_with_timeout(old_writer_id, pool.force_close_timeout(), false)
-        .await;
     pool.stats
         .increment_me_single_endpoint_shadow_rotate_total();
     shadow_rotate_deadline.insert(key, now + interval);

src/transport/middle_proxy/health/tests.rs

@@ -7,7 +7,7 @@ use std::time::{Duration, Instant};
 use tokio::sync::mpsc;
 use tokio_util::sync::CancellationToken;
 
-use super::{ScheduledReconnects, reap_draining_writers};
+use super::{ScheduledReconnects, maybe_refresh_idle_writer_for_dc, reap_draining_writers};
 use crate::config::{GeneralConfig, MeRouteNoWriterMode, MeSocksKdfPolicy, MeWriterPickMode};
 use crate::crypto::SecureRandom;
 use crate::network::IpFamily;
@@ -226,6 +226,79 @@ async fn insert_live_writer(pool: &Arc<MePool>, writer_id: u64, writer_dc: i32)
     pool.conn_count.fetch_add(1, Ordering::Relaxed);
 }
 
+async fn insert_active_writer_at(
+    pool: &Arc<MePool>,
+    writer_id: u64,
+    writer_dc: i32,
+    addr: SocketAddr,
+) -> MeWriter {
+    let (tx, _writer_rx) = mpsc::channel::<WriterCommand>(8);
+    let byte_budget = pool.new_writer_byte_budget();
+    let writer = MeWriter {
+        id: writer_id,
+        addr,
+        source_ip: addr.ip(),
+        writer_dc,
+        generation: pool.current_generation(),
+        contour: Arc::new(AtomicU8::new(WriterContour::Active.as_u8())),
+        created_at: Instant::now(),
+        tx: tx.clone(),
+        byte_budget: byte_budget.clone(),
+        cancel: CancellationToken::new(),
+        degraded: Arc::new(AtomicBool::new(false)),
+        rtt_ema_ms_x10: Arc::new(AtomicU32::new(0)),
+        draining: Arc::new(AtomicBool::new(false)),
+        draining_started_at_epoch_secs: Arc::new(AtomicU64::new(0)),
+        drain_deadline_epoch_secs: Arc::new(AtomicU64::new(0)),
+        allow_drain_fallback: Arc::new(AtomicBool::new(false)),
+    };
+    pool.writers.write().await.push(writer.clone());
+    pool.registry
+        .register_writer(writer_id, tx, byte_budget)
+        .await;
+    pool.conn_count.fetch_add(1, Ordering::Relaxed);
+    writer
+}
+
+#[tokio::test]
+async fn under_floor_idle_writer_still_enters_transactional_refresh() {
+    let pool = make_pool(128).await;
+    let listener = std::net::TcpListener::bind((Ipv4Addr::LOCALHOST, 0)).unwrap();
+    let endpoint = listener.local_addr().unwrap();
+    drop(listener);
+    let writer_id = 7001;
+    let writer = insert_active_writer_at(&pool, writer_id, 2, endpoint).await;
+    let key = (2, IpFamily::V4);
+    let live_writer_ids_by_addr = HashMap::from([((2, endpoint), vec![writer_id])]);
+    let writer_idle_since = HashMap::from([(
+        writer_id,
+        MePool::now_epoch_secs().saturating_sub(60),
+    )]);
+    let bound_clients_by_writer = HashMap::from([(writer_id, 0)]);
+    let mut next_attempt = HashMap::new();
+    let rng = Arc::new(SecureRandom::new());
+
+    maybe_refresh_idle_writer_for_dc(
+        &pool,
+        &rng,
+        key,
+        2,
+        IpFamily::V4,
+        &[endpoint],
+        1,
+        10,
+        &live_writer_ids_by_addr,
+        &writer_idle_since,
+        &bound_clients_by_writer,
+        &mut next_attempt,
+    )
+    .await;
+
+    assert!(next_attempt.contains_key(&key));
+    assert!(!writer.draining.load(Ordering::Acquire));
+    assert_eq!(pool.registry.writer_replacement_counts(), (0, 0));
+}
+
 #[tokio::test]
 async fn reap_draining_writers_force_closes_oldest_over_threshold() {
     let pool = make_pool(2).await;

src/transport/middle_proxy/health/zombie_watchdog.rs

@@ -35,7 +35,7 @@ pub async fn me_zombie_writer_watchdog(pool: Arc<MePool>) {
                 continue;
             };
             ws.iter()
-                .filter(|w| w.draining.load(std::sync::atomic::Ordering::Relaxed))
+                .filter(|w| w.draining.load(std::sync::atomic::Ordering::Acquire))
                 .filter_map(|w| {
                     let deadline = w
                         .drain_deadline_epoch_secs

src/transport/middle_proxy/mod.rs

@@ -64,6 +64,7 @@ pub use ping::{
     MePingFamily, MePingReport, MePingSample, format_me_route, format_sample_line, run_me_ping,
 };
 pub use pool::MePool;
+pub(crate) use pool_status::MeApiHardswapSnapshot;
 #[allow(unused_imports)]
 pub use pool_nat::{detect_public_ip, stun_probe};
 pub(crate) use registry::ConnLease;

src/transport/middle_proxy/pool.rs

@@ -29,9 +29,25 @@ use super::pool_lifecycle::MePoolLifecycle;
 const ME_FORCE_CLOSE_SAFETY_FALLBACK_SECS: u64 = 300;
 
 #[derive(Debug, Clone, Copy, PartialEq, Eq, Hash)]
-pub(super) struct RefillDcKey {
+/// Exact lifecycle role used to coalesce refill work without cross-generation drift.
+pub(super) struct RefillTargetKey {
+    /// Telegram DC owning the writer.
     pub dc: i32,
+    /// Address family of the writer endpoint.
     pub family: IpFamily,
+    /// Generation that retains publication authority.
+    pub generation: u64,
+    /// Lifecycle contour that the replacement must preserve.
+    pub contour: WriterContour,
+}
+
+#[derive(Debug, Clone, Copy, Default)]
+/// Bounded queued-loss state for one exact refill target.
+pub(super) struct RefillTargetState {
+    /// Additional lost writers waiting behind the active refill producer.
+    pub(super) pending_count: usize,
+    /// Most recently lost endpoint, used as the next same-endpoint preference.
+    pub(super) next_addr: Option<SocketAddr>,
 }
 
 #[derive(Clone)]
@@ -121,6 +137,13 @@ impl DerefMut for WritersWriteGuard<'_> {
     }
 }
 
+impl WritersWriteGuard<'_> {
+    /// Publishes the current vector while retaining exclusive mutation ownership.
+    pub(super) fn publish_current(&self) {
+        self.state.store_guarded(self.writers.clone());
+    }
+}
+
 impl Drop for WritersWriteGuard<'_> {
     fn drop(&mut self) {
         let writers = std::mem::take(&mut self.writers);
@@ -128,7 +151,7 @@ impl Drop for WritersWriteGuard<'_> {
     }
 }
 
-#[derive(Debug, Clone, Copy, PartialEq, Eq)]
+#[derive(Debug, Clone, Copy, PartialEq, Eq, Hash)]
 #[repr(u8)]
 pub(super) enum WriterContour {
     Warm = 0,
@@ -136,18 +159,6 @@ pub(super) enum WriterContour {
     Draining = 2,
 }
 
-pub(super) struct WriterOpenReservation<'a> {
-    counter: Option<&'a AtomicUsize>,
-}
-
-impl Drop for WriterOpenReservation<'_> {
-    fn drop(&mut self) {
-        if let Some(counter) = self.counter {
-            counter.fetch_sub(1, Ordering::AcqRel);
-        }
-    }
-}
-
 impl WriterContour {
     pub(super) fn as_u8(self) -> u8 {
         self as u8
@@ -471,8 +482,9 @@ pub struct MePool {
     pub(super) next_writer_id: AtomicU64,
     pub(super) writer_connect_active_reserved: AtomicUsize,
     pub(super) writer_connect_warm_reserved: AtomicUsize,
+    pub(super) writer_replacement_open_reserved: AtomicUsize,
     pub(super) rtt_stats: Arc<Mutex<HashMap<u64, (f64, f64)>>>,
-    pub(super) refill_states: Arc<ParkingMutex<HashMap<RefillDcKey, Option<SocketAddr>>>>,
+    pub(super) refill_states: Arc<ParkingMutex<HashMap<RefillTargetKey, RefillTargetState>>>,
     pub(super) refill_running: AtomicUsize,
     pub(super) refill_pending: AtomicUsize,
     pub(super) conn_count: AtomicUsize,
@@ -508,5 +520,6 @@ mod transport_policy;
 mod selection_policy;
 // Bounded writer-open admission and coverage accounting.
 mod writer_admission;
+pub(super) use writer_admission::{WriterOpenIntent, WriterOpenReservation, WriterRole};
 // Endpoint-to-DC routing and health timing policy.
 mod routing;

src/transport/middle_proxy/pool/construction.rs

@@ -398,6 +398,7 @@ impl MePool {
             next_writer_id: AtomicU64::new(1),
             writer_connect_active_reserved: AtomicUsize::new(0),
             writer_connect_warm_reserved: AtomicUsize::new(0),
+            writer_replacement_open_reserved: AtomicUsize::new(0),
             rtt_stats: Arc::new(Mutex::new(HashMap::new())),
             refill_states: Arc::new(ParkingMutex::new(HashMap::new())),
             refill_running: AtomicUsize::new(0),

src/transport/middle_proxy/pool/writer_admission.rs

@@ -1,42 +1,110 @@
 use super::*;
 
+const WRITER_REPLACEMENT_OPEN_LIMIT_MAX: usize = 128;
+
+#[derive(Debug, Clone, Copy, PartialEq, Eq, Hash)]
+/// Immutable identity of a writer lifecycle role at replacement reservation time.
+pub(in crate::transport::middle_proxy) struct WriterRole {
+    /// Telegram DC owning the writer.
+    pub(in crate::transport::middle_proxy) dc: i32,
+    /// Address family of the writer endpoint.
+    pub(in crate::transport::middle_proxy) family: IpFamily,
+    /// Pool generation owning the writer.
+    pub(in crate::transport::middle_proxy) generation: u64,
+    /// Lifecycle contour assigned to the writer.
+    pub(in crate::transport::middle_proxy) contour: WriterContour,
+}
+
+impl WriterRole {
+    /// Captures the current role of an installed writer.
+    pub(in crate::transport::middle_proxy) fn from_writer(writer: &MeWriter) -> Self {
+        Self {
+            dc: writer.writer_dc,
+            family: if writer.addr.is_ipv4() {
+                IpFamily::V4
+            } else {
+                IpFamily::V6
+            },
+            generation: writer.generation,
+            contour: WriterContour::from_u8(writer.contour.load(Ordering::Acquire)),
+        }
+    }
+
+    /// Revalidates that an installed writer still has this exact role.
+    pub(in crate::transport::middle_proxy) fn matches(self, writer: &MeWriter) -> bool {
+        self == Self::from_writer(writer)
+    }
+}
+
+#[derive(Debug, Clone, Copy, PartialEq, Eq)]
+/// Capacity policy applied while opening a writer.
+pub(in crate::transport::middle_proxy) enum WriterOpenIntent {
+    /// Ordinary pool growth constrained by the configured contour cap.
+    Normal,
+    /// Required active coverage allowed to exceed an undersized configured cap temporarily.
+    Coverage,
+    /// Replacement-before-drain capacity owned by an existing victim reservation.
+    Replacement,
+}
+
+/// RAII ownership of one bounded in-flight writer open.
+pub(in crate::transport::middle_proxy) struct WriterOpenReservation<'a> {
+    counter: Option<&'a AtomicUsize>,
+}
+
+impl Drop for WriterOpenReservation<'_> {
+    fn drop(&mut self) {
+        if let Some(counter) = self.counter {
+            counter.fetch_sub(1, Ordering::AcqRel);
+        }
+    }
+}
+
 impl MePool {
+    /// Computes the authoritative active-writer floor across enabled families and DCs.
     pub(in crate::transport::middle_proxy) async fn active_coverage_required_total(&self) -> usize {
         let now_epoch_secs = Self::now_epoch_secs();
-        let mut endpoints_by_dc = HashMap::<i32, HashSet<SocketAddr>>::new();
+        let mut required_total = 0usize;
 
         if self.family_enabled_for_drain_coverage(IpFamily::V4, now_epoch_secs) {
             let map = self.proxy_map_v4.read().await;
-            for (dc, addrs) in map.iter() {
+            for addrs in map.values() {
-                let entry = endpoints_by_dc.entry(*dc).or_default();
+                let mut endpoints = HashSet::<SocketAddr>::new();
                 for (ip, port) in addrs.iter().copied() {
-                    entry.insert(SocketAddr::new(ip, port));
+                    endpoints.insert(SocketAddr::new(ip, port));
                 }
+                required_total = required_total.saturating_add(
+                    self.required_writers_for_dc_with_floor_mode(endpoints.len(), false),
+                );
             }
         }
 
         if self.family_enabled_for_drain_coverage(IpFamily::V6, now_epoch_secs) {
             let map = self.proxy_map_v6.read().await;
-            for (dc, addrs) in map.iter() {
+            for addrs in map.values() {
-                let entry = endpoints_by_dc.entry(*dc).or_default();
+                let mut endpoints = HashSet::<SocketAddr>::new();
                 for (ip, port) in addrs.iter().copied() {
-                    entry.insert(SocketAddr::new(ip, port));
+                    endpoints.insert(SocketAddr::new(ip, port));
                 }
+                required_total = required_total.saturating_add(
+                    self.required_writers_for_dc_with_floor_mode(endpoints.len(), false),
+                );
             }
         }
 
-        endpoints_by_dc
+        required_total
-            .values()
-            .map(|endpoints| self.required_writers_for_dc_with_floor_mode(endpoints.len(), false))
-            .sum()
     }
 
+    /// Reports whether one writer may be opened under the selected contour policy.
     pub(in crate::transport::middle_proxy) async fn can_open_writer_for_contour(
         &self,
         contour: WriterContour,
-        allow_coverage_override: bool,
+        intent: WriterOpenIntent,
         writer_dc: i32,
     ) -> bool {
+        if intent == WriterOpenIntent::Replacement {
+            return true;
+        }
         let (active_writers, warm_writers, _) = self.non_draining_writer_counts_by_contour().await;
         match contour {
             WriterContour::Active => {
@@ -44,7 +112,7 @@ impl MePool {
                 if active_writers < active_cap {
                     return true;
                 }
-                if !allow_coverage_override {
+                if intent != WriterOpenIntent::Coverage {
                     return false;
                 }
 
@@ -64,12 +132,14 @@ impl MePool {
                 if endpoints_len > 0 {
                     let base_req =
                         self.required_writers_for_dc_with_floor_mode(endpoints_len, false);
+                    let active_generation = self.reinit.status.load().active_generation;
                     let active_for_dc = {
                         let ws = self.writers.read().await;
                         ws.iter()
                             .filter(|w| {
                                 !w.draining.load(std::sync::atomic::Ordering::Relaxed)
                                     && w.writer_dc == writer_dc
+                                    && w.generation == active_generation
                                     && matches!(
                                         WriterContour::from_u8(
                                             w.contour.load(std::sync::atomic::Ordering::Relaxed),
@@ -92,10 +162,11 @@ impl MePool {
         }
     }
 
+    /// Reserves bounded transient capacity for a writer open attempt.
     pub(in crate::transport::middle_proxy) async fn reserve_writer_open(
         &self,
         contour: WriterContour,
-        allow_coverage_override: bool,
+        intent: WriterOpenIntent,
         writer_dc: i32,
     ) -> Option<WriterOpenReservation<'_>> {
         let counter = match contour {
@@ -106,9 +177,52 @@ impl MePool {
             }
         };
 
+        if intent == WriterOpenIntent::Replacement {
+            let configured_cap = match contour {
+                WriterContour::Active => self.adaptive_floor_active_cap_configured_total(),
+                WriterContour::Warm => self.adaptive_floor_warm_cap_configured_total(),
+                WriterContour::Draining => usize::MAX,
+            };
+            let effective_cap = match contour {
+                WriterContour::Active => self
+                    .floor_runtime
+                    .me_adaptive_floor_active_cap_effective
+                    .load(Ordering::Acquire) as usize,
+                WriterContour::Warm => self
+                    .floor_runtime
+                    .me_adaptive_floor_warm_cap_effective
+                    .load(Ordering::Acquire) as usize,
+                WriterContour::Draining => usize::MAX,
+            };
+            let replacement_limit = configured_cap
+                .max(effective_cap)
+                .max(1)
+                .min(WRITER_REPLACEMENT_OPEN_LIMIT_MAX);
+            loop {
+                let reserved = self.writer_replacement_open_reserved.load(Ordering::Acquire);
+                if reserved >= replacement_limit {
+                    return None;
+                }
+                if self
+                    .writer_replacement_open_reserved
+                    .compare_exchange_weak(
+                        reserved,
+                        reserved + 1,
+                        Ordering::AcqRel,
+                        Ordering::Acquire,
+                    )
+                    .is_ok()
+                {
+                    return Some(WriterOpenReservation {
+                        counter: Some(&self.writer_replacement_open_reserved),
+                    });
+                }
+            }
+        }
+
         loop {
             if !self
-                .can_open_writer_for_contour(contour, allow_coverage_override, writer_dc)
+                .can_open_writer_for_contour(contour, intent, writer_dc)
                 .await
             {
                 return None;
@@ -125,7 +239,7 @@ impl MePool {
                 WriterContour::Warm => self.adaptive_floor_warm_cap_configured_total(),
                 WriterContour::Draining => usize::MAX,
             };
-            if contour == WriterContour::Active && allow_coverage_override {
+            if contour == WriterContour::Active && intent == WriterOpenIntent::Coverage {
                 limit = limit
                     .max(self.active_coverage_required_total().await)
                     .saturating_add(
@@ -150,6 +264,7 @@ impl MePool {
         }
     }
 
+    /// Resolves a DC writer floor for static or adaptive-idle operation.
     pub(in crate::transport::middle_proxy) fn required_writers_for_dc_with_floor_mode(
         &self,
         endpoint_count: usize,

src/transport/middle_proxy/pool_config.rs

@@ -5,7 +5,8 @@ use std::time::Duration;
 
 use tracing::warn;
 
-use super::pool::MePool;
+use super::pool::{MePool, WriterContour, WriterRole};
+use super::pool_writer::WriterReplacementPurpose;
 
 #[derive(Debug, Clone, Copy, PartialEq, Eq)]
 pub enum SnapshotApplyOutcome {
@@ -113,11 +114,33 @@ impl MePool {
     pub async fn reconnect_all(self: &Arc<Self>) {
         let ws = self.writers.read().await.clone();
         for w in ws.iter() {
-            if let Ok(()) = self
+            let role = WriterRole::from_writer(w);
-                .connect_one_for_dc(w.addr, w.writer_dc, self.rng.as_ref())
+            if w.draining.load(std::sync::atomic::Ordering::Acquire)
-                .await
+                || role.contour == WriterContour::Draining
+            {
+                continue;
+            }
+            let Some(mut reservation) = self
+                .registry
+                .try_reserve_writer_replacement_preserving_clients(w.id)
+                .await
+            else {
+                continue;
+            };
+            if self
+                .replace_writer_with_generation_contour_for_dc(
+                    w.addr,
+                    self.rng.as_ref(),
+                    role.generation,
+                    role.contour,
+                    role.dc,
+                    role,
+                    WriterReplacementPurpose::SecretRotation,
+                    &mut reservation,
+                )
+                .await
+                .is_ok()
             {
-                self.mark_writer_draining(w.id).await;
                 tokio::time::sleep(Duration::from_secs(2)).await;
             }
         }

src/transport/middle_proxy/pool_init.rs

@@ -9,7 +9,7 @@ use tracing::{debug, info, warn};
 use crate::crypto::SecureRandom;
 use crate::error::{ProxyError, Result};
 
-use super::pool::MePool;
+use super::pool::{MePool, WriterOpenIntent};
 
 impl MePool {
     pub async fn init(self: &Arc<Self>, pool_size: usize, rng: &Arc<SecureRandom>) -> Result<()> {
@@ -76,7 +76,7 @@ impl MePool {
                         target_writers,
                         rng_clone,
                         connect_concurrency,
-                        true,
+                        WriterOpenIntent::Coverage,
                     )
                     .await
                 });
@@ -125,7 +125,7 @@ impl MePool {
                                 target_writers,
                                 rng_clone_local,
                                 connect_concurrency,
-                                false,
+                                WriterOpenIntent::Normal,
                             )
                             .await
                     });
@@ -162,7 +162,7 @@ impl MePool {
         target_writers: usize,
         rng: Arc<SecureRandom>,
         connect_concurrency: usize,
-        allow_coverage_override: bool,
+        intent: WriterOpenIntent,
     ) -> bool {
         if addrs.is_empty() {
             return false;
@@ -204,7 +204,7 @@ impl MePool {
                         rng_clone.as_ref(),
                         generation,
                         super::pool::WriterContour::Active,
-                        allow_coverage_override,
+                        intent,
                     )
                     .await
                 });
@@ -238,7 +238,9 @@ impl MePool {
             if !progress {
                 let active_writers_current = self.active_contour_writer_count_total().await;
                 let active_cap_configured = self.adaptive_floor_active_cap_configured_total();
-                if !allow_coverage_override && active_writers_current >= active_cap_configured {
+                if intent != WriterOpenIntent::Coverage
+                    && active_writers_current >= active_cap_configured
+                {
                     info!(
                         dc = %dc,
                         alive = alive_after,

src/transport/middle_proxy/pool_refill.rs

@@ -9,26 +9,42 @@ use tracing::{debug, info, warn};
 use crate::crypto::SecureRandom;
 use crate::network::IpFamily;
 
-use super::pool::{MePool, RefillDcKey, WriterContour};
+use super::pool::{
+    MePool, RefillTargetKey, RefillTargetState, WriterContour, WriterOpenIntent, WriterRole,
+};
 
 const ME_FLAP_UPTIME_THRESHOLD_SECS: u64 = 20;
 const ME_FLAP_QUARANTINE_SECS: u64 = 25;
 const ME_FLAP_MIN_UPTIME_MILLIS: u64 = 500;
 const ME_REFILL_TOTAL_ATTEMPT_CAP: u32 = 20;
+const ME_REFILL_PENDING_PER_TARGET_MAX: usize = 128;
+
+#[derive(Debug, Clone, Copy, PartialEq, Eq)]
+enum RefillOutcome {
+    Restored,
+    Failed,
+    Obsolete,
+}
 
 struct RefillRunGuard {
     pool: Arc<MePool>,
-    key: RefillDcKey,
+    key: RefillTargetKey,
     active: bool,
 }
 
 impl RefillRunGuard {
     fn next_or_finish(&mut self) -> Option<SocketAddr> {
         let mut states = self.pool.refill_states.lock();
-        let next = states.get_mut(&self.key).and_then(Option::take);
+        let next = states.get_mut(&self.key).and_then(|state| {
-        if next.is_some() {
+            if state.pending_count == 0 {
+                return None;
+            }
+            state.pending_count -= 1;
+            state.next_addr
+        });
+        if let Some(next) = next {
             self.pool.refill_pending.fetch_sub(1, Ordering::AcqRel);
-            return next;
+            return Some(next);
         }
         states.remove(&self.key);
         self.pool.refill_running.fetch_sub(1, Ordering::AcqRel);
@@ -42,10 +58,10 @@ impl Drop for RefillRunGuard {
         if !self.active {
             return;
         }
-        if let Some(pending) = self.pool.refill_states.lock().remove(&self.key)
+        if let Some(state) = self.pool.refill_states.lock().remove(&self.key) {
-            && pending.is_some()
+            self.pool
-        {
+                .refill_pending
-            self.pool.refill_pending.fetch_sub(1, Ordering::AcqRel);
+                .fetch_sub(state.pending_count, Ordering::AcqRel);
         }
         self.pool.refill_running.fetch_sub(1, Ordering::AcqRel);
     }
@@ -165,7 +181,8 @@ impl MePool {
         self.connectable_endpoints(endpoints).await
     }
 
-    pub(super) async fn has_refill_inflight_for_dc_key(&self, key: RefillDcKey) -> bool {
+    /// Reports whether the exact generation and contour already has a refill producer.
+    pub(super) async fn has_refill_inflight_for_target(&self, key: RefillTargetKey) -> bool {
         self.refill_states.lock().contains_key(&key)
     }
 
@@ -181,7 +198,7 @@ impl MePool {
             rng,
             self.current_generation(),
             WriterContour::Active,
-            false,
+            WriterOpenIntent::Normal,
         )
         .await
     }
@@ -193,14 +210,14 @@ impl MePool {
         rng: &SecureRandom,
         generation: u64,
         contour: WriterContour,
-        allow_coverage_override: bool,
+        intent: WriterOpenIntent,
     ) -> bool {
         let mut candidates = self.connectable_endpoints(endpoints).await;
         if candidates.is_empty() {
             return false;
         }
         if candidates.len() > 1 {
-            let mut active_by_endpoint = HashMap::<SocketAddr, usize>::new();
+            let mut matching_by_endpoint = HashMap::<SocketAddr, usize>::new();
             let ws = self.writers.read().await;
             for writer in ws.iter() {
                 if writer.draining.load(Ordering::Relaxed) {
@@ -209,32 +226,31 @@ impl MePool {
                 if writer.writer_dc != dc {
                     continue;
                 }
-                if !matches!(
+                if writer.generation != generation
-                    super::pool::WriterContour::from_u8(writer.contour.load(Ordering::Relaxed),),
+                    || WriterContour::from_u8(writer.contour.load(Ordering::Acquire)) != contour
-                    super::pool::WriterContour::Active
+                {
-                ) {
                     continue;
                 }
                 if candidates.contains(&writer.addr) {
-                    *active_by_endpoint.entry(writer.addr).or_insert(0) += 1;
+                    *matching_by_endpoint.entry(writer.addr).or_insert(0) += 1;
                 }
             }
             drop(ws);
             candidates
-                .sort_by_key(|addr| (active_by_endpoint.get(addr).copied().unwrap_or(0), *addr));
+                .sort_by_key(|addr| (matching_by_endpoint.get(addr).copied().unwrap_or(0), *addr));
         }
         let start = (self.rr.fetch_add(1, Ordering::Relaxed) as usize) % candidates.len();
         for offset in 0..candidates.len() {
             let idx = (start + offset) % candidates.len();
             let addr = candidates[idx];
             match self
-                .connect_one_with_generation_contour_for_dc_with_cap_policy(
+                .connect_one_with_generation_contour_for_dc_with_intent(
                     addr,
                     rng,
                     generation,
                     contour,
                     dc,
-                    allow_coverage_override,
+                    intent,
                 )
                 .await
             {
@@ -245,43 +261,74 @@ impl MePool {
         false
     }
 
-    async fn endpoints_for_dc(&self, target_dc: i32) -> Vec<SocketAddr> {
+    async fn endpoints_for_refill_target(&self, target: RefillTargetKey) -> Vec<SocketAddr> {
         let now_epoch_secs = Self::now_epoch_secs();
-        let mut endpoints = HashSet::<SocketAddr>::new();
+        if !self.family_enabled_for_drain_coverage(target.family, now_epoch_secs) {
-
+            return Vec::new();
-        if self.family_enabled_for_drain_coverage(IpFamily::V4, now_epoch_secs) {
-            let map = self.proxy_map_v4.read().await;
-            if let Some(addrs) = map.get(&target_dc) {
-                for (ip, port) in addrs {
-                    endpoints.insert(SocketAddr::new(*ip, *port));
-                }
         }
+        let map = match target.family {
+            IpFamily::V4 => self.proxy_map_v4.read().await,
+            IpFamily::V6 => self.proxy_map_v6.read().await,
+        };
+        let mut endpoints = map
+            .get(&target.dc)
+            .into_iter()
+            .flatten()
+            .map(|(ip, port)| SocketAddr::new(*ip, *port))
+            .collect::<Vec<_>>();
+        endpoints.sort_unstable();
+        endpoints.dedup();
+        endpoints
     }
 
-        if self.family_enabled_for_drain_coverage(IpFamily::V6, now_epoch_secs) {
+    fn refill_target_is_authoritative(&self, target: RefillTargetKey) -> bool {
-            let map = self.proxy_map_v6.read().await;
+        if !self.family_enabled_for_drain_coverage(target.family, Self::now_epoch_secs()) {
-            if let Some(addrs) = map.get(&target_dc) {
+            return false;
-                for (ip, port) in addrs {
-                    endpoints.insert(SocketAddr::new(*ip, *port));
-                }
         }
+        let status = self.reinit.status.load();
+        let role_is_authoritative = match target.contour {
+            WriterContour::Active => target.generation == status.active_generation,
+            WriterContour::Warm => status.pending_hardswap_generation != 0
+                && target.generation == status.pending_hardswap_generation,
+            WriterContour::Draining => false,
+        };
+        role_is_authoritative
+            && self
+                .preferred_endpoints_by_dc
+                .load()
+                .get(&target.dc)
+                .is_some_and(|endpoints| {
+                    endpoints.iter().any(|endpoint| match target.family {
+                        IpFamily::V4 => endpoint.is_ipv4(),
+                        IpFamily::V6 => endpoint.is_ipv6(),
+                    })
+                })
     }
 
-        let mut sorted: Vec<SocketAddr> = endpoints.into_iter().collect();
+    async fn refill_writer_after_loss(
-        sorted.sort_unstable();
+        self: &Arc<Self>,
-        sorted
+        addr: SocketAddr,
+        target: RefillTargetKey,
+    ) -> RefillOutcome {
+        if !self.refill_target_is_authoritative(target) {
+            return RefillOutcome::Obsolete;
         }
-
+        let open_intent = if target.contour == WriterContour::Active {
-    async fn refill_writer_after_loss(self: &Arc<Self>, addr: SocketAddr, writer_dc: i32) -> bool {
+            WriterOpenIntent::Coverage
+        } else {
+            WriterOpenIntent::Normal
+        };
         let fast_retries = self.reconnect_runtime.me_reconnect_fast_retry_count.max(1);
         let mut total_attempts = 0u32;
         let same_endpoint_quarantined = self.is_endpoint_quarantined(addr).await;
-        let dc_endpoints = self.endpoints_for_dc(writer_dc).await;
+        let dc_endpoints = self.endpoints_for_refill_target(target).await;
         let single_endpoint_dc = dc_endpoints.len() == 1 && dc_endpoints[0] == addr;
         let bypass_quarantine_for_single_endpoint =
             single_endpoint_dc && self.single_endpoint_outage_disable_quarantine();
 
-        if !same_endpoint_quarantined || bypass_quarantine_for_single_endpoint {
+        if dc_endpoints.contains(&addr)
+            && (!same_endpoint_quarantined || bypass_quarantine_for_single_endpoint)
+        {
             if same_endpoint_quarantined && bypass_quarantine_for_single_endpoint {
                 debug!(
                     %addr,
@@ -289,13 +336,23 @@ impl MePool {
                 );
             }
             for attempt in 0..fast_retries {
+                if !self.refill_target_is_authoritative(target) {
+                    return RefillOutcome::Obsolete;
+                }
                 if total_attempts >= ME_REFILL_TOTAL_ATTEMPT_CAP {
                     break;
                 }
                 total_attempts = total_attempts.saturating_add(1);
                 self.stats.increment_me_reconnect_attempt();
                 match self
-                    .connect_one_for_dc(addr, writer_dc, self.rng.as_ref())
+                    .connect_one_with_generation_contour_for_dc_with_intent(
+                        addr,
+                        self.rng.as_ref(),
+                        target.generation,
+                        target.contour,
+                        target.dc,
+                        open_intent,
+                    )
                     .await
                 {
                     Ok(()) => {
@@ -307,7 +364,7 @@ impl MePool {
                             attempt = attempt + 1,
                             "ME writer restored on the same endpoint"
                         );
-                        return true;
+                        return RefillOutcome::Restored;
                     }
                     Err(e) => {
                         debug!(
@@ -328,17 +385,27 @@ impl MePool {
 
         if dc_endpoints.is_empty() {
             self.stats.increment_me_refill_failed_total();
-            return false;
+            return RefillOutcome::Failed;
         }
 
         for attempt in 0..fast_retries {
+            if !self.refill_target_is_authoritative(target) {
+                return RefillOutcome::Obsolete;
+            }
             if total_attempts >= ME_REFILL_TOTAL_ATTEMPT_CAP {
                 break;
             }
             total_attempts = total_attempts.saturating_add(1);
             self.stats.increment_me_reconnect_attempt();
             if self
-                .connect_endpoints_round_robin(writer_dc, &dc_endpoints, self.rng.as_ref())
+                .connect_endpoints_round_robin_with_generation_contour(
+                    target.dc,
+                    &dc_endpoints,
+                    self.rng.as_ref(),
+                    target.generation,
+                    target.contour,
+                    open_intent,
+                )
                 .await
             {
                 self.stats.increment_me_reconnect_success();
@@ -348,12 +415,12 @@ impl MePool {
                     attempt = attempt + 1,
                     "ME writer restored via DC fallback endpoint"
                 );
-                return true;
+                return RefillOutcome::Restored;
             }
         }
 
         self.stats.increment_me_refill_failed_total();
-        false
+        RefillOutcome::Failed
     }
 
     pub(crate) fn trigger_immediate_refill_for_dc(
@@ -361,35 +428,58 @@ impl MePool {
         addr: SocketAddr,
         writer_dc: i32,
     ) {
-        let Some(registration) = self.lifecycle.try_register() else {
+        self.trigger_immediate_refill_for_role(
-            return;
+            addr,
-        };
+            WriterRole {
-        let dc_key = RefillDcKey {
                 dc: writer_dc,
                 family: if addr.is_ipv4() {
                     IpFamily::V4
                 } else {
                     IpFamily::V6
                 },
+                generation: self.current_generation(),
+                contour: WriterContour::Active,
+            },
+        );
+    }
+
+    /// Coalesces unexpected writer loss without changing its generation or contour role.
+    pub(super) fn trigger_immediate_refill_for_role(
+        self: &Arc<Self>,
+        addr: SocketAddr,
+        role: WriterRole,
+    ) {
+        let target = RefillTargetKey {
+            dc: role.dc,
+            family: role.family,
+            generation: role.generation,
+            contour: role.contour,
+        };
+        if !self.refill_target_is_authoritative(target) {
+            return;
+        }
+        let Some(registration) = self.lifecycle.try_register() else {
+            return;
         };
         {
             let mut states = self.refill_states.lock();
-            if let Some(pending) = states.get_mut(&dc_key) {
+            if let Some(state) = states.get_mut(&target) {
-                if pending.is_none() {
+                if state.pending_count < ME_REFILL_PENDING_PER_TARGET_MAX {
+                    state.pending_count += 1;
+                    state.next_addr = Some(addr);
                     self.refill_pending.fetch_add(1, Ordering::AcqRel);
                 }
-                *pending = Some(addr);
                 self.stats.increment_me_refill_skipped_inflight_total();
                 return;
             }
-            states.insert(dc_key, None);
+            states.insert(target, RefillTargetState::default());
             self.refill_running.fetch_add(1, Ordering::AcqRel);
         }
 
         let pool = Arc::clone(self);
         let mut run_guard = RefillRunGuard {
             pool: Arc::clone(&pool),
-            key: dc_key,
+            key: target,
             active: true,
         };
         self.lifecycle
@@ -397,9 +487,24 @@ impl MePool {
                 let mut current_addr = addr;
                 loop {
                     pool.stats.increment_me_refill_triggered_total();
-                    let restored = pool.refill_writer_after_loss(current_addr, writer_dc).await;
+                    let outcome = pool.refill_writer_after_loss(current_addr, target).await;
-                    if !restored {
+                    if outcome == RefillOutcome::Failed {
-                        warn!(%current_addr, dc = writer_dc, "ME immediate refill failed");
+                        warn!(
+                            %current_addr,
+                            dc = target.dc,
+                            generation = target.generation,
+                            contour = ?target.contour,
+                            "ME immediate refill failed"
+                        );
+                    } else if outcome == RefillOutcome::Obsolete {
+                        debug!(
+                            %current_addr,
+                            dc = target.dc,
+                            generation = target.generation,
+                            contour = ?target.contour,
+                            "ME immediate refill target is no longer authoritative"
+                        );
+                        return;
                     }
 
                     let Some(next_addr) = run_guard.next_or_finish() else {

src/transport/middle_proxy/pool_reinit.rs

@@ -11,11 +11,12 @@ use std::collections::hash_map::DefaultHasher;
 use tracing::{debug, info, warn};
 
 use crate::crypto::SecureRandom;
+use crate::config::MeBindStaleMode;
 use crate::network::IpFamily;
 
 use super::pool::{
     MeDrainGateReason, MePool, ReinitAttemptState, ReinitCoordinatorState, ReinitCore,
-    ReinitPendingState, ReinitStatusSnapshot, WriterContour,
+    ReinitPendingState, ReinitStatusSnapshot, WriterContour, WriterOpenIntent,
 };
 
 // Reinitialization admission, generation state, and coverage checks.
@@ -51,6 +52,26 @@ struct ReinitReservation {
     pending_age_secs: u64,
 }
 
+struct ReinitCommitOutcome {
+    coverage_ratio: f32,
+    missing_dc: Vec<i32>,
+    stale_writer_ids: Vec<u64>,
+    force_close_writer_ids: Vec<u64>,
+}
+
+#[derive(Debug)]
+enum ReinitCommitFailure {
+    Superseded,
+    Coverage {
+        coverage_ratio: f32,
+        missing_dc: Vec<i32>,
+    },
+    Redundancy {
+        coverage_ratio: f32,
+        missing_dc: Vec<i32>,
+    },
+}
+
 fn publish_reinit_state(reinit: &ReinitCore, state: &ReinitCoordinatorState) {
     let mut warm_generations = state
         .attempts
@@ -58,6 +79,9 @@ fn publish_reinit_state(reinit: &ReinitCore, state: &ReinitCoordinatorState) {
         .filter(|attempt| attempt.hardswap && !attempt.committed)
         .map(|attempt| attempt.generation)
         .collect::<Vec<_>>();
+    if let Some(pending) = state.pending {
+        warm_generations.push(pending.generation);
+    }
     warm_generations.sort_unstable();
     warm_generations.dedup();
     let pending = state.pending;

src/transport/middle_proxy/pool_reinit/coordination.rs

@@ -1,7 +1,10 @@
 use super::*;
 
 impl MePool {
-    pub(super) fn desired_map_hash(desired_by_dc: &HashMap<i32, HashSet<SocketAddr>>) -> u64 {
+    /// Hashes the sorted desired endpoint map for generation authority checks.
+    pub(in crate::transport::middle_proxy) fn desired_map_hash(
+        desired_by_dc: &HashMap<i32, HashSet<SocketAddr>>,
+    ) -> u64 {
         let mut hasher = DefaultHasher::new();
         let mut dcs: Vec<i32> = desired_by_dc.keys().copied().collect();
         dcs.sort_unstable();
@@ -19,6 +22,7 @@ impl MePool {
         hasher.finish()
     }
 
+    /// Reserves one generation attempt and publishes its pending ownership snapshot.
     pub(super) fn reserve_reinit_attempt(
         self: &Arc<Self>,
         hardswap: bool,
@@ -85,8 +89,62 @@ impl MePool {
         }
     }
 
-    pub(super) fn commit_reinit_attempt(&self, attempt: &ReinitAttemptGuard) -> bool {
+    /// Revalidates coverage and commits generation ownership under the publication barrier.
+    pub(super) async fn commit_reinit_attempt(
+        &self,
+        attempt: &ReinitAttemptGuard,
+        desired_by_dc: &HashMap<i32, HashSet<SocketAddr>>,
+        min_ratio: f32,
+    ) -> std::result::Result<ReinitCommitOutcome, ReinitCommitFailure> {
+        let writers = self.writers.write().await;
+        let mut registry_registration = self.registry.prepare_writer_registration().await;
         let mut state = self.reinit.coordinator.lock();
+        let Some(record) = state.attempts.get(&attempt.attempt_id).copied() else {
+            return Err(ReinitCommitFailure::Superseded);
+        };
+        if record.generation != attempt.generation
+            || record.map_hash != state.desired_map_hash
+            || record.map_hash != attempt.map_hash
+            || (attempt.hardswap
+                && !state.pending.is_some_and(|pending| {
+                    pending.generation == attempt.generation
+                        && pending.map_hash == attempt.map_hash
+                }))
+        {
+            return Err(ReinitCommitFailure::Superseded);
+        }
+
+        let authoritative_writer_addrs = writers
+            .iter()
+            .filter(|writer| !writer.draining.load(Ordering::Acquire))
+            .filter(|writer| {
+                if attempt.hardswap {
+                    writer.generation == attempt.generation
+                } else {
+                    writer.generation == state.active_generation
+                        && WriterContour::from_u8(writer.contour.load(Ordering::Acquire))
+                            == WriterContour::Active
+                }
+            })
+            .map(|writer| (writer.writer_dc, writer.addr))
+            .collect::<HashSet<_>>();
+        let (coverage_ratio, missing_dc) =
+            Self::coverage_ratio(desired_by_dc, &authoritative_writer_addrs);
+        if coverage_ratio < min_ratio {
+            return Err(ReinitCommitFailure::Coverage {
+                coverage_ratio,
+                missing_dc,
+            });
+        }
+        if attempt.hardswap
+            && !missing_dc.is_empty()
+            && self.bind_stale_mode() == MeBindStaleMode::Never
+        {
+            return Err(ReinitCommitFailure::Redundancy {
+                coverage_ratio,
+                missing_dc,
+            });
+        }
         if !commit_reinit_state(
             &mut state,
             attempt.attempt_id,
@@ -94,12 +152,12 @@ impl MePool {
             attempt.map_hash,
             attempt.hardswap,
         ) {
-            return false;
+            return Err(ReinitCommitFailure::Superseded);
         }
+
         if attempt.hardswap {
-            let writers = self.writers.snapshot();
             for writer in writers.iter() {
-                if !writer.draining.load(Ordering::Relaxed)
+                if !writer.draining.load(Ordering::Acquire)
                     && writer.generation == attempt.generation
                 {
                     writer
@@ -108,10 +166,59 @@ impl MePool {
                 }
             }
         }
-        publish_reinit_state(self.reinit.as_ref(), &state);
+
-        true
+        let desired_addrs = desired_by_dc
+            .iter()
+            .flat_map(|(dc, endpoints)| endpoints.iter().copied().map(|addr| (*dc, addr)))
+            .collect::<HashSet<_>>();
+        let missing_dc_set = missing_dc.iter().copied().collect::<HashSet<_>>();
+        let mut stale_writer_ids = Vec::<u64>::new();
+        let mut force_close_writer_ids = Vec::<u64>::new();
+        for writer in writers.iter() {
+            if writer.draining.load(Ordering::Acquire) {
+                continue;
+            }
+            let stale = if attempt.hardswap {
+                writer.generation < attempt.generation
+            } else {
+                !desired_addrs.contains(&(writer.writer_dc, writer.addr))
+            };
+            if !stale {
+                continue;
             }
 
+            let preserve_fallback = attempt.hardswap
+                && missing_dc_set.contains(&writer.writer_dc);
+            if !preserve_fallback && attempt.hardswap {
+                registry_registration.retire(writer.id);
+            }
+            self.apply_writer_draining_state(
+                writer,
+                self.force_close_timeout(),
+                preserve_fallback || !attempt.hardswap,
+            );
+            stale_writer_ids.push(writer.id);
+            if (attempt.hardswap && !preserve_fallback)
+                || (!attempt.hardswap && missing_dc.is_empty())
+            {
+                force_close_writer_ids.push(writer.id);
+            }
+        }
+        publish_reinit_state(self.reinit.as_ref(), &state);
+        drop(state);
+        drop(registry_registration);
+        drop(writers);
+        self.notify_writer_epoch();
+
+        Ok(ReinitCommitOutcome {
+            coverage_ratio,
+            missing_dc,
+            stale_writer_ids,
+            force_close_writer_ids,
+        })
+    }
+
+    /// Computes desired DC-group coverage and returns missing groups in stable order.
     pub(super) fn coverage_ratio(
         desired_by_dc: &HashMap<i32, HashSet<SocketAddr>>,
         active_writer_addrs: &HashSet<(i32, SocketAddr)>,
@@ -146,6 +253,7 @@ impl MePool {
         (ratio, missing_dc)
     }
 
+    /// Restores at least one active writer for every enabled desired DC group.
     pub async fn reconcile_connections(self: &Arc<Self>, rng: &SecureRandom) {
         for family in self.family_order() {
             let map = self.proxy_map_for_family(family).await;
@@ -175,7 +283,10 @@ impl MePool {
         }
     }
 
-    pub(super) async fn desired_dc_endpoints(&self) -> HashMap<i32, HashSet<SocketAddr>> {
+    /// Returns the currently authoritative endpoint set for drain and coverage decisions.
+    pub(in crate::transport::middle_proxy) async fn desired_dc_endpoints(
+        &self,
+    ) -> HashMap<i32, HashSet<SocketAddr>> {
         let now_epoch_secs = Self::now_epoch_secs();
         let mut out: HashMap<i32, HashSet<SocketAddr>> = HashMap::new();
 
@@ -202,38 +313,63 @@ impl MePool {
         out
     }
 
-    pub(in crate::transport::middle_proxy) async fn has_non_draining_writer_per_desired_dc_group(
+    /// Promotes authoritative warm writers and drains warm or active generation orphans.
-        &self,
+    pub(super) async fn reconcile_writer_generation_roles(&self) -> usize {
-    ) -> bool {
+        let writers = self.writers.write().await;
-        let desired_by_dc = self.desired_dc_endpoints().await;
+        let mut registry_registration = self.registry.prepare_writer_registration().await;
-        let required_dcs: HashSet<i32> = desired_by_dc
+        let state = self.reinit.coordinator.lock();
-            .iter()
+        let active_generation = state.active_generation;
-            .filter_map(|(dc, endpoints)| {
+        let pending_generation = state.pending.map(|pending| pending.generation);
-                if endpoints.is_empty() {
+        let preferred = self.preferred_endpoints_by_dc.load();
-                    None
+        let now_epoch_secs = Self::now_epoch_secs();
-                } else {
+        let mut changed = 0usize;
-                    Some(*dc)
-                }
-            })
-            .collect();
-        if required_dcs.is_empty() {
-            return true;
-        }
 
-        let ws = self.writers.read().await;
+        for writer in writers.iter() {
-        let mut covered_dcs = HashSet::<i32>::with_capacity(required_dcs.len());
+            if writer.draining.load(Ordering::Acquire) {
-        for writer in ws.iter() {
-            if writer.draining.load(Ordering::Relaxed) {
                 continue;
             }
-            if required_dcs.contains(&writer.writer_dc) {
+            let contour = WriterContour::from_u8(writer.contour.load(Ordering::Acquire));
-                covered_dcs.insert(writer.writer_dc);
+            let family = if writer.addr.is_ipv4() {
-                if covered_dcs.len() == required_dcs.len() {
+                IpFamily::V4
-                    return true;
+            } else {
+                IpFamily::V6
+            };
+            let endpoint_is_current = self
+                .family_enabled_for_drain_coverage(family, now_epoch_secs)
+                && preferred
+                    .get(&writer.writer_dc)
+                    .is_some_and(|endpoints| endpoints.contains(&writer.addr));
+            if contour == WriterContour::Warm
+                && writer.generation == active_generation
+                && endpoint_is_current
+            {
+                writer
+                    .contour
+                    .store(WriterContour::Active.as_u8(), Ordering::Release);
+                changed = changed.saturating_add(1);
+                continue;
             }
+            let authoritative_warm = contour == WriterContour::Warm
+                && pending_generation == Some(writer.generation)
+                && endpoint_is_current;
+            let stale_active = contour == WriterContour::Active
+                && writer.generation != active_generation;
+            if authoritative_warm || (contour == WriterContour::Active && !stale_active) {
+                continue;
             }
+
+            registry_registration.retire(writer.id);
+            self.apply_writer_draining_state(writer, self.force_close_timeout(), false);
+            changed = changed.saturating_add(1);
         }
-        false
+        drop(preferred);
+        drop(state);
+        drop(registry_registration);
+        drop(writers);
+        if changed > 0 {
+            self.notify_writer_epoch();
+        }
+        changed
     }
 
     pub(super) fn hardswap_warmup_connect_delay_ms(&self) -> u64 {
@@ -270,6 +406,7 @@ impl MePool {
         core.saturating_add(rand::rng().random_range(0..=jitter))
     }
 
+    /// Counts non-draining writers owned by one generation and desired DC endpoint set.
     pub(super) async fn fresh_writer_count_for_dc_endpoints(
         &self,
         generation: u64,
@@ -285,14 +422,21 @@ impl MePool {
             .count()
     }
 
+    /// Counts authoritative active writers for one desired DC endpoint set.
     pub(in crate::transport::middle_proxy) async fn active_writer_count_for_dc_endpoints(
         &self,
         dc: i32,
         endpoints: &HashSet<SocketAddr>,
     ) -> usize {
+        let generation = self.current_generation();
         let ws = self.writers.read().await;
         ws.iter()
             .filter(|w| !w.draining.load(Ordering::Relaxed))
+            .filter(|w| w.generation == generation)
+            .filter(|w| {
+                WriterContour::from_u8(w.contour.load(Ordering::Acquire))
+                    == WriterContour::Active
+            })
             .filter(|w| w.writer_dc == dc)
             .filter(|w| endpoints.contains(&w.addr))
             .count()

src/transport/middle_proxy/pool_reinit/reconcile.rs

@@ -56,7 +56,7 @@ impl MePool {
                             rng,
                             generation,
                             WriterContour::Warm,
-                            false,
+                            WriterOpenIntent::Normal,
                         )
                         .await;
                     debug!(
@@ -141,6 +141,15 @@ impl MePool {
         let attempt = reservation.attempt;
         let previous_generation = attempt.previous_generation;
         let generation = attempt.generation;
+        let reconciled_roles = self.reconcile_writer_generation_roles().await;
+        if reconciled_roles > 0 {
+            info!(
+                reconciled_roles,
+                active_generation = previous_generation,
+                pending_generation = generation,
+                "ME writer generation roles reconciled"
+            );
+        }
         if reservation.pending_reused {
             self.stats.increment_me_hardswap_pending_reuse_total();
             debug!(
@@ -180,14 +189,10 @@ impl MePool {
         );
         let (coverage_ratio, missing_dc) =
             Self::coverage_ratio(&desired_by_dc, &active_writer_addrs);
-        let mut route_quorum_ok = coverage_ratio >= min_ratio;
-        let mut redundancy_ok = missing_dc.is_empty();
-        let mut redundancy_missing_dc = missing_dc.clone();
-        let mut gate_coverage_ratio = coverage_ratio;
         if !hardswap && coverage_ratio < min_ratio {
             self.set_last_drain_gate(
                 false,
-                redundancy_ok,
+                missing_dc.is_empty(),
                 MeDrainGateReason::CoverageQuorum,
                 now_epoch_secs,
             );
@@ -211,14 +216,10 @@ impl MePool {
                 .collect();
             let (fresh_coverage_ratio, fresh_missing_dc) =
                 Self::coverage_ratio(&desired_by_dc, &fresh_writer_addrs);
-            route_quorum_ok = fresh_coverage_ratio >= min_ratio;
-            redundancy_ok = fresh_missing_dc.is_empty();
-            redundancy_missing_dc = fresh_missing_dc.clone();
-            gate_coverage_ratio = fresh_coverage_ratio;
             if fresh_coverage_ratio < min_ratio {
                 self.set_last_drain_gate(
                     false,
-                    redundancy_ok,
+                    fresh_missing_dc.is_empty(),
                     MeDrainGateReason::CoverageQuorum,
                     now_epoch_secs,
                 );
@@ -233,49 +234,77 @@ impl MePool {
             }
         }
 
-        self.set_last_drain_gate(
+        drop(writers);
-            route_quorum_ok,
+        let commit = self
-            redundancy_ok,
+            .commit_reinit_attempt(&attempt, &desired_by_dc, min_ratio)
-            MeDrainGateReason::Open,
+            .await;
-            now_epoch_secs,
+        let outcome = match commit {
-        );
+            Ok(outcome) => outcome,
-        if !redundancy_ok {
+            Err(ReinitCommitFailure::Superseded) => {
-            warn!(
-                missing_dc = ?redundancy_missing_dc,
-                coverage_ratio = format_args!("{gate_coverage_ratio:.3}"),
-                min_ratio = format_args!("{min_ratio:.3}"),
-                "ME reinit proceeds with weighted quorum while some DC groups remain uncovered"
-            );
-        }
-
-        if !self.commit_reinit_attempt(&attempt) {
                 debug!(
                     previous_generation,
-                generation, "ME reinit result discarded after a newer desired-map attempt"
+                    generation,
+                    "ME reinit result discarded after a newer desired-map attempt"
                 );
                 return false;
             }
-
+            Err(ReinitCommitFailure::Coverage {
-        let desired_addrs: HashSet<(i32, SocketAddr)> = desired_by_dc
+                coverage_ratio,
-            .iter()
+                missing_dc,
-            .flat_map(|(dc, set)| set.iter().copied().map(|addr| (*dc, addr)))
+            }) => {
-            .collect();
+                self.set_last_drain_gate(
-
+                    false,
-        let stale_writer_ids: Vec<u64> = writers
+                    missing_dc.is_empty(),
-            .iter()
+                    MeDrainGateReason::CoverageQuorum,
-            .filter(|w| !w.draining.load(Ordering::Relaxed))
+                    now_epoch_secs,
-            .filter(|w| {
+                );
-                if hardswap {
+                warn!(
-                    w.generation < generation
+                    previous_generation,
-                } else {
+                    generation,
-                    !desired_addrs.contains(&(w.writer_dc, w.addr))
+                    coverage_ratio = format_args!("{coverage_ratio:.3}"),
+                    min_ratio = format_args!("{min_ratio:.3}"),
+                    missing_dc = ?missing_dc,
+                    "ME reinit coverage changed before commit; keeping current generation"
+                );
+                return false;
+            }
+            Err(ReinitCommitFailure::Redundancy {
+                coverage_ratio,
+                missing_dc,
+            }) => {
+                self.set_last_drain_gate(
+                    true,
+                    false,
+                    MeDrainGateReason::Redundancy,
+                    now_epoch_secs,
+                );
+                warn!(
+                    previous_generation,
+                    generation,
+                    coverage_ratio = format_args!("{coverage_ratio:.3}"),
+                    min_ratio = format_args!("{min_ratio:.3}"),
+                    missing_dc = ?missing_dc,
+                    "ME hardswap weighted quorum requires stale-binding fallback"
+                );
+                return false;
+            }
+        };
+        self.set_last_drain_gate(
+            true,
+            outcome.missing_dc.is_empty(),
+            MeDrainGateReason::Open,
+            now_epoch_secs,
+        );
+        if !outcome.missing_dc.is_empty() {
+            warn!(
+                missing_dc = ?outcome.missing_dc,
+                coverage_ratio = format_args!("{:.3}", outcome.coverage_ratio),
+                min_ratio = format_args!("{min_ratio:.3}"),
+                "ME reinit committed with bounded stale fallback for uncovered DC groups"
+            );
         }
-            })
-            .map(|w| w.id)
-            .collect();
-        drop(writers);
 
-        if stale_writer_ids.is_empty() {
+        if outcome.stale_writer_ids.is_empty() {
             debug!("ME reinit cycle completed with no stale writers");
             return true;
         }
@@ -283,36 +312,21 @@ impl MePool {
         let drain_timeout = self.force_close_timeout();
         let drain_timeout_secs = drain_timeout.map(|d| d.as_secs()).unwrap_or(0);
         info!(
-            stale_writers = stale_writer_ids.len(),
+            stale_writers = outcome.stale_writer_ids.len(),
+            force_close_writers = outcome.force_close_writer_ids.len(),
             previous_generation,
             generation,
             hardswap,
-            coverage_ratio = format_args!("{coverage_ratio:.3}"),
+            coverage_ratio = format_args!("{:.3}", outcome.coverage_ratio),
             min_ratio = format_args!("{min_ratio:.3}"),
             drain_timeout_secs,
             "ME reinit cycle covered; processing stale writers"
         );
         self.stats.increment_pool_swap_total();
-        let can_drop_with_replacement = self.has_non_draining_writer_per_desired_dc_group().await;
+        for writer_id in outcome.force_close_writer_ids {
-        if can_drop_with_replacement {
-            info!(
-                stale_writers = stale_writer_ids.len(),
-                "ME reinit stale writers: replacement coverage ready, force-closing clients for fast rebind"
-            );
-        } else {
-            warn!(
-                stale_writers = stale_writer_ids.len(),
-                "ME reinit stale writers: replacement coverage incomplete, keeping draining fallback"
-            );
-        }
-        for writer_id in stale_writer_ids {
-            self.mark_writer_draining_with_timeout(writer_id, drain_timeout, !hardswap)
-                .await;
-            if can_drop_with_replacement {
             self.stats.increment_pool_force_close_total();
             self.remove_writer_and_close_clients(writer_id).await;
         }
-        }
         true
     }
 

src/transport/middle_proxy/pool_reinit/tests.rs

@@ -1,15 +1,68 @@
 use std::collections::{HashMap, HashSet};
 use std::net::{IpAddr, Ipv4Addr, SocketAddr};
+use std::sync::Arc;
+use std::sync::atomic::{AtomicBool, AtomicU8, AtomicU32, AtomicU64, Ordering};
+use std::time::Instant;
 
-use super::{MePool, commit_reinit_state};
+use tokio::sync::mpsc;
+use tokio_util::sync::CancellationToken;
+
+use super::{MePool, ReinitCommitFailure, commit_reinit_state};
+use crate::config::MeBindStaleMode;
+use crate::transport::middle_proxy::codec::WriterCommand;
 use crate::transport::middle_proxy::pool::{
-    ReinitAttemptState, ReinitCoordinatorState, ReinitPendingState,
+    MeWriter, ReinitAttemptState, ReinitCoordinatorState, ReinitPendingState, WriterContour,
 };
+use crate::transport::middle_proxy::pool_writer_security_tests::make_pool;
 
 fn addr(octet: u8, port: u16) -> SocketAddr {
     SocketAddr::new(IpAddr::V4(Ipv4Addr::new(127, 0, 0, octet)), port)
 }
 
+async fn insert_writer(
+    pool: &Arc<MePool>,
+    writer_id: u64,
+    writer_dc: i32,
+    endpoint: SocketAddr,
+    generation: u64,
+    contour: WriterContour,
+) -> MeWriter {
+    let (tx, _rx) = mpsc::channel::<WriterCommand>(8);
+    let byte_budget = pool.new_writer_byte_budget();
+    let writer = MeWriter {
+        id: writer_id,
+        addr: endpoint,
+        source_ip: endpoint.ip(),
+        writer_dc,
+        generation,
+        contour: Arc::new(AtomicU8::new(contour.as_u8())),
+        created_at: Instant::now(),
+        tx: tx.clone(),
+        byte_budget: byte_budget.clone(),
+        cancel: CancellationToken::new(),
+        degraded: Arc::new(AtomicBool::new(false)),
+        rtt_ema_ms_x10: Arc::new(AtomicU32::new(0)),
+        draining: Arc::new(AtomicBool::new(false)),
+        draining_started_at_epoch_secs: Arc::new(AtomicU64::new(0)),
+        drain_deadline_epoch_secs: Arc::new(AtomicU64::new(0)),
+        allow_drain_fallback: Arc::new(AtomicBool::new(false)),
+    };
+
+    pool.registry
+        .register_writer(writer_id, tx, byte_budget)
+        .await;
+    pool.writers.write().await.push(writer.clone());
+    pool.conn_count.fetch_add(1, Ordering::Relaxed);
+    writer
+}
+
+fn desired_two_dcs() -> HashMap<i32, HashSet<SocketAddr>> {
+    HashMap::from([
+        (1, HashSet::from([addr(1, 2001)])),
+        (2, HashSet::from([addr(2, 2002)])),
+    ])
+}
+
 #[test]
 fn coverage_ratio_counts_dc_coverage_not_floor() {
     let dc1 = addr(1, 2001);
@@ -95,3 +148,226 @@ fn stale_concurrent_attempt_cannot_regress_active_generation() {
     assert_eq!(state.active_generation, 3);
     assert!(state.pending.is_none());
 }
+
+#[tokio::test]
+async fn partial_hardswap_is_rejected_when_stale_binding_is_disabled() {
+    let pool = make_pool().await;
+    let desired_by_dc = desired_two_dcs();
+    let active_generation = pool.current_generation();
+    let old_dc1 = insert_writer(
+        &pool,
+        101,
+        1,
+        addr(1, 2001),
+        active_generation,
+        WriterContour::Active,
+    )
+    .await;
+    let old_dc2 = insert_writer(
+        &pool,
+        102,
+        2,
+        addr(2, 2002),
+        active_generation,
+        WriterContour::Active,
+    )
+    .await;
+    let map_hash = MePool::desired_map_hash(&desired_by_dc);
+    let reservation = pool.reserve_reinit_attempt(true, map_hash, 100);
+    insert_writer(
+        &pool,
+        201,
+        1,
+        addr(1, 2001),
+        reservation.attempt.generation,
+        WriterContour::Warm,
+    )
+    .await;
+
+    let result = pool
+        .commit_reinit_attempt(&reservation.attempt, &desired_by_dc, 0.5)
+        .await;
+
+    assert!(matches!(result, Err(ReinitCommitFailure::Redundancy { .. })));
+    assert_eq!(pool.current_generation(), active_generation);
+    assert!(!old_dc1.draining.load(Ordering::Acquire));
+    assert!(!old_dc2.draining.load(Ordering::Acquire));
+}
+
+#[tokio::test]
+async fn partial_hardswap_preserves_fallback_only_for_missing_dc() {
+    let pool = make_pool().await;
+    pool.binding_policy
+        .me_bind_stale_mode
+        .store(MeBindStaleMode::Ttl.as_u8(), Ordering::Release);
+    let desired_by_dc = desired_two_dcs();
+    let active_generation = pool.current_generation();
+    let old_dc1 = insert_writer(
+        &pool,
+        301,
+        1,
+        addr(1, 2001),
+        active_generation,
+        WriterContour::Active,
+    )
+    .await;
+    let old_dc2 = insert_writer(
+        &pool,
+        302,
+        2,
+        addr(9, 2999),
+        active_generation,
+        WriterContour::Active,
+    )
+    .await;
+    let map_hash = MePool::desired_map_hash(&desired_by_dc);
+    let reservation = pool.reserve_reinit_attempt(true, map_hash, 100);
+    let fresh_dc1 = insert_writer(
+        &pool,
+        401,
+        1,
+        addr(1, 2001),
+        reservation.attempt.generation,
+        WriterContour::Warm,
+    )
+    .await;
+
+    let outcome = pool
+        .commit_reinit_attempt(&reservation.attempt, &desired_by_dc, 0.5)
+        .await
+        .expect("partial hardswap must commit when bounded stale fallback is enabled");
+
+    assert_eq!(pool.current_generation(), reservation.attempt.generation);
+    assert_eq!(outcome.missing_dc, vec![2]);
+    assert_eq!(outcome.force_close_writer_ids, vec![301]);
+    assert!(old_dc1.draining.load(Ordering::Acquire));
+    assert!(!old_dc1.allow_drain_fallback.load(Ordering::Acquire));
+    assert!(old_dc2.draining.load(Ordering::Acquire));
+    assert!(old_dc2.allow_drain_fallback.load(Ordering::Acquire));
+    assert_eq!(
+        WriterContour::from_u8(fresh_dc1.contour.load(Ordering::Acquire)),
+        WriterContour::Active
+    );
+}
+
+#[tokio::test]
+async fn complete_hardswap_promotes_fresh_generation_and_retires_old_writers() {
+    let pool = make_pool().await;
+    let desired_by_dc = desired_two_dcs();
+    let active_generation = pool.current_generation();
+    let old_dc1 = insert_writer(
+        &pool,
+        501,
+        1,
+        addr(1, 2001),
+        active_generation,
+        WriterContour::Active,
+    )
+    .await;
+    let old_dc2 = insert_writer(
+        &pool,
+        502,
+        2,
+        addr(2, 2002),
+        active_generation,
+        WriterContour::Active,
+    )
+    .await;
+    let map_hash = MePool::desired_map_hash(&desired_by_dc);
+    let reservation = pool.reserve_reinit_attempt(true, map_hash, 100);
+    let fresh_dc1 = insert_writer(
+        &pool,
+        601,
+        1,
+        addr(1, 2001),
+        reservation.attempt.generation,
+        WriterContour::Warm,
+    )
+    .await;
+    let fresh_dc2 = insert_writer(
+        &pool,
+        602,
+        2,
+        addr(2, 2002),
+        reservation.attempt.generation,
+        WriterContour::Warm,
+    )
+    .await;
+
+    let outcome = pool
+        .commit_reinit_attempt(&reservation.attempt, &desired_by_dc, 1.0)
+        .await
+        .expect("fully covered hardswap must commit");
+
+    assert_eq!(pool.current_generation(), reservation.attempt.generation);
+    assert!(outcome.missing_dc.is_empty());
+    assert_eq!(outcome.force_close_writer_ids, vec![501, 502]);
+    assert!(old_dc1.draining.load(Ordering::Acquire));
+    assert!(old_dc2.draining.load(Ordering::Acquire));
+    assert_eq!(
+        WriterContour::from_u8(fresh_dc1.contour.load(Ordering::Acquire)),
+        WriterContour::Active
+    );
+    assert_eq!(
+        WriterContour::from_u8(fresh_dc2.contour.load(Ordering::Acquire)),
+        WriterContour::Active
+    );
+}
+
+#[tokio::test]
+async fn generation_role_reconciliation_promotes_active_warm_and_drains_orphans() {
+    let pool = make_pool().await;
+    let endpoint = addr(1, 2001);
+    pool.preferred_endpoints_by_dc
+        .store(Arc::new(HashMap::from([(1, vec![endpoint])])));
+    let desired_by_dc = HashMap::from([(1, HashSet::from([endpoint]))]);
+    let map_hash = MePool::desired_map_hash(&desired_by_dc);
+    let reservation = pool.reserve_reinit_attempt(true, map_hash, 100);
+    let active_warm = insert_writer(
+        &pool,
+        701,
+        1,
+        endpoint,
+        pool.current_generation(),
+        WriterContour::Warm,
+    )
+    .await;
+    let pending_warm = insert_writer(
+        &pool,
+        702,
+        1,
+        endpoint,
+        reservation.attempt.generation,
+        WriterContour::Warm,
+    )
+    .await;
+    let orphan_warm = insert_writer(
+        &pool,
+        703,
+        1,
+        endpoint,
+        reservation.attempt.generation + 10,
+        WriterContour::Warm,
+    )
+    .await;
+
+    let changed = pool.reconcile_writer_generation_roles().await;
+
+    assert_eq!(changed, 2);
+    assert_eq!(
+        WriterContour::from_u8(active_warm.contour.load(Ordering::Acquire)),
+        WriterContour::Active
+    );
+    assert_eq!(
+        WriterContour::from_u8(pending_warm.contour.load(Ordering::Acquire)),
+        WriterContour::Warm
+    );
+    assert!(orphan_warm.draining.load(Ordering::Acquire));
+    assert!(!orphan_warm.allow_drain_fallback.load(Ordering::Acquire));
+    assert_eq!(
+        pool.api_hardswap_snapshot()
+            .await
+            .orphan_warm_writers_current,
+        0
+    );
+}

src/transport/middle_proxy/pool_runtime_api.rs

@@ -1,7 +1,7 @@
 use std::collections::HashMap;
 use std::time::Instant;
 
-use super::pool::{MeDrainGateReason, MePool, RefillDcKey};
+use super::pool::{MeDrainGateReason, MePool, RefillTargetKey};
 use crate::network::IpFamily;
 
 #[derive(Clone, Debug)]
@@ -61,14 +61,17 @@ impl MePool {
         let refill_states = self.refill_states.lock();
         let inflight_endpoints_total = refill_states
             .values()
-            .map(|pending| 1usize + usize::from(pending.is_some()))
+            .map(|state| 1usize.saturating_add(state.pending_count))
             .sum();
         let running_dc_total = refill_states.len();
         let pending_dc_total = refill_states
             .values()
-            .filter(|pending| pending.is_some())
+            .filter(|state| state.pending_count > 0)
             .count();
-        let inflight_dc_keys = refill_states.keys().copied().collect::<Vec<RefillDcKey>>();
+        let inflight_dc_keys = refill_states
+            .keys()
+            .copied()
+            .collect::<Vec<RefillTargetKey>>();
         drop(refill_states);
 
         let mut by_dc_map = HashMap::<(i16, &'static str), usize>::new();

src/transport/middle_proxy/pool_status.rs

@@ -12,6 +12,9 @@ use crate::transport::upstream::IpPreference;
 mod status_snapshot;
 // ME runtime policy and coherent snapshot assembly.
 mod runtime_snapshot;
+// Hardswap ownership, coverage, and writer-replacement lifecycle state.
+mod hardswap_snapshot;
+pub(crate) use hardswap_snapshot::MeApiHardswapSnapshot;
 #[derive(Clone, Debug)]
 pub(crate) struct MeApiWriterStatusSnapshot {
     pub writer_id: u64,
@@ -98,6 +101,20 @@ pub(crate) struct MeApiRuntimeSnapshot {
     pub reinit_inflight: usize,
     pub reinit_max_concurrency_effective: usize,
     pub hardswap_enabled: bool,
+    /// Number of authoritative warm writers in the pending generation.
+    pub pending_writers_current: usize,
+    /// Number of writers still required to reach the pending generation floor.
+    pub pending_writer_deficit: usize,
+    /// Number of desired DC groups without pending-generation coverage.
+    pub pending_missing_dc_groups: usize,
+    /// Whether the pending generation targets the current desired endpoint map.
+    pub pending_map_current: Option<bool>,
+    /// Number of warm writers not owned by the current pending generation.
+    pub orphan_warm_writers_current: usize,
+    /// Number of writer replacements in the preparatory phase.
+    pub replacement_preparing_current: usize,
+    /// Number of replacement victims closed to new client binds.
+    pub replacement_retiring_current: usize,
     pub floor_mode: &'static str,
     pub adaptive_floor_idle_secs: u64,
     pub adaptive_floor_min_writers_single_endpoint: u8,

src/transport/middle_proxy/pool_status/hardswap_snapshot.rs

@@ -0,0 +1,102 @@
+use super::*;
+
+#[derive(Clone, Debug)]
+/// Bounded control-plane snapshot of hardswap and replacement progress.
+pub(crate) struct MeApiHardswapSnapshot {
+    /// Whether a hardswap generation is pending.
+    pub pending: bool,
+    /// Age of the pending generation when one exists.
+    pub pending_age_secs: Option<u64>,
+    /// Number of authoritative warm writers in the pending generation.
+    pub pending_writers_current: usize,
+    /// Number of writers still required to reach the pending generation floor.
+    pub pending_writer_deficit: usize,
+    /// Number of desired DC groups without a pending-generation writer.
+    pub pending_missing_dc_groups: usize,
+    /// Whether the pending generation targets the current desired endpoint map.
+    pub pending_map_current: Option<bool>,
+    /// Number of warm writers not owned by the current pending generation.
+    pub orphan_warm_writers_current: usize,
+    /// Number of writer replacements still in the preparatory phase.
+    pub replacement_preparing_current: usize,
+    /// Number of replacement victims already closed to new bindings.
+    pub replacement_retiring_current: usize,
+}
+
+impl MePool {
+    /// Returns bounded hardswap progress without exposing generation or endpoint labels.
+    pub(crate) async fn api_hardswap_snapshot(&self) -> MeApiHardswapSnapshot {
+        let reinit = self.reinit.status.load_full();
+        self.api_hardswap_snapshot_for_reinit(reinit.as_ref()).await
+    }
+
+    pub(super) async fn api_hardswap_snapshot_for_reinit(
+        &self,
+        reinit: &ReinitStatusSnapshot,
+    ) -> MeApiHardswapSnapshot {
+        let desired_by_dc = self.desired_dc_endpoints().await;
+        let desired_hash = Self::desired_map_hash(&desired_by_dc);
+        let writers = self.writers.read().await;
+        let pending_generation = reinit.pending_hardswap_generation;
+        let pending = pending_generation != 0;
+        let mut pending_writers_current = 0usize;
+        let mut pending_by_dc = HashMap::<i32, usize>::new();
+        let mut orphan_warm_writers_current = 0usize;
+
+        for writer in writers.iter() {
+            if writer.draining.load(Ordering::Acquire) {
+                continue;
+            }
+            let contour = WriterContour::from_u8(writer.contour.load(Ordering::Acquire));
+            if contour == WriterContour::Warm && writer.generation != pending_generation {
+                orphan_warm_writers_current = orphan_warm_writers_current.saturating_add(1);
+            }
+            if pending
+                && writer.generation == pending_generation
+                && contour == WriterContour::Warm
+                && desired_by_dc
+                    .get(&writer.writer_dc)
+                    .is_some_and(|endpoints| endpoints.contains(&writer.addr))
+            {
+                pending_writers_current = pending_writers_current.saturating_add(1);
+                *pending_by_dc.entry(writer.writer_dc).or_insert(0) += 1;
+            }
+        }
+
+        let mut pending_writer_deficit = 0usize;
+        let mut pending_missing_dc_groups = 0usize;
+        if pending {
+            for (dc, endpoints) in &desired_by_dc {
+                if endpoints.is_empty() {
+                    continue;
+                }
+                let alive = pending_by_dc.get(dc).copied().unwrap_or(0);
+                let required = self.required_writers_for_dc(endpoints.len());
+                pending_writer_deficit = pending_writer_deficit
+                    .saturating_add(required.saturating_sub(alive));
+                if alive == 0 {
+                    pending_missing_dc_groups = pending_missing_dc_groups.saturating_add(1);
+                }
+            }
+        }
+        let (replacement_preparing_current, replacement_retiring_current) =
+            self.registry.writer_replacement_counts();
+        let pending_age_secs = pending.then(|| {
+            Self::now_epoch_secs()
+                .saturating_sub(reinit.pending_hardswap_started_at_epoch_secs)
+        });
+
+        MeApiHardswapSnapshot {
+            pending,
+            pending_age_secs,
+            pending_writers_current,
+            pending_writer_deficit,
+            pending_missing_dc_groups,
+            pending_map_current: pending
+                .then_some(reinit.pending_hardswap_map_hash == desired_hash),
+            orphan_warm_writers_current,
+            replacement_preparing_current,
+            replacement_retiring_current,
+        }
+    }
+}

src/transport/middle_proxy/pool_status/runtime_snapshot.rs

@@ -16,6 +16,7 @@ impl MePool {
         let pending_started_at = reinit.pending_hardswap_started_at_epoch_secs;
         let pending_hardswap_age_secs =
             (pending_started_at > 0).then_some(now_epoch_secs.saturating_sub(pending_started_at));
+        let hardswap = self.api_hardswap_snapshot_for_reinit(reinit).await;
 
         let mut quarantined_endpoints = Vec::<MeApiQuarantinedEndpointSnapshot>::new();
         {
@@ -64,6 +65,13 @@ impl MePool {
                 .max_concurrency_effective
                 .load(Ordering::Acquire),
             hardswap_enabled: self.reinit.hardswap.load(Ordering::Relaxed),
+            pending_writers_current: hardswap.pending_writers_current,
+            pending_writer_deficit: hardswap.pending_writer_deficit,
+            pending_missing_dc_groups: hardswap.pending_missing_dc_groups,
+            pending_map_current: hardswap.pending_map_current,
+            orphan_warm_writers_current: hardswap.orphan_warm_writers_current,
+            replacement_preparing_current: hardswap.replacement_preparing_current,
+            replacement_retiring_current: hardswap.replacement_retiring_current,
             floor_mode: floor_mode_label(self.floor_mode()),
             adaptive_floor_idle_secs: self
                 .floor_runtime

src/transport/middle_proxy/pool_status/status_snapshot.rs

@@ -16,10 +16,15 @@ impl MePool {
             return false;
         }
 
+        let active_generation = self.reinit.status.load().active_generation;
         let writers = self.writers.read().await.clone();
         let mut live_writers_by_dc = HashMap::<i16, usize>::new();
         for writer in writers.iter() {
-            if writer.draining.load(Ordering::Relaxed) {
+            if writer.draining.load(Ordering::Relaxed)
+                || writer.generation != active_generation
+                || WriterContour::from_u8(writer.contour.load(Ordering::Relaxed))
+                    != WriterContour::Active
+            {
                 continue;
             }
             if let Ok(dc) = i16::try_from(writer.writer_dc) {
@@ -53,10 +58,15 @@ impl MePool {
             return false;
         }
 
+        let active_generation = self.reinit.status.load().active_generation;
         let writers = self.writers.read().await.clone();
         let mut live_writers_by_dc = HashMap::<i16, usize>::new();
         for writer in writers.iter() {
-            if writer.draining.load(Ordering::Relaxed) {
+            if writer.draining.load(Ordering::Relaxed)
+                || writer.generation != active_generation
+                || WriterContour::from_u8(writer.contour.load(Ordering::Relaxed))
+                    != WriterContour::Active
+            {
                 continue;
             }
             if let Ok(dc) = i16::try_from(writer.writer_dc) {
@@ -127,7 +137,7 @@ impl MePool {
         for writer in writers.iter() {
             let endpoint = writer.addr;
             let dc = i16::try_from(writer.writer_dc).ok();
-            let draining = writer.draining.load(Ordering::Relaxed);
+            let draining = writer.draining.load(Ordering::Acquire);
             let degraded = writer.degraded.load(Ordering::Relaxed);
             let matches_active_generation = writer.generation == active_generation;
             let in_desired_map = dc
@@ -156,13 +166,18 @@ impl MePool {
                 && drain_ttl_secs > 0
                 && drain_started_at_epoch_secs
                     .is_some_and(|started| now_epoch_secs.saturating_sub(started) > drain_ttl_secs);
-            let state = match WriterContour::from_u8(writer.contour.load(Ordering::Relaxed)) {
+            let contour = WriterContour::from_u8(writer.contour.load(Ordering::Relaxed));
+            let state = match contour {
                 WriterContour::Warm => "warm",
                 WriterContour::Active => "active",
                 WriterContour::Draining => "draining",
             };
 
-            if !draining && let Some(dc_idx) = dc {
+            let authoritative_active = !draining
+                && contour == WriterContour::Active
+                && matches_active_generation
+                && in_desired_map;
+            if authoritative_active && let Some(dc_idx) = dc {
                 *live_writers_by_dc_endpoint
                     .entry((dc_idx, endpoint))
                     .or_insert(0) += 1;
@@ -172,10 +187,8 @@ impl MePool {
                     entry.0 += ema_ms;
                     entry.1 += 1;
                 }
-                if matches_active_generation && in_desired_map {
                 *fresh_writers_by_dc.entry(dc_idx).or_insert(0) += 1;
             }
-            }
 
             writer_rows.push(MeApiWriterStatusSnapshot {
                 writer_id: writer.id,

src/transport/middle_proxy/pool_writer.rs

@@ -2,6 +2,7 @@ use std::collections::HashMap;
 use std::future::Future;
 use std::io::ErrorKind;
 use std::net::SocketAddr;
+use std::pin::Pin;
 use std::sync::Arc;
 use std::sync::atomic::{AtomicBool, AtomicU8, AtomicU32, AtomicU64, Ordering};
 use std::time::{Duration, Instant};
@@ -19,14 +20,61 @@ use crate::error::{ProxyError, Result};
 use crate::protocol::constants::{RPC_CLOSE_EXT_U32, RPC_PING_U32};
 
 use super::codec::{RpcWriter, WriterCommand, build_control_payload};
-use super::pool::{MePool, MeWriter, WriterContour};
+use super::pool::{
+    MePool, MeWriter, WriterContour, WriterOpenIntent, WriterOpenReservation, WriterRole,
+};
 use super::pool_lifecycle::MeTaskRegistration;
 use super::reader::reader_loop;
+use super::registry::WriterReplacementReservation;
 use super::wire::build_proxy_req_payload;
 
 // Writer admission, teardown, and drain-state transitions.
+mod publication;
+mod replacement;
 mod runtime;
 
+struct PreparedWriter<'a> {
+    writer: MeWriter,
+    tx: mpsc::Sender<WriterCommand>,
+    byte_budget: Arc<tokio::sync::Semaphore>,
+    task_registration: MeTaskRegistration<'a>,
+    writer_task: Pin<Box<dyn Future<Output = ()> + Send + 'static>>,
+    intent: WriterOpenIntent,
+    _open_reservation: WriterOpenReservation<'a>,
+}
+
+#[derive(Debug, Clone, Copy, PartialEq, Eq)]
+/// Commit-time invariant applied to a transactional writer replacement.
+pub(super) enum WriterReplacementPurpose {
+    /// Replaces an idle writer before the upstream idle lifetime expires.
+    IdleRefresh,
+    /// Moves one idle slot from a donor above floor to a receiver below floor.
+    FloorRebalance {
+        /// Minimum writer count that must remain in the donor group.
+        donor_floor: usize,
+        /// Target count above which the receiver no longer needs the slot.
+        receiver_floor: usize,
+    },
+    /// Replaces a stale-generation writer with an active-generation writer.
+    GenerationConvergence,
+    /// Rotates a single-endpoint shadow while retaining coverage.
+    ShadowRotation,
+    /// Replaces a writer after cryptographic secret rotation.
+    SecretRotation,
+}
+
+impl WriterReplacementPurpose {
+    fn as_str(self) -> &'static str {
+        match self {
+            Self::IdleRefresh => "idle_refresh",
+            Self::FloorRebalance { .. } => "floor_rebalance",
+            Self::GenerationConvergence => "generation_convergence",
+            Self::ShadowRotation => "shadow_rotation",
+            Self::SecretRotation => "secret_rotation",
+        }
+    }
+}
+
 const ME_ACTIVE_PING_SECS: u64 = 25;
 const ME_ACTIVE_PING_JITTER_SECS: i64 = 5;
 const ME_IDLE_KEEPALIVE_MAX_SECS: u64 = 5;

src/transport/middle_proxy/pool_writer/publication.rs

@@ -0,0 +1,180 @@
+use super::*;
+
+impl MePool {
+    /// Publishes a connected writer only while its generation owns the requested role.
+    pub(super) async fn publish_connected_writer(
+        self: &Arc<Self>,
+        prepared: PreparedWriter<'_>,
+    ) -> Result<()> {
+        let PreparedWriter {
+            writer,
+            tx,
+            byte_budget,
+            task_registration,
+            writer_task,
+            intent,
+            _open_reservation,
+        } = prepared;
+        let writer_id = writer.id;
+        let mut writers = self.writers.write().await;
+        let mut registry_registration = self.registry.prepare_writer_registration().await;
+        let coordinator = self.reinit.coordinator.lock();
+        let contour = self.authorize_writer_publication(&writer, &coordinator)?;
+        self.authorize_writer_publication_capacity(&writer, contour, intent, writers.as_slice())?;
+        writer.contour.store(contour.as_u8(), Ordering::Release);
+        registry_registration.install(writer_id, tx, byte_budget);
+        writers.push(writer);
+        self.conn_count.fetch_add(1, Ordering::Relaxed);
+        self.lifecycle
+            .spawn_registered_writer(task_registration, writer_task);
+        drop(coordinator);
+        drop(registry_registration);
+        drop(writers);
+        self.notify_writer_epoch();
+        Ok(())
+    }
+
+    /// Resolves the writer role against the linearized reinitialization authority.
+    pub(super) fn authorize_writer_publication(
+        &self,
+        writer: &MeWriter,
+        coordinator: &crate::transport::middle_proxy::pool::ReinitCoordinatorState,
+    ) -> Result<WriterContour> {
+        let endpoint_is_current = self
+            .preferred_endpoints_by_dc
+            .load()
+            .get(&writer.writer_dc)
+            .is_some_and(|endpoints| endpoints.contains(&writer.addr));
+        if !endpoint_is_current {
+            return Err(ProxyError::Proxy(
+                "ME writer target changed before publication".into(),
+            ));
+        }
+
+        let requested = WriterContour::from_u8(writer.contour.load(Ordering::Acquire));
+        if writer.generation == coordinator.active_generation
+            && matches!(requested, WriterContour::Active | WriterContour::Warm)
+        {
+            return Ok(WriterContour::Active);
+        }
+        if requested == WriterContour::Warm
+            && coordinator.pending.is_some_and(|pending| {
+                pending.generation == writer.generation
+                    && pending.map_hash == coordinator.desired_map_hash
+            })
+        {
+            return Ok(WriterContour::Warm);
+        }
+        Err(ProxyError::Proxy(
+            "ME writer generation lost publication authority".into(),
+        ))
+    }
+
+    /// Revalidates role-local capacity at the serialized publication boundary.
+    pub(in crate::transport::middle_proxy) fn authorize_writer_publication_capacity(
+        &self,
+        writer: &MeWriter,
+        contour: WriterContour,
+        intent: WriterOpenIntent,
+        writers: &[MeWriter],
+    ) -> Result<()> {
+        let family = if writer.addr.is_ipv4() {
+            crate::network::IpFamily::V4
+        } else {
+            crate::network::IpFamily::V6
+        };
+        let now_epoch_secs = Self::now_epoch_secs();
+        if !self.family_enabled_for_drain_coverage(family, now_epoch_secs) {
+            return Err(ProxyError::Proxy(
+                "ME writer family lost publication authority".into(),
+            ));
+        }
+        if intent == WriterOpenIntent::Replacement || contour == WriterContour::Draining {
+            return Ok(());
+        }
+        let preferred = self.preferred_endpoints_by_dc.load();
+        let Some(endpoints) = preferred.get(&writer.writer_dc) else {
+            return Err(ProxyError::Proxy(
+                "ME writer target changed before publication".into(),
+            ));
+        };
+        let required = match contour {
+            WriterContour::Active => self.required_writers_for_dc(
+                endpoints
+                    .iter()
+                    .filter(|endpoint| endpoint.is_ipv4() == writer.addr.is_ipv4())
+                    .count(),
+            ),
+            WriterContour::Warm => self.required_writers_for_dc(
+                endpoints
+                    .iter()
+                    .filter(|endpoint| {
+                        let endpoint_family = if endpoint.is_ipv4() {
+                            crate::network::IpFamily::V4
+                        } else {
+                            crate::network::IpFamily::V6
+                        };
+                        self.family_enabled_for_drain_coverage(
+                            endpoint_family,
+                            now_epoch_secs,
+                        )
+                    })
+                    .count(),
+            ),
+            WriterContour::Draining => 0,
+        };
+        let current = writers
+            .iter()
+            .filter(|candidate| {
+                !candidate.draining.load(Ordering::Acquire)
+                    && candidate.writer_dc == writer.writer_dc
+                    && candidate.generation == writer.generation
+                    && WriterContour::from_u8(candidate.contour.load(Ordering::Acquire))
+                        == contour
+                    && (contour == WriterContour::Warm
+                        || candidate.addr.is_ipv4() == writer.addr.is_ipv4())
+                    && endpoints.contains(&candidate.addr)
+                    && (contour != WriterContour::Warm
+                        || self.family_enabled_for_drain_coverage(
+                            if candidate.addr.is_ipv4() {
+                                crate::network::IpFamily::V4
+                            } else {
+                                crate::network::IpFamily::V6
+                            },
+                            now_epoch_secs,
+                        ))
+            })
+            .count();
+        if current >= required {
+            return Err(ProxyError::Proxy(
+                "ME writer floor was restored before publication".into(),
+            ));
+        }
+        Ok(())
+    }
+
+    /// Commits writer visibility and lifecycle ownership after all cancellation points.
+    #[allow(clippy::too_many_arguments)]
+    pub(in crate::transport::middle_proxy) async fn publish_prepared_writer<F>(
+        self: &Arc<Self>,
+        writer: MeWriter,
+        tx: mpsc::Sender<WriterCommand>,
+        byte_budget: Arc<tokio::sync::Semaphore>,
+        task_registration: MeTaskRegistration<'_>,
+        writer_task: F,
+    ) where
+        F: Future<Output = ()> + Send + 'static,
+    {
+        // Writer publication follows the global writers -> registry binding lock order.
+        let mut writers = self.writers.write().await;
+        let mut registry_registration = self.registry.prepare_writer_registration().await;
+        registry_registration.install(writer.id, tx, byte_budget);
+        writers.push(writer);
+        self.conn_count.fetch_add(1, Ordering::Relaxed);
+        self.lifecycle
+            .spawn_registered_writer(task_registration, writer_task);
+        drop(writers);
+        drop(registry_registration);
+        self.notify_writer_epoch();
+    }
+}

src/transport/middle_proxy/pool_writer/replacement.rs

@@ -0,0 +1,376 @@
+use super::*;
+
+impl MePool {
+    /// Opens a replacement and atomically publishes it before retiring its reserved victim.
+    pub(in crate::transport::middle_proxy) async fn replace_writer_with_generation_contour_for_dc(
+        self: &Arc<Self>,
+        addr: SocketAddr,
+        rng: &SecureRandom,
+        generation: u64,
+        contour: WriterContour,
+        writer_dc: i32,
+        expected_victim_role: WriterRole,
+        purpose: WriterReplacementPurpose,
+        reservation: &mut WriterReplacementReservation<'_>,
+    ) -> Result<()> {
+        let prepared = self
+            .prepare_writer_with_intent(
+                addr,
+                rng,
+                generation,
+                contour,
+                writer_dc,
+                WriterOpenIntent::Replacement,
+            )
+            .await?;
+        self.publish_prepared_replacement_writer(
+            prepared,
+            expected_victim_role,
+            purpose,
+            reservation,
+        )
+        .await
+    }
+
+    async fn publish_prepared_replacement_writer(
+        self: &Arc<Self>,
+        prepared: PreparedWriter<'_>,
+        expected_victim_role: WriterRole,
+        purpose: WriterReplacementPurpose,
+        reservation: &mut WriterReplacementReservation<'_>,
+    ) -> Result<()> {
+        let PreparedWriter {
+            writer,
+            tx,
+            byte_budget,
+            task_registration,
+            writer_task,
+            intent,
+            _open_reservation,
+        } = prepared;
+        let replacement_writer_id = writer.id;
+        let victim_writer_id = reservation.writer_id();
+
+        // Lock order is writers -> registry binding. No cancellation point follows acquisition of
+        // the registry guard, so publication and victim retirement commit as one state change.
+        let mut writers = self.writers.write().await;
+        let mut registry_registration = self.registry.prepare_writer_registration().await;
+        let coordinator = self.reinit.coordinator.lock();
+        let contour = self.authorize_writer_publication(&writer, &coordinator)?;
+        self.authorize_writer_publication_capacity(&writer, contour, intent, writers.as_slice())?;
+        writer.contour.store(contour.as_u8(), Ordering::Release);
+        let Some(victim_pos) = writers
+            .iter()
+            .position(|candidate| candidate.id == victim_writer_id)
+        else {
+            return Err(ProxyError::Proxy(
+                "ME replacement victim disappeared before commit".into(),
+            ));
+        };
+        let victim = &writers[victim_pos];
+        if victim.draining.load(Ordering::Acquire) || !expected_victim_role.matches(victim) {
+            return Err(ProxyError::Proxy(
+                "ME replacement victim changed role before commit".into(),
+            ));
+        }
+        if let WriterReplacementPurpose::FloorRebalance {
+            donor_floor,
+            receiver_floor,
+        } = purpose
+        {
+            if expected_victim_role.generation != coordinator.active_generation
+                || writer.generation != coordinator.active_generation
+            {
+                return Err(ProxyError::Proxy(
+                    "ME floor rebalance lost active-generation authority".into(),
+                ));
+            }
+            let preferred = self.preferred_endpoints_by_dc.load();
+            let donor_count = writers
+                .iter()
+                .filter(|candidate| {
+                    !candidate.draining.load(Ordering::Acquire)
+                        && candidate.writer_dc == expected_victim_role.dc
+                        && candidate.generation == expected_victim_role.generation
+                        && WriterContour::from_u8(candidate.contour.load(Ordering::Acquire))
+                            == WriterContour::Active
+                        && preferred
+                            .get(&candidate.writer_dc)
+                            .is_some_and(|endpoints| endpoints.contains(&candidate.addr))
+                        && (candidate.addr.is_ipv4()
+                            == matches!(expected_victim_role.family, crate::network::IpFamily::V4))
+                })
+                .count();
+            let receiver_family = if writer.addr.is_ipv4() {
+                crate::network::IpFamily::V4
+            } else {
+                crate::network::IpFamily::V6
+            };
+            let receiver_count = writers
+                .iter()
+                .filter(|candidate| {
+                    !candidate.draining.load(Ordering::Acquire)
+                        && candidate.writer_dc == writer.writer_dc
+                        && candidate.generation == writer.generation
+                        && WriterContour::from_u8(candidate.contour.load(Ordering::Acquire))
+                            == WriterContour::Active
+                        && preferred
+                            .get(&candidate.writer_dc)
+                            .is_some_and(|endpoints| endpoints.contains(&candidate.addr))
+                        && (if candidate.addr.is_ipv4() {
+                            crate::network::IpFamily::V4
+                        } else {
+                            crate::network::IpFamily::V6
+                        }) == receiver_family
+                })
+                .count();
+            if donor_count <= donor_floor || receiver_count >= receiver_floor {
+                return Err(ProxyError::Proxy(
+                    "ME floor rebalance became unnecessary before commit".into(),
+                ));
+            }
+        }
+        if !registry_registration.prepare_replacement_commit(reservation) {
+            return Err(ProxyError::Proxy(
+                "ME replacement victim became active before commit".into(),
+            ));
+        }
+
+        registry_registration.install(replacement_writer_id, tx, byte_budget);
+        writers.push(writer);
+        self.conn_count.fetch_add(1, Ordering::Relaxed);
+        writers.publish_current();
+        self.apply_writer_draining_state(
+            &writers[victim_pos],
+            self.force_close_timeout(),
+            false,
+        );
+        self.lifecycle
+            .spawn_registered_writer(task_registration, writer_task);
+        reservation.mark_committed();
+        drop(coordinator);
+        drop(registry_registration);
+        drop(writers);
+        self.notify_writer_epoch();
+        info!(
+            victim_writer_id,
+            replacement_writer_id,
+            purpose = purpose.as_str(),
+            "ME writer replacement committed"
+        );
+        Ok(())
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use std::collections::HashMap;
+    use std::net::{IpAddr, Ipv4Addr};
+
+    use crate::transport::middle_proxy::pool_writer_security_tests::make_pool;
+
+    use super::*;
+
+    fn endpoint(octet: u8) -> SocketAddr {
+        SocketAddr::new(IpAddr::V4(Ipv4Addr::new(127, 0, 0, octet)), 443)
+    }
+
+    async fn install_writer(
+        pool: &Arc<MePool>,
+        writer_id: u64,
+        writer_dc: i32,
+        addr: SocketAddr,
+    ) -> MeWriter {
+        let (tx, _rx) = mpsc::channel::<WriterCommand>(8);
+        let byte_budget = pool.new_writer_byte_budget();
+        let writer = MeWriter {
+            id: writer_id,
+            addr,
+            source_ip: addr.ip(),
+            writer_dc,
+            generation: pool.current_generation(),
+            contour: Arc::new(AtomicU8::new(WriterContour::Active.as_u8())),
+            created_at: Instant::now(),
+            tx: tx.clone(),
+            byte_budget: byte_budget.clone(),
+            cancel: CancellationToken::new(),
+            degraded: Arc::new(AtomicBool::new(false)),
+            rtt_ema_ms_x10: Arc::new(AtomicU32::new(0)),
+            draining: Arc::new(AtomicBool::new(false)),
+            draining_started_at_epoch_secs: Arc::new(AtomicU64::new(0)),
+            drain_deadline_epoch_secs: Arc::new(AtomicU64::new(0)),
+            allow_drain_fallback: Arc::new(AtomicBool::new(false)),
+        };
+        let mut writers = pool.writers.write().await;
+        let mut registry_registration = pool.registry.prepare_writer_registration().await;
+        registry_registration.install(writer_id, tx, byte_budget);
+        writers.push(writer.clone());
+        pool.conn_count.fetch_add(1, Ordering::Relaxed);
+        drop(registry_registration);
+        drop(writers);
+        writer
+    }
+
+    async fn prepared_writer<'a>(
+        pool: &'a Arc<MePool>,
+        writer_id: u64,
+        writer_dc: i32,
+        addr: SocketAddr,
+    ) -> PreparedWriter<'a> {
+        let open_reservation = pool
+            .reserve_writer_open(
+                WriterContour::Active,
+                WriterOpenIntent::Replacement,
+                writer_dc,
+            )
+            .await
+            .expect("replacement open must be admitted");
+        let task_registration = pool
+            .lifecycle
+            .try_register()
+            .expect("test pool lifecycle must be open");
+        let (tx, rx) = mpsc::channel::<WriterCommand>(8);
+        let byte_budget = pool.new_writer_byte_budget();
+        PreparedWriter {
+            writer: MeWriter {
+                id: writer_id,
+                addr,
+                source_ip: addr.ip(),
+                writer_dc,
+                generation: pool.current_generation(),
+                contour: Arc::new(AtomicU8::new(WriterContour::Active.as_u8())),
+                created_at: Instant::now(),
+                tx: tx.clone(),
+                byte_budget: byte_budget.clone(),
+                cancel: CancellationToken::new(),
+                degraded: Arc::new(AtomicBool::new(false)),
+                rtt_ema_ms_x10: Arc::new(AtomicU32::new(0)),
+                draining: Arc::new(AtomicBool::new(false)),
+                draining_started_at_epoch_secs: Arc::new(AtomicU64::new(0)),
+                drain_deadline_epoch_secs: Arc::new(AtomicU64::new(0)),
+                allow_drain_fallback: Arc::new(AtomicBool::new(false)),
+            },
+            tx,
+            byte_budget,
+            task_registration,
+            writer_task: Box::pin(async move {
+                drop(rx);
+            }),
+            intent: WriterOpenIntent::Replacement,
+            _open_reservation: open_reservation,
+        }
+    }
+
+    #[tokio::test]
+    async fn replacement_commit_publishes_successor_before_draining_victim() {
+        let pool = make_pool().await;
+        let addr = endpoint(1);
+        pool.preferred_endpoints_by_dc
+            .store(Arc::new(HashMap::from([(2, vec![addr])])));
+        let victim = install_writer(&pool, 1001, 2, addr).await;
+        let expected_role = WriterRole::from_writer(&victim);
+        let mut reservation = pool
+            .registry
+            .try_reserve_writer_replacement(victim.id)
+            .await
+            .expect("idle victim must be reservable");
+        let prepared = prepared_writer(&pool, 1002, 2, addr).await;
+
+        pool.publish_prepared_replacement_writer(
+            prepared,
+            expected_role,
+            WriterReplacementPurpose::IdleRefresh,
+            &mut reservation,
+        )
+        .await
+        .expect("replacement commit must succeed");
+
+        let writers = pool.writers.read().await;
+        assert_eq!(writers.len(), 2);
+        assert_eq!(
+            writers
+                .iter()
+                .filter(|writer| !writer.draining.load(Ordering::Acquire))
+                .count(),
+            1
+        );
+        assert!(victim.draining.load(Ordering::Acquire));
+        assert!(writers.iter().any(|writer| writer.id == 1002));
+        drop(writers);
+        assert_eq!(pool.conn_count.load(Ordering::Acquire), 2);
+        assert_eq!(pool.registry.writer_replacement_counts(), (0, 1));
+    }
+
+    #[tokio::test]
+    async fn cancelled_replacement_waiting_for_publication_restores_all_reservations() {
+        let pool = make_pool().await;
+        let addr = endpoint(2);
+        pool.preferred_endpoints_by_dc
+            .store(Arc::new(HashMap::from([(2, vec![addr])])));
+        let victim = install_writer(&pool, 2001, 2, addr).await;
+        let expected_role = WriterRole::from_writer(&victim);
+        let mut reservation = pool
+            .registry
+            .try_reserve_writer_replacement(victim.id)
+            .await
+            .expect("idle victim must be reservable");
+        let prepared = prepared_writer(&pool, 2002, 2, addr).await;
+        let writers_guard = pool.writers.write().await;
+
+        let result = tokio::time::timeout(
+            Duration::from_millis(10),
+            pool.publish_prepared_replacement_writer(
+                prepared,
+                expected_role,
+                WriterReplacementPurpose::IdleRefresh,
+                &mut reservation,
+            ),
+        )
+        .await;
+
+        assert!(result.is_err());
+        drop(writers_guard);
+        drop(reservation);
+        assert_eq!(pool.writer_replacement_open_reserved.load(Ordering::Acquire), 0);
+        assert_eq!(pool.registry.writer_replacement_counts(), (0, 0));
+        assert!(!victim.draining.load(Ordering::Acquire));
+        assert!(!pool.writers.read().await.iter().any(|writer| writer.id == 2002));
+    }
+
+    #[tokio::test]
+    async fn floor_rebalance_commit_rejects_a_donor_without_surplus() {
+        let pool = make_pool().await;
+        let donor_addr = endpoint(3);
+        let receiver_addr = endpoint(4);
+        pool.preferred_endpoints_by_dc.store(Arc::new(HashMap::from([
+            (1, vec![donor_addr]),
+            (2, vec![receiver_addr]),
+        ])));
+        let victim = install_writer(&pool, 3001, 1, donor_addr).await;
+        let expected_role = WriterRole::from_writer(&victim);
+        let mut reservation = pool
+            .registry
+            .try_reserve_writer_replacement(victim.id)
+            .await
+            .expect("idle victim must be reservable");
+        let prepared = prepared_writer(&pool, 3002, 2, receiver_addr).await;
+
+        let result = pool
+            .publish_prepared_replacement_writer(
+                prepared,
+                expected_role,
+                WriterReplacementPurpose::FloorRebalance {
+                    donor_floor: 1,
+                    receiver_floor: 1,
+                },
+                &mut reservation,
+            )
+            .await;
+
+        assert!(result.is_err());
+        drop(reservation);
+        assert!(!victim.draining.load(Ordering::Acquire));
+        assert!(!pool.writers.read().await.iter().any(|writer| writer.id == 3002));
+        assert_eq!(pool.registry.writer_replacement_counts(), (0, 0));
+    }
+}

src/transport/middle_proxy/pool_writer/runtime.rs

@@ -54,23 +54,45 @@ impl MePool {
         contour: WriterContour,
         writer_dc: i32,
     ) -> Result<()> {
-        self.connect_one_with_generation_contour_for_dc_with_cap_policy(
+        self.connect_one_with_generation_contour_for_dc_with_intent(
-            addr, rng, generation, contour, writer_dc, false,
+            addr,
+            rng,
+            generation,
+            contour,
+            writer_dc,
+            WriterOpenIntent::Normal,
         )
         .await
     }
 
-    pub(in crate::transport::middle_proxy) async fn connect_one_with_generation_contour_for_dc_with_cap_policy(
+    /// Connects and publishes one writer under an explicit capacity intent.
+    pub(in crate::transport::middle_proxy) async fn connect_one_with_generation_contour_for_dc_with_intent(
         self: &Arc<Self>,
         addr: SocketAddr,
         rng: &SecureRandom,
         generation: u64,
         contour: WriterContour,
         writer_dc: i32,
-        allow_coverage_override: bool,
+        intent: WriterOpenIntent,
     ) -> Result<()> {
-        let Some(_writer_open_reservation) = self
+        let prepared = self
-            .reserve_writer_open(contour, allow_coverage_override, writer_dc)
+            .prepare_writer_with_intent(addr, rng, generation, contour, writer_dc, intent)
+            .await?;
+        self.publish_connected_writer(prepared).await
+    }
+
+    /// Completes all cancellable connection work without publishing pool visibility.
+    pub(super) async fn prepare_writer_with_intent<'a>(
+        self: &'a Arc<Self>,
+        addr: SocketAddr,
+        rng: &SecureRandom,
+        generation: u64,
+        contour: WriterContour,
+        writer_dc: i32,
+        intent: WriterOpenIntent,
+    ) -> Result<PreparedWriter<'a>> {
+        let Some(writer_open_reservation) = self
+            .reserve_writer_open(contour, intent, writer_dc)
             .await
         else {
             return Err(ProxyError::Proxy(format!(
@@ -264,36 +286,15 @@ impl MePool {
             })
         };
 
-        self.publish_prepared_writer(writer, tx, byte_budget, task_registration, writer_task)
+        Ok(PreparedWriter {
-            .await;
+            writer,
-
+            tx,
-        Ok(())
+            byte_budget,
-    }
+            task_registration,
-
+            writer_task,
-    /// Commits writer visibility and lifecycle ownership after all cancellation points.
+            intent,
-    #[allow(clippy::too_many_arguments)]
+            _open_reservation: writer_open_reservation,
-    pub(in crate::transport::middle_proxy) async fn publish_prepared_writer<F>(
+        })
-        self: &Arc<Self>,
-        writer: MeWriter,
-        tx: mpsc::Sender<WriterCommand>,
-        byte_budget: Arc<tokio::sync::Semaphore>,
-        task_registration: MeTaskRegistration<'_>,
-        writer_task: F,
-    ) where
-        F: Future<Output = ()> + Send + 'static,
-    {
-        let (mut writers, mut registry_registration) = tokio::join!(
-            self.writers.write(),
-            self.registry.prepare_writer_registration()
-        );
-        registry_registration.install(writer.id, tx, byte_budget);
-        writers.push(writer);
-        self.conn_count.fetch_add(1, Ordering::Relaxed);
-        self.lifecycle
-            .spawn_registered_writer(task_registration, writer_task);
-        drop(writers);
-        drop(registry_registration);
-        self.notify_writer_epoch();
     }
 
     pub(crate) async fn remove_writer_and_close_clients(self: &Arc<Self>, writer_id: u64) {
@@ -331,7 +332,7 @@ impl MePool {
     ) -> bool {
         let mut close_tx: Option<mpsc::Sender<WriterCommand>> = None;
         let mut removed_addr: Option<SocketAddr> = None;
-        let mut removed_dc: Option<i32> = None;
+        let mut removed_role: Option<WriterRole> = None;
         let mut removed_uptime: Option<Duration> = None;
         let mut trigger_refill = false;
         let mut removed = false;
@@ -352,7 +353,7 @@ impl MePool {
                 self.stats.increment_me_writer_removed_total();
                 w.cancel.cancel();
                 removed_addr = Some(w.addr);
-                removed_dc = Some(w.writer_dc);
+                removed_role = Some(WriterRole::from_writer(&w));
                 removed_uptime = Some(w.created_at.elapsed());
                 trigger_refill = !was_draining;
                 if trigger_refill {
@@ -397,8 +398,8 @@ impl MePool {
                     );
                 }
             }
-            if trigger_refill && let Some(writer_dc) = removed_dc {
+            if trigger_refill && let Some(role) = removed_role {
-                self.trigger_immediate_refill_for_dc(addr, writer_dc);
+                self.trigger_immediate_refill_for_role(addr, role);
             }
         }
         if removed {
@@ -415,26 +416,13 @@ impl MePool {
     ) {
         let timeout = timeout.filter(|d| !d.is_zero());
         let found = {
-            let mut ws = self.writers.write().await;
+            let ws = self.writers.write().await;
-            if let Some(w) = ws.iter_mut().find(|w| w.id == writer_id) {
+            let mut registry_registration = self.registry.prepare_writer_registration().await;
-                let already_draining = w.draining.swap(true, Ordering::Relaxed);
+            if let Some(w) = ws.iter().find(|w| w.id == writer_id) {
-                w.allow_drain_fallback
+                if !allow_drain_fallback {
-                    .store(allow_drain_fallback, Ordering::Relaxed);
+                    registry_registration.retire(writer_id);
-                let now_epoch_secs = Self::now_epoch_secs();
-                w.draining_started_at_epoch_secs
-                    .store(now_epoch_secs, Ordering::Relaxed);
-                let drain_deadline_epoch_secs = timeout
-                    .map(|duration| now_epoch_secs.saturating_add(duration.as_secs()))
-                    .unwrap_or(0);
-                w.drain_deadline_epoch_secs
-                    .store(drain_deadline_epoch_secs, Ordering::Relaxed);
-                if !already_draining {
-                    self.stats.increment_pool_drain_active();
-                    self.increment_draining_active_runtime();
                 }
-                w.contour
+                self.apply_writer_draining_state(w, timeout, allow_drain_fallback);
-                    .store(WriterContour::Draining.as_u8(), Ordering::Relaxed);
-                w.draining.store(true, Ordering::Relaxed);
                 true
             } else {
                 false
@@ -452,6 +440,39 @@ impl MePool {
         );
     }
 
+    /// Publishes drain metadata before the release-store that makes draining observable.
+    pub(in crate::transport::middle_proxy) fn apply_writer_draining_state(
+        &self,
+        writer: &MeWriter,
+        timeout: Option<Duration>,
+        allow_drain_fallback: bool,
+    ) -> bool {
+        let timeout = timeout.filter(|duration| !duration.is_zero());
+        let already_draining = writer.draining.load(Ordering::Acquire);
+        writer
+            .allow_drain_fallback
+            .store(allow_drain_fallback, Ordering::Release);
+        let now_epoch_secs = Self::now_epoch_secs();
+        writer
+            .draining_started_at_epoch_secs
+            .store(now_epoch_secs, Ordering::Release);
+        let drain_deadline_epoch_secs = timeout
+            .map(|duration| now_epoch_secs.saturating_add(duration.as_secs()))
+            .unwrap_or(0);
+        writer
+            .drain_deadline_epoch_secs
+            .store(drain_deadline_epoch_secs, Ordering::Release);
+        writer
+            .contour
+            .store(WriterContour::Draining.as_u8(), Ordering::Release);
+        writer.draining.store(true, Ordering::Release);
+        if !already_draining {
+            self.stats.increment_pool_drain_active();
+            self.increment_draining_active_runtime();
+        }
+        !already_draining
+    }
+
     pub(crate) async fn mark_writer_draining(self: &Arc<Self>, writer_id: u64) {
         self.mark_writer_draining_with_timeout(writer_id, Some(Duration::from_secs(300)), false)
             .await;
@@ -461,7 +482,7 @@ impl MePool {
         &self,
         writer: &MeWriter,
     ) -> bool {
-        if !writer.draining.load(Ordering::Relaxed) {
+        if !writer.draining.load(Ordering::Acquire) {
             return true;
         }
         if !writer.allow_drain_fallback.load(Ordering::Relaxed) {

src/transport/middle_proxy/registry.rs

@@ -20,8 +20,14 @@ const ROUTE_QUEUED_MAX_FRAME_PERMITS: usize = 1024;
 
 // Transactional writer registry publication.
 mod publication;
+// Cancellation-safe idle-writer replacement reservations.
+mod replacement;
 mod writer;
 
+pub(in crate::transport::middle_proxy) use replacement::{
+    WriterBindOutcome, WriterReplacementReservation,
+};
+
 #[derive(Debug, Clone, Copy, PartialEq, Eq)]
 pub enum RouteResult {
     Routed,
@@ -74,6 +80,7 @@ struct WriterTable {
 struct WriterRoute {
     tx: mpsc::Sender<WriterCommand>,
     byte_budget: Arc<Semaphore>,
+    replacement_state: Arc<AtomicU8>,
 }
 
 #[derive(Clone)]
@@ -94,6 +101,15 @@ struct BindingState {
     last_meta_for_writer: DashMap<u64, ConnMeta>,
 }
 
+#[derive(Debug, Clone, Copy, PartialEq, Eq)]
+#[repr(u8)]
+enum WriterReplacementState {
+    Open = 0,
+    Preparing,
+    Retiring,
+    Draining,
+}
+
 struct BindingInner {
     writer_for_conn: HashMap<u64, u64>,
     conns_for_writer: HashMap<u64, HashSet<u64>>,

src/transport/middle_proxy/registry/publication.rs

@@ -1,9 +1,11 @@
 use std::sync::Arc;
+use std::sync::atomic::{AtomicU8, Ordering};
 
 use tokio::sync::{MutexGuard, Semaphore, mpsc};
 
 use super::super::codec::WriterCommand;
-use super::{BindingInner, ConnRegistry, WriterRoute};
+use super::replacement::WriterReplacementReservation;
+use super::{BindingInner, ConnRegistry, WriterReplacementState, WriterRoute};
 
 /// Holds registry binding ownership until pool writer visibility is published.
 pub(in crate::transport::middle_proxy) struct WriterRegistrationGuard<'a> {
@@ -24,6 +26,73 @@ impl ConnRegistry {
 }
 
 impl WriterRegistrationGuard<'_> {
+    /// Prevents new bindings while preserving all existing writer associations.
+    pub(in crate::transport::middle_proxy) fn retire(&mut self, writer_id: u64) -> bool {
+        let Some(state) = self
+            .registry
+            .writers
+            .map
+            .get(&writer_id)
+            .map(|route| Arc::clone(&route.replacement_state))
+        else {
+            return false;
+        };
+        loop {
+            let current = state.load(Ordering::Acquire);
+            if current == WriterReplacementState::Retiring as u8
+                || current == WriterReplacementState::Draining as u8
+            {
+                return true;
+            }
+            if state
+                .compare_exchange_weak(
+                    current,
+                    WriterReplacementState::Draining as u8,
+                    Ordering::AcqRel,
+                    Ordering::Acquire,
+                )
+                .is_ok()
+            {
+                return true;
+            }
+        }
+    }
+
+    /// Revalidates an idle replacement victim and prevents subsequent client bindings.
+    pub(in crate::transport::middle_proxy) fn prepare_replacement_commit(
+        &mut self,
+        reservation: &WriterReplacementReservation<'_>,
+    ) -> bool {
+        let Some(route_state) = self
+            .registry
+            .writers
+            .map
+            .get(&reservation.writer_id())
+            .map(|route| Arc::clone(&route.replacement_state))
+        else {
+            return false;
+        };
+        if !std::ptr::eq(self.registry, reservation.registry())
+            || !Arc::ptr_eq(&route_state, reservation.state())
+            || reservation.requires_idle()
+                && self
+                .binding
+                .conns_for_writer
+                .get(&reservation.writer_id())
+                .is_none_or(|conn_ids| !conn_ids.is_empty())
+        {
+            return false;
+        }
+        route_state
+            .compare_exchange(
+                WriterReplacementState::Preparing as u8,
+                WriterReplacementState::Retiring as u8,
+                Ordering::AcqRel,
+                Ordering::Acquire,
+            )
+            .is_ok()
+    }
+
     /// Installs registry state while retaining binding ownership for pool publication.
     pub(in crate::transport::middle_proxy) fn install(
         &mut self,
@@ -42,9 +111,13 @@ impl WriterRegistrationGuard<'_> {
             .writer_idle_since_epoch_secs
             .entry(writer_id)
             .or_insert_with(ConnRegistry::now_epoch_secs);
-        self.registry
+        self.registry.writers.map.insert(
-            .writers
+            writer_id,
-            .map
+            WriterRoute {
-            .insert(writer_id, WriterRoute { tx, byte_budget });
+                tx,
+                byte_budget,
+                replacement_state: Arc::new(AtomicU8::new(WriterReplacementState::Open as u8)),
+            },
+        );
     }
 }

src/transport/middle_proxy/registry/replacement.rs

@@ -0,0 +1,142 @@
+use std::sync::Arc;
+use std::sync::atomic::{AtomicU8, Ordering};
+
+use super::{ConnRegistry, WriterReplacementState};
+
+/// Result of atomically committing a client-to-writer binding.
+#[derive(Debug, Clone, Copy, PartialEq, Eq)]
+pub(in crate::transport::middle_proxy) enum WriterBindOutcome {
+    /// The client route was bound to the selected writer.
+    Bound,
+    /// The client route disappeared before the binding commit.
+    RouteMissing,
+    /// The selected writer disappeared before the binding commit.
+    WriterMissing,
+    /// The selected writer is retiring and no longer accepts new clients.
+    WriterRetiring,
+}
+
+/// Cancellation-safe ownership of one prospective writer replacement.
+pub(in crate::transport::middle_proxy) struct WriterReplacementReservation<'a> {
+    registry: &'a ConnRegistry,
+    writer_id: u64,
+    state: Arc<AtomicU8>,
+    require_idle: bool,
+    committed: bool,
+}
+
+impl WriterReplacementReservation<'_> {
+    /// Returns the stable identifier of the prospective victim.
+    pub(in crate::transport::middle_proxy) fn writer_id(&self) -> u64 {
+        self.writer_id
+    }
+
+    /// Returns the registry whose binding lock linearizes this reservation.
+    pub(super) fn registry(&self) -> &ConnRegistry {
+        self.registry
+    }
+
+    /// Returns the writer-local replacement state identity captured at reservation time.
+    pub(super) fn state(&self) -> &Arc<AtomicU8> {
+        &self.state
+    }
+
+    /// Reports whether commit must revalidate that the victim remains unbound.
+    pub(super) fn requires_idle(&self) -> bool {
+        self.require_idle
+    }
+
+    /// Transfers retirement ownership to the published replacement.
+    pub(in crate::transport::middle_proxy) fn mark_committed(&mut self) {
+        self.committed = true;
+    }
+}
+
+impl Drop for WriterReplacementReservation<'_> {
+    fn drop(&mut self) {
+        if self.committed {
+            return;
+        }
+        let preparing = WriterReplacementState::Preparing as u8;
+        let open = WriterReplacementState::Open as u8;
+        let _ = self
+            .state
+            .compare_exchange(preparing, open, Ordering::AcqRel, Ordering::Acquire);
+    }
+}
+
+impl ConnRegistry {
+    /// Claims an idle writer as a prospective replacement victim.
+    ///
+    /// Preparing prevents duplicate replacement work but intentionally permits new client binds.
+    /// Commit revalidates idleness under the existing binding lock before blocking future binds.
+    pub(in crate::transport::middle_proxy) async fn try_reserve_writer_replacement(
+        &self,
+        writer_id: u64,
+    ) -> Option<WriterReplacementReservation<'_>> {
+        self.reserve_writer_replacement(writer_id, true).await
+    }
+
+    /// Claims a writer while retaining its existing client associations through replacement.
+    pub(in crate::transport::middle_proxy) async fn try_reserve_writer_replacement_preserving_clients(
+        &self,
+        writer_id: u64,
+    ) -> Option<WriterReplacementReservation<'_>> {
+        self.reserve_writer_replacement(writer_id, false).await
+    }
+
+    async fn reserve_writer_replacement(
+        &self,
+        writer_id: u64,
+        require_idle: bool,
+    ) -> Option<WriterReplacementReservation<'_>> {
+        let binding = self.binding.inner.lock().await;
+        let state = self
+            .writers
+            .map
+            .get(&writer_id)
+            .map(|route| Arc::clone(&route.replacement_state))?;
+        if require_idle
+            && binding
+            .conns_for_writer
+            .get(&writer_id)
+            .is_none_or(|conn_ids| !conn_ids.is_empty())
+        {
+            return None;
+        }
+        state
+            .compare_exchange(
+                WriterReplacementState::Open as u8,
+                WriterReplacementState::Preparing as u8,
+                Ordering::AcqRel,
+                Ordering::Acquire,
+            )
+            .ok()?;
+
+        Some(WriterReplacementReservation {
+            registry: self,
+            writer_id,
+            state,
+            require_idle,
+            committed: false,
+        })
+    }
+
+    /// Returns fixed-cardinality gauges for preparing and retiring replacements.
+    pub(in crate::transport::middle_proxy) fn writer_replacement_counts(&self) -> (usize, usize) {
+        let mut preparing = 0usize;
+        let mut retiring = 0usize;
+        for route in &self.writers.map {
+            match route.replacement_state.load(Ordering::Acquire) {
+                state if state == WriterReplacementState::Preparing as u8 => {
+                    preparing = preparing.saturating_add(1);
+                }
+                state if state == WriterReplacementState::Retiring as u8 => {
+                    retiring = retiring.saturating_add(1);
+                }
+                _ => {}
+            }
+        }
+        (preparing, retiring)
+    }
+}

src/transport/middle_proxy/registry/tests.rs

@@ -4,13 +4,90 @@ use std::sync::Arc;
 use bytes::Bytes;
 use tokio::sync::Semaphore;
 
-use super::{ConnMeta, ConnRegistry, RouteResult};
+use super::{ConnMeta, ConnRegistry, RouteResult, WriterBindOutcome};
 use crate::transport::middle_proxy::MeResponse;
 
 fn writer_byte_budget() -> Arc<Semaphore> {
     Arc::new(Semaphore::new(2049))
 }
 
+fn conn_meta() -> ConnMeta {
+    let addr = SocketAddr::new(IpAddr::V4(Ipv4Addr::LOCALHOST), 443);
+    ConnMeta {
+        target_dc: 2,
+        client_addr: addr,
+        our_addr: addr,
+        proto_flags: 0,
+    }
+}
+
+#[tokio::test]
+async fn preparing_replacement_allows_bind_and_idle_commit_revalidates() {
+    let registry = ConnRegistry::new();
+    let (writer_tx, _writer_rx) = tokio::sync::mpsc::channel(8);
+    registry
+        .register_writer(10, writer_tx, writer_byte_budget())
+        .await;
+    let reservation = registry
+        .try_reserve_writer_replacement(10)
+        .await
+        .expect("idle writer must be reservable");
+    let (conn_id, _rx) = registry.register().await;
+
+    assert_eq!(
+        registry
+            .bind_writer_with_outcome(conn_id, 10, conn_meta())
+            .await,
+        WriterBindOutcome::Bound
+    );
+    let mut registration = registry.prepare_writer_registration().await;
+    assert!(!registration.prepare_replacement_commit(&reservation));
+    drop(registration);
+}
+
+#[tokio::test]
+async fn retiring_replacement_rejects_new_bind() {
+    let registry = ConnRegistry::new();
+    let (writer_tx, _writer_rx) = tokio::sync::mpsc::channel(8);
+    registry
+        .register_writer(10, writer_tx, writer_byte_budget())
+        .await;
+    let mut reservation = registry
+        .try_reserve_writer_replacement(10)
+        .await
+        .expect("idle writer must be reservable");
+    let mut registration = registry.prepare_writer_registration().await;
+    assert!(registration.prepare_replacement_commit(&reservation));
+    drop(registration);
+    reservation.mark_committed();
+    let (conn_id, _rx) = registry.register().await;
+
+    assert_eq!(
+        registry
+            .bind_writer_with_outcome(conn_id, 10, conn_meta())
+            .await,
+        WriterBindOutcome::WriterRetiring
+    );
+}
+
+#[tokio::test]
+async fn dropped_preparing_reservation_restores_writer_admission() {
+    let registry = ConnRegistry::new();
+    let (writer_tx, _writer_rx) = tokio::sync::mpsc::channel(8);
+    registry
+        .register_writer(10, writer_tx, writer_byte_budget())
+        .await;
+    let reservation = registry
+        .try_reserve_writer_replacement(10)
+        .await
+        .expect("idle writer must be reservable");
+    assert!(registry.try_reserve_writer_replacement(10).await.is_none());
+
+    drop(reservation);
+
+    assert!(registry.try_reserve_writer_replacement(10).await.is_some());
+}
+
 #[tokio::test]
 async fn writer_activity_snapshot_tracks_writer_and_dc_load() {
     let registry = ConnRegistry::new();

src/transport/middle_proxy/registry/writer.rs

@@ -12,6 +12,7 @@ use super::{
     BoundConn, ConnMeta, ConnRegistry, ConnWriter, HotConnBinding, RouteResult,
     WriterActivitySnapshot,
 };
+use super::replacement::WriterBindOutcome;
 
 impl ConnRegistry {
     fn set_writer_bound_count(&self, writer_id: u64, count: usize) {
@@ -286,17 +287,30 @@ impl ConnRegistry {
         }
     }
 
-    pub async fn bind_writer(&self, conn_id: u64, writer_id: u64, meta: ConnMeta) -> bool {
+    /// Atomically binds one client route while rejecting retiring writer generations.
+    pub(in crate::transport::middle_proxy) async fn bind_writer_with_outcome(
+        &self,
+        conn_id: u64,
+        writer_id: u64,
+        meta: ConnMeta,
+    ) -> WriterBindOutcome {
         let mut binding = self.binding.inner.lock().await;
         // ROUTING IS THE SOURCE OF TRUTH:
         // never keep/attach writer binding for a connection that is already
         // absent from the routing table.
         if !self.routing.map.contains_key(&conn_id) {
-            return false;
+            return WriterBindOutcome::RouteMissing;
         }
-        if !self.writers.map.contains_key(&writer_id) {
+        let Some(writer_route) = self.writers.map.get(&writer_id) else {
-            return false;
+            return WriterBindOutcome::WriterMissing;
+        };
+        let writer_state = writer_route.replacement_state.load(Ordering::Acquire);
+        if writer_state == super::WriterReplacementState::Retiring as u8
+            || writer_state == super::WriterReplacementState::Draining as u8
+        {
+            return WriterBindOutcome::WriterRetiring;
         }
+        drop(writer_route);
 
         let previous_writer_id = binding.writer_for_conn.insert(conn_id, writer_id);
         if let Some(previous_writer_id) = previous_writer_id
@@ -331,7 +345,15 @@ impl ConnRegistry {
         self.hot_binding
             .map
             .insert(conn_id, HotConnBinding { writer_id, meta });
-        true
+        WriterBindOutcome::Bound
+    }
+
+    pub async fn bind_writer(&self, conn_id: u64, writer_id: u64, meta: ConnMeta) -> bool {
+        matches!(
+            self.bind_writer_with_outcome(conn_id, writer_id, meta)
+                .await,
+            WriterBindOutcome::Bound
+        )
     }
 
     pub async fn get_last_writer_meta(&self, writer_id: u64) -> Option<ConnMeta> {

src/transport/middle_proxy/send.rs

@@ -12,7 +12,7 @@ use tracing::{debug, warn};
 
 use super::MePool;
 use super::codec::{ProxyReqCommand, WriterBytePermit, WriterCommand};
-use super::registry::ConnMeta;
+use super::registry::{ConnMeta, WriterBindOutcome};
 use super::wire::{build_proxy_req_payload, proxy_req_payload_len};
 use crate::config::defaults::ME_WRITER_BYTE_PERMIT_UNIT_BYTES;
 use crate::config::{MeRouteNoWriterMode, MeWriterPickMode};
@@ -648,13 +648,30 @@ impl MePool {
                         // Keep the advertised proxy IP aligned with the selected ME writer source.
                         let effective_our_addr = SocketAddr::new(w.source_ip, our_addr.port());
                         let (payload, meta) = build_routed_payload(effective_our_addr);
-                        if !self.registry.bind_writer(conn_id, w.id, meta).await {
+                        let bind_outcome = self
+                            .registry
+                            .bind_writer_with_outcome(conn_id, w.id, meta)
+                            .await;
+                        if bind_outcome != WriterBindOutcome::Bound {
+                            drop(permit);
+                            if bind_outcome == WriterBindOutcome::WriterRetiring {
+                                debug!(
+                                    conn_id,
+                                    writer_id = w.id,
+                                    "ME writer entered replacement retirement before bind commit"
+                                );
+                                continue;
+                            }
+                            if bind_outcome == WriterBindOutcome::RouteMissing {
+                                return Err(ProxyError::Proxy(
+                                    "ME client route disappeared before writer bind".into(),
+                                ));
+                            }
                             debug!(
                                 conn_id,
                                 writer_id = w.id,
                                 "ME writer disappeared before bind commit, pruning stale writer"
                             );
-                            drop(permit);
                             self.remove_writer_and_close_clients(w.id).await;
                             continue;
                         }
@@ -742,13 +759,30 @@ impl MePool {
             // Keep the advertised proxy IP aligned with the selected ME writer source.
             let effective_our_addr = SocketAddr::new(w.source_ip, our_addr.port());
             let (payload, meta) = build_routed_payload(effective_our_addr);
-            if !self.registry.bind_writer(conn_id, w.id, meta).await {
+            let bind_outcome = self
+                .registry
+                .bind_writer_with_outcome(conn_id, w.id, meta)
+                .await;
+            if bind_outcome != WriterBindOutcome::Bound {
+                drop(permit);
+                if bind_outcome == WriterBindOutcome::WriterRetiring {
+                    debug!(
+                        conn_id,
+                        writer_id = w.id,
+                        "ME writer entered replacement retirement before fallback bind commit"
+                    );
+                    continue;
+                }
+                if bind_outcome == WriterBindOutcome::RouteMissing {
+                    return Err(ProxyError::Proxy(
+                        "ME client route disappeared before writer bind".into(),
+                    ));
+                }
                 debug!(
                     conn_id,
                     writer_id = w.id,
                     "ME writer disappeared before fallback bind commit, pruning stale writer"
                 );
-                drop(permit);
                 self.remove_writer_and_close_clients(w.id).await;
                 continue;
             }

src/transport/middle_proxy/tests/pool_refill_security_tests.rs

@@ -30,7 +30,10 @@ async fn make_pool() -> Arc<MePool> {
         HashMap::new(),
         HashMap::new(),
         None,
-        NetworkDecision::default(),
+        NetworkDecision {
+            ipv4_me: true,
+            ..NetworkDecision::default()
+        },
         None,
         Arc::new(SecureRandom::new()),
         Arc::new(Stats::default()),
@@ -162,12 +165,26 @@ async fn connectable_endpoints_releases_quarantine_lock_before_sleep() {
     assert_eq!(endpoints, vec![addr]);
 }
 
+#[tokio::test]
+async fn refill_does_not_queue_a_removed_dc_target() {
+    let pool = make_pool().await;
+    let addr = SocketAddr::new(IpAddr::V4(Ipv4Addr::new(127, 31, 0, 20)), 443);
+
+    pool.trigger_immediate_refill_for_dc(addr, 2);
+
+    assert!(pool.refill_states.lock().is_empty());
+    assert_eq!(pool.refill_running.load(Ordering::Acquire), 0);
+    assert_eq!(pool.refill_pending.load(Ordering::Acquire), 0);
+}
+
 #[tokio::test(flavor = "current_thread")]
-async fn refill_coalesces_one_pending_endpoint_and_cleans_up_before_first_poll() {
+async fn refill_preserves_bounded_pending_cardinality_and_cleans_up_before_first_poll() {
     let pool = make_pool().await;
     let first = SocketAddr::new(IpAddr::V4(Ipv4Addr::new(127, 31, 0, 21)), 443);
     let second = SocketAddr::new(IpAddr::V4(Ipv4Addr::new(127, 31, 0, 22)), 443);
     let latest = SocketAddr::new(IpAddr::V4(Ipv4Addr::new(127, 31, 0, 23)), 443);
+    pool.preferred_endpoints_by_dc
+        .store(Arc::new(HashMap::from([(2, vec![first, second, latest])])));
 
     pool.trigger_immediate_refill_for_dc(first, 2);
     pool.trigger_immediate_refill_for_dc(second, 2);
@@ -175,7 +192,16 @@ async fn refill_coalesces_one_pending_endpoint_and_cleans_up_before_first_poll()
 
     assert_eq!(pool.refill_states.lock().len(), 1);
     assert_eq!(pool.refill_running.load(Ordering::Acquire), 1);
-    assert_eq!(pool.refill_pending.load(Ordering::Acquire), 1);
+    assert_eq!(pool.refill_pending.load(Ordering::Acquire), 2);
+    let state = pool
+        .refill_states
+        .lock()
+        .values()
+        .copied()
+        .next()
+        .expect("refill state");
+    assert_eq!(state.pending_count, 2);
+    assert_eq!(state.next_addr, Some(latest));
 
     pool.begin_shutdown();
     tokio::time::timeout(Duration::from_secs(1), async {

src/transport/middle_proxy/tests/pool_writer_publication_tests.rs

@@ -7,10 +7,121 @@ use tokio::sync::mpsc;
 use tokio_util::sync::CancellationToken;
 
 use super::codec::WriterCommand;
-use super::pool::{MeWriter, WriterContour};
+use super::pool::{MeWriter, WriterContour, WriterOpenIntent};
 use super::pool_writer_security_tests::make_pool;
 use super::registry::ConnMeta;
 
+fn unregistered_writer(
+    pool: &Arc<super::pool::MePool>,
+    writer_id: u64,
+    addr: SocketAddr,
+    generation: u64,
+    contour: WriterContour,
+) -> MeWriter {
+    let (tx, _rx) = mpsc::channel::<WriterCommand>(8);
+    MeWriter {
+        id: writer_id,
+        addr,
+        source_ip: addr.ip(),
+        writer_dc: 2,
+        generation,
+        contour: Arc::new(AtomicU8::new(contour.as_u8())),
+        created_at: Instant::now(),
+        tx,
+        byte_budget: pool.new_writer_byte_budget(),
+        cancel: CancellationToken::new(),
+        degraded: Arc::new(AtomicBool::new(false)),
+        rtt_ema_ms_x10: Arc::new(AtomicU32::new(0)),
+        draining: Arc::new(AtomicBool::new(false)),
+        draining_started_at_epoch_secs: Arc::new(AtomicU64::new(0)),
+        drain_deadline_epoch_secs: Arc::new(AtomicU64::new(0)),
+        allow_drain_fallback: Arc::new(AtomicBool::new(false)),
+    }
+}
+
+#[tokio::test]
+async fn normal_warm_publication_cannot_race_past_the_dc_floor() {
+    let pool = make_pool().await;
+    let addr = SocketAddr::new(IpAddr::V4(Ipv4Addr::LOCALHOST), 443);
+    pool.preferred_endpoints_by_dc
+        .store(Arc::new(std::collections::HashMap::from([(2, vec![addr])])));
+    let generation = 2;
+    let writers = (1..=3)
+        .map(|writer_id| {
+            unregistered_writer(&pool, writer_id, addr, generation, WriterContour::Warm)
+        })
+        .collect::<Vec<_>>();
+    let candidate = unregistered_writer(&pool, 4, addr, generation, WriterContour::Warm);
+
+    assert!(
+        pool.authorize_writer_publication_capacity(
+            &candidate,
+            WriterContour::Warm,
+            WriterOpenIntent::Normal,
+            &writers[..2],
+        )
+        .is_ok()
+    );
+    assert!(
+        pool.authorize_writer_publication_capacity(
+            &candidate,
+            WriterContour::Warm,
+            WriterOpenIntent::Normal,
+            &writers,
+        )
+        .is_err()
+    );
+    assert!(
+        pool.authorize_writer_publication_capacity(
+            &candidate,
+            WriterContour::Warm,
+            WriterOpenIntent::Replacement,
+            &writers,
+        )
+        .is_ok()
+    );
+}
+
+#[tokio::test]
+async fn normal_active_publication_cannot_race_past_the_family_floor() {
+    let pool = make_pool().await;
+    let addr = SocketAddr::new(IpAddr::V4(Ipv4Addr::LOCALHOST), 443);
+    pool.preferred_endpoints_by_dc
+        .store(Arc::new(std::collections::HashMap::from([(2, vec![addr])])));
+    let generation = pool.current_generation();
+    let writers = (1..=3)
+        .map(|writer_id| {
+            unregistered_writer(
+                &pool,
+                writer_id,
+                addr,
+                generation,
+                WriterContour::Active,
+            )
+        })
+        .collect::<Vec<_>>();
+    let candidate = unregistered_writer(&pool, 4, addr, generation, WriterContour::Active);
+
+    assert!(
+        pool.authorize_writer_publication_capacity(
+            &candidate,
+            WriterContour::Active,
+            WriterOpenIntent::Coverage,
+            &writers[..2],
+        )
+        .is_ok()
+    );
+    assert!(
+        pool.authorize_writer_publication_capacity(
+            &candidate,
+            WriterContour::Active,
+            WriterOpenIntent::Coverage,
+            &writers,
+        )
+        .is_err()
+    );
+}
+
 #[tokio::test]
 async fn successful_writer_publication_is_fully_visible_and_removable() {
     let pool = make_pool().await;

src/transport/middle_proxy/tests/pool_writer_security_tests.rs

@@ -35,7 +35,10 @@ pub(super) async fn make_pool() -> Arc<MePool> {
         HashMap::new(),
         HashMap::new(),
         None,
-        NetworkDecision::default(),
+        NetworkDecision {
+            ipv4_me: true,
+            ..NetworkDecision::default()
+        },
         None,
         Arc::new(SecureRandom::new()),
         Arc::new(Stats::new()),

src/transport/middle_proxy/tests/send_adversarial_tests.rs

@@ -324,6 +324,36 @@ async fn send_proxy_req_does_not_replay_when_first_bind_commit_fails() {
     assert_eq!(bound.expect("writer should be bound").writer_id, 11);
 }
 
+#[tokio::test]
+async fn missing_client_route_does_not_prune_a_healthy_writer() {
+    let (pool, _rng) = make_pool().await;
+    let writer_id = 12;
+    let addr = SocketAddr::new(IpAddr::V4(Ipv4Addr::new(127, 0, 0, 12)), 443);
+    let _writer_rx = insert_writer(&pool, writer_id, 2, addr, true).await;
+
+    let result = pool
+        .send_proxy_req(
+            999_999,
+            2,
+            SocketAddr::new(IpAddr::V4(Ipv4Addr::LOCALHOST), 30005),
+            SocketAddr::new(IpAddr::V4(Ipv4Addr::LOCALHOST), 443),
+            b"cancelled-route",
+            0,
+            None,
+            None,
+        )
+        .await;
+
+    assert!(result.is_err());
+    assert!(
+        pool.writers
+            .read()
+            .await
+            .iter()
+            .any(|writer| writer.id == writer_id)
+    );
+}
+
 #[tokio::test]
 async fn send_proxy_req_prunes_iterative_stale_bind_failures_without_data_replay() {
     let (pool, _rng) = make_pool().await;