Arc-swap for ME Writer Snapshots

Co-Authored-By: brekotis <93345790+brekotis@users.noreply.github.com>

src/transport/middle_proxy/health.rs

@@ -189,14 +189,14 @@ pub(super) async fn reap_draining_writers(
     let drain_threshold = pool
         .me_pool_drain_threshold
         .load(std::sync::atomic::Ordering::Relaxed);
-    let writers = pool.writers.read().await.clone();
+    let writers = pool.writers.load_full();
     let activity = pool.registry.writer_activity_snapshot().await;
     let mut draining_writers = Vec::new();
     let mut empty_writer_ids = Vec::<u64>::new();
     let mut timeout_expired_writer_ids = Vec::<u64>::new();
     let mut force_close_writer_ids = Vec::<u64>::new();
-    for writer in writers {
-        if !writer.draining.load(std::sync::atomic::Ordering::Relaxed) {
+    for writer in writers.iter().cloned() {
+        if !writer.draining.load(std::sync::atomic::Ordering::Acquire) {
             continue;
         }
         if draining_writer_timeout_expired(pool, &writer, now_epoch_secs, drain_ttl_secs) {
@@ -497,12 +497,13 @@ 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();
-    for writer in pool.writers.read().await.iter().filter(|w| {
-        !w.draining.load(std::sync::atomic::Ordering::Relaxed)
+    let writers_snapshot = pool.writers.load_full();
+    for writer in writers_snapshot.iter().filter(|w| {
+        !w.draining.load(std::sync::atomic::Ordering::Acquire)
     }) {
         if !matches!(
             super::pool::WriterContour::from_u8(
-                writer.contour.load(std::sync::atomic::Ordering::Relaxed),
+                writer.contour.load(std::sync::atomic::Ordering::Acquire),
             ),
             super::pool::WriterContour::Active
         ) {
@@ -1566,20 +1567,20 @@ async fn maybe_rotate_single_endpoint_shadow(
             let now_epoch_secs = MePool::now_epoch_secs();
     
             // Collect zombie IDs under a short read-lock.
-            let zombie_ids: Vec<u64> = {
-                let ws = pool.writers.read().await;
-                ws.iter()
-                    .filter(|w| w.draining.load(std::sync::atomic::Ordering::Relaxed))
+            let zombie_ids: Vec<u64> = pool
+                .writers
+                .load_full()
+                .iter()
+                .filter(|w| w.draining.load(std::sync::atomic::Ordering::Acquire))
                 .filter(|w| {
                     let deadline = w
                         .drain_deadline_epoch_secs
-                            .load(std::sync::atomic::Ordering::Relaxed);
+                        .load(std::sync::atomic::Ordering::Acquire);
                     deadline != 0
                         && now_epoch_secs.saturating_sub(deadline) > ZOMBIE_THRESHOLD_SECS
                 })
                 .map(|w| w.id)
-                    .collect()
-            };
+                .collect();
     
             if zombie_ids.is_empty() {
                 continue;
@@ -1737,7 +1738,9 @@ mod tests {
             drain_deadline_epoch_secs: Arc::new(AtomicU64::new(0)),
             allow_drain_fallback: Arc::new(AtomicBool::new(false)),
         };
-        pool.writers.write().await.push(writer);
+        let mut writers = (*pool.writers.load_full()).clone();
+        writers.push(writer);
+        pool.writers.store(Arc::new(writers));
         pool.registry.register_writer(writer_id, tx).await;
         pool.conn_count.fetch_add(1, Ordering::Relaxed);
         assert!(
@@ -1769,7 +1772,7 @@ mod tests {
 
         reap_draining_writers(&pool, &mut warn_next_allowed, &mut soft_evict_next_allowed).await;
 
-        let writer_ids: Vec<u64> = pool.writers.read().await.iter().map(|writer| writer.id).collect();
+        let writer_ids: Vec<u64> = pool.writers.load_full().iter().map(|writer| writer.id).collect();
         assert_eq!(writer_ids, vec![20, 30]);
         assert!(pool.registry.get_writer(conn_a).await.is_none());
         assert_eq!(pool.registry.get_writer(conn_b).await.unwrap().writer_id, 20);
@@ -1788,7 +1791,7 @@ mod tests {
 
         reap_draining_writers(&pool, &mut warn_next_allowed, &mut soft_evict_next_allowed).await;
 
-        let writer_ids: Vec<u64> = pool.writers.read().await.iter().map(|writer| writer.id).collect();
+        let writer_ids: Vec<u64> = pool.writers.load_full().iter().map(|writer| writer.id).collect();
         assert_eq!(writer_ids, vec![10, 20, 30]);
         assert_eq!(pool.registry.get_writer(conn_a).await.unwrap().writer_id, 10);
         assert_eq!(pool.registry.get_writer(conn_b).await.unwrap().writer_id, 20);

src/transport/middle_proxy/health_adversarial_tests.rs

@@ -147,7 +147,9 @@ async fn insert_draining_writer(
         allow_drain_fallback: Arc::new(AtomicBool::new(false)),
     };
 
-    pool.writers.write().await.push(writer);
+    let mut writers = (*pool.writers.load_full()).clone();
+    writers.push(writer);
+    pool.writers.store(Arc::new(writers));
     pool.registry.register_writer(writer_id, tx).await;
     pool.conn_count.fetch_add(1, Ordering::Relaxed);
 
@@ -174,7 +176,7 @@ async fn insert_draining_writer(
 }
 
 async fn writer_count(pool: &Arc<MePool>) -> usize {
-    pool.writers.read().await.len()
+    pool.writers.load_full().len()
 }
 
 async fn sorted_writer_ids(pool: &Arc<MePool>) -> Vec<u64> {

src/transport/middle_proxy/health_integration_tests.rs

@@ -144,7 +144,9 @@ async fn insert_draining_writer(
         drain_deadline_epoch_secs: Arc::new(AtomicU64::new(drain_deadline_epoch_secs)),
         allow_drain_fallback: Arc::new(AtomicBool::new(false)),
     };
-    pool.writers.write().await.push(writer);
+    let mut writers = (*pool.writers.load_full()).clone();
+    writers.push(writer);
+    pool.writers.store(Arc::new(writers));
     pool.registry.register_writer(writer_id, tx).await;
     pool.conn_count.fetch_add(1, Ordering::Relaxed);
     for idx in 0..bound_clients {
@@ -190,7 +192,7 @@ async fn me_health_monitor_drains_expired_backlog_over_multiple_cycles() {
     monitor.abort();
     let _ = monitor.await;
 
-    assert!(pool.writers.read().await.is_empty());
+    assert!(pool.writers.load_full().is_empty());
 }
 
 #[tokio::test]
@@ -206,7 +208,7 @@ async fn me_health_monitor_cleans_empty_draining_writers_without_force_close() {
     monitor.abort();
     let _ = monitor.await;
 
-    assert!(pool.writers.read().await.is_empty());
+    assert!(pool.writers.load_full().is_empty());
 }
 
 #[tokio::test]
@@ -231,5 +233,5 @@ async fn me_health_monitor_converges_retry_like_threshold_backlog_to_empty() {
     monitor.abort();
     let _ = monitor.await;
 
-    assert!(pool.writers.read().await.is_empty());
+    assert!(pool.writers.load_full().is_empty());
 }

src/transport/middle_proxy/health_regression_tests.rs

@@ -138,7 +138,9 @@ async fn insert_draining_writer(
         drain_deadline_epoch_secs: Arc::new(AtomicU64::new(drain_deadline_epoch_secs)),
         allow_drain_fallback: Arc::new(AtomicBool::new(false)),
     };
-    pool.writers.write().await.push(writer);
+    let mut writers = (*pool.writers.load_full()).clone();
+    writers.push(writer);
+    pool.writers.store(Arc::new(writers));
     pool.registry.register_writer(writer_id, tx).await;
     pool.conn_count.fetch_add(1, Ordering::Relaxed);
     for idx in 0..bound_clients {
@@ -256,7 +258,9 @@ async fn reap_draining_writers_does_not_block_on_stuck_writer_close_signal() {
         drain_deadline_epoch_secs: Arc::new(AtomicU64::new(0)),
         allow_drain_fallback: Arc::new(AtomicBool::new(false)),
     };
-    pool.writers.write().await.push(blocked_writer);
+    let mut writers = (*pool.writers.load_full()).clone();
+    writers.push(blocked_writer);
+    pool.writers.store(Arc::new(writers));
     pool.registry
         .register_writer(blocked_writer_id, blocked_tx)
         .await;
@@ -357,7 +361,7 @@ async fn reap_draining_writers_limits_closes_per_health_tick() {
 
     reap_draining_writers(&pool, &mut warn_next_allowed, &mut soft_evict_next_allowed).await;
 
-    assert_eq!(pool.writers.read().await.len(), writer_total - close_budget);
+    assert_eq!(pool.writers.load_full().len(), writer_total - close_budget);
 }
 
 #[tokio::test]
@@ -380,13 +384,13 @@ async fn reap_draining_writers_backlog_drains_across_ticks() {
     let mut soft_evict_next_allowed = HashMap::new();
 
     for _ in 0..8 {
-        if pool.writers.read().await.is_empty() {
+        if pool.writers.load_full().is_empty() {
             break;
         }
         reap_draining_writers(&pool, &mut warn_next_allowed, &mut soft_evict_next_allowed).await;
     }
 
-    assert!(pool.writers.read().await.is_empty());
+    assert!(pool.writers.load_full().is_empty());
 }
 
 #[tokio::test]
@@ -411,12 +415,12 @@ async fn reap_draining_writers_threshold_backlog_converges_to_threshold() {
 
     for _ in 0..16 {
         reap_draining_writers(&pool, &mut warn_next_allowed, &mut soft_evict_next_allowed).await;
-        if pool.writers.read().await.len() <= threshold as usize {
+        if pool.writers.load_full().len() <= threshold as usize {
             break;
         }
     }
 
-    assert_eq!(pool.writers.read().await.len(), threshold as usize);
+    assert_eq!(pool.writers.load_full().len(), threshold as usize);
 }
 
 #[tokio::test]
@@ -521,14 +525,14 @@ async fn reap_draining_writers_warn_state_never_exceeds_live_draining_population
             .await;
         }
         reap_draining_writers(&pool, &mut warn_next_allowed, &mut soft_evict_next_allowed).await;
-        assert!(warn_next_allowed.len() <= pool.writers.read().await.len());
+        assert!(warn_next_allowed.len() <= pool.writers.load_full().len());
 
         let existing_writer_ids = current_writer_ids(&pool).await;
         for writer_id in existing_writer_ids.into_iter().take(4) {
             let _ = pool.remove_writer_and_close_clients(writer_id).await;
         }
         reap_draining_writers(&pool, &mut warn_next_allowed, &mut soft_evict_next_allowed).await;
-        assert!(warn_next_allowed.len() <= pool.writers.read().await.len());
+        assert!(warn_next_allowed.len() <= pool.writers.load_full().len());
     }
 }
 
@@ -558,13 +562,13 @@ async fn reap_draining_writers_mixed_backlog_converges_without_leaking_warn_stat
 
     for _ in 0..16 {
         reap_draining_writers(&pool, &mut warn_next_allowed, &mut soft_evict_next_allowed).await;
-        if pool.writers.read().await.len() <= 6 {
+        if pool.writers.load_full().len() <= 6 {
             break;
         }
     }
 
-    assert!(pool.writers.read().await.len() <= 6);
-    assert!(warn_next_allowed.len() <= pool.writers.read().await.len());
+    assert!(pool.writers.load_full().len() <= 6);
+    assert!(warn_next_allowed.len() <= pool.writers.load_full().len());
 }
 
 #[tokio::test]

src/transport/middle_proxy/pool.rs

@@ -5,6 +5,7 @@ use std::sync::atomic::{AtomicBool, AtomicI32, AtomicU8, AtomicU32, AtomicU64, A
 use std::time::{Duration, Instant, SystemTime, UNIX_EPOCH};
 
 use tokio::sync::{Mutex, Notify, RwLock, mpsc};
+use arc_swap::ArcSwap;
 use tokio_util::sync::CancellationToken;
 
 use crate::config::{
@@ -82,7 +83,7 @@ pub struct SecretSnapshot {
 #[allow(dead_code)]
 pub struct MePool {
     pub(super) registry: Arc<ConnRegistry>,
-    pub(super) writers: Arc<RwLock<Vec<MeWriter>>>,
+    pub(super) writers: Arc<ArcSwap<Arc<Vec<MeWriter>>>>,
     pub(super) rr: AtomicU64,
     pub(super) decision: NetworkDecision,
     pub(super) upstream: Option<Arc<UpstreamManager>>,
@@ -329,7 +330,7 @@ impl MePool {
         );
         Arc::new(Self {
             registry,
-            writers: Arc::new(RwLock::new(Vec::new())),
+            writers: Arc::new(ArcSwap::from_pointee(Vec::new())),
             rr: AtomicU64::new(0),
             decision,
             upstream,
@@ -512,7 +513,7 @@ impl MePool {
     }
 
     pub fn current_generation(&self) -> u64 {
-        self.active_generation.load(Ordering::Relaxed)
+        self.active_generation.load(Ordering::Acquire)
     }
 
     pub fn set_runtime_ready(&self, ready: bool) {
@@ -728,7 +729,7 @@ impl MePool {
         MeSocksKdfPolicy::from_u8(self.me_socks_kdf_policy.load(Ordering::Relaxed))
     }
 
-    pub(super) fn writers_arc(&self) -> Arc<RwLock<Vec<MeWriter>>> {
+    pub(super) fn writers_arc(&self) -> Arc<ArcSwap<Arc<Vec<MeWriter>>>> {
         self.writers.clone()
     }
 
@@ -776,14 +777,14 @@ impl MePool {
     }
 
     pub(super) async fn non_draining_writer_counts_by_contour(&self) -> (usize, usize, usize) {
-        let ws = self.writers.read().await;
+        let ws = self.writers.load_full();
         let mut active = 0usize;
         let mut warm = 0usize;
         for writer in ws.iter() {
-            if writer.draining.load(Ordering::Relaxed) {
+            if writer.draining.load(Ordering::Acquire) {
                 continue;
             }
-            match WriterContour::from_u8(writer.contour.load(Ordering::Relaxed)) {
+            match WriterContour::from_u8(writer.contour.load(Ordering::Acquire)) {
                 WriterContour::Active => active = active.saturating_add(1),
                 WriterContour::Warm => warm = warm.saturating_add(1),
                 WriterContour::Draining => {}

src/transport/middle_proxy/pool_config.rs

@@ -108,8 +108,8 @@ impl MePool {
     }
 
     pub async fn reconnect_all(self: &Arc<Self>) {
-        let ws = self.writers.read().await.clone();
-        for w in ws {
+        let ws = self.writers.load_full();
+        for w in ws.iter().cloned() {
             if let Ok(()) = self
                 .connect_one_for_dc(w.addr, w.writer_dc, self.rng.as_ref())
                 .await

src/transport/middle_proxy/pool_init.rs

@@ -132,7 +132,7 @@ impl MePool {
             }
         }
 
-        if self.writers.read().await.is_empty() {
+        if self.writers.load_full().is_empty() {
             return Err(ProxyError::Proxy("No ME connections".into()));
         }
         info!(

src/transport/middle_proxy/pool_refill.rs

@@ -119,9 +119,9 @@ impl MePool {
         }
         if candidates.len() > 1 {
             let mut active_by_endpoint = HashMap::<SocketAddr, usize>::new();
-            let ws = self.writers.read().await;
+            let ws = self.writers.load_full();
             for writer in ws.iter() {
-                if writer.draining.load(Ordering::Relaxed) {
+                if writer.draining.load(Ordering::Acquire) {
                     continue;
                 }
                 if writer.writer_dc != dc {
@@ -129,7 +129,7 @@ impl MePool {
                 }
                 if !matches!(
                     super::pool::WriterContour::from_u8(
-                        writer.contour.load(Ordering::Relaxed),
+                        writer.contour.load(Ordering::Acquire),
                     ),
                     super::pool::WriterContour::Active
                 ) {
@@ -139,7 +139,6 @@ impl MePool {
                     *active_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));
         }
         let start = (self.rr.fetch_add(1, Ordering::Relaxed) as usize) % candidates.len();

src/transport/middle_proxy/pool_reinit.rs

@@ -36,26 +36,26 @@ impl MePool {
     }
 
     fn clear_pending_hardswap_state(&self) {
-        self.pending_hardswap_generation.store(0, Ordering::Relaxed);
+        self.pending_hardswap_generation.store(0, Ordering::Release);
         self.pending_hardswap_started_at_epoch_secs
-            .store(0, Ordering::Relaxed);
-        self.pending_hardswap_map_hash.store(0, Ordering::Relaxed);
-        self.warm_generation.store(0, Ordering::Relaxed);
+            .store(0, Ordering::Release);
+        self.pending_hardswap_map_hash.store(0, Ordering::Release);
+        self.warm_generation.store(0, Ordering::Release);
     }
 
     async fn promote_warm_generation_to_active(&self, generation: u64) {
-        self.active_generation.store(generation, Ordering::Relaxed);
-        self.warm_generation.store(0, Ordering::Relaxed);
+        self.active_generation.store(generation, Ordering::Release);
+        self.warm_generation.store(0, Ordering::Release);
 
-        let ws = self.writers.read().await;
+        let ws = self.writers.load_full();
         for writer in ws.iter() {
-            if writer.draining.load(Ordering::Relaxed) {
+            if writer.draining.load(Ordering::Acquire) {
                 continue;
             }
             if writer.generation == generation {
                 writer
                     .contour
-                    .store(WriterContour::Active.as_u8(), Ordering::Relaxed);
+                    .store(WriterContour::Active.as_u8(), Ordering::Release);
             }
         }
     }
@@ -177,9 +177,9 @@ impl MePool {
         dc: i32,
         endpoints: &HashSet<SocketAddr>,
     ) -> usize {
-        let ws = self.writers.read().await;
+        let ws = self.writers.load_full();
         ws.iter()
-            .filter(|w| !w.draining.load(Ordering::Relaxed))
+            .filter(|w| !w.draining.load(Ordering::Acquire))
             .filter(|w| w.generation == generation)
             .filter(|w| w.writer_dc == dc)
             .filter(|w| endpoints.contains(&w.addr))
@@ -191,9 +191,9 @@ impl MePool {
         dc: i32,
         endpoints: &HashSet<SocketAddr>,
     ) -> usize {
-        let ws = self.writers.read().await;
+        let ws = self.writers.load_full();
         ws.iter()
-            .filter(|w| !w.draining.load(Ordering::Relaxed))
+            .filter(|w| !w.draining.load(Ordering::Acquire))
             .filter(|w| w.writer_dc == dc)
             .filter(|w| endpoints.contains(&w.addr))
             .count()
@@ -358,12 +358,12 @@ impl MePool {
                 }
                 let next_generation = self.generation.fetch_add(1, Ordering::Relaxed) + 1;
                 self.pending_hardswap_generation
-                    .store(next_generation, Ordering::Relaxed);
+                    .store(next_generation, Ordering::Release);
                 self.pending_hardswap_started_at_epoch_secs
-                    .store(now_epoch_secs, Ordering::Relaxed);
+                    .store(now_epoch_secs, Ordering::Release);
                 self.pending_hardswap_map_hash
-                    .store(desired_map_hash, Ordering::Relaxed);
-                self.warm_generation.store(next_generation, Ordering::Relaxed);
+                    .store(desired_map_hash, Ordering::Release);
+                self.warm_generation.store(next_generation, Ordering::Release);
                 next_generation
             }
         } else {
@@ -372,17 +372,17 @@ impl MePool {
         };
 
         if hardswap {
-            self.warm_generation.store(generation, Ordering::Relaxed);
+            self.warm_generation.store(generation, Ordering::Release);
             self.warmup_generation_for_all_dcs(rng, generation, &desired_by_dc)
                 .await;
         } else {
             self.reconcile_connections(rng).await;
         }
 
-        let writers = self.writers.read().await;
+        let writers = self.writers.load_full();
         let active_writer_addrs: HashSet<(i32, SocketAddr)> = writers
             .iter()
-            .filter(|w| !w.draining.load(Ordering::Relaxed))
+            .filter(|w| !w.draining.load(Ordering::Acquire))
             .map(|w| (w.writer_dc, w.addr))
             .collect();
         let min_ratio = Self::permille_to_ratio(
@@ -405,7 +405,7 @@ impl MePool {
         if hardswap {
             let fresh_writer_addrs: HashSet<(i32, SocketAddr)> = writers
                 .iter()
-                .filter(|w| !w.draining.load(Ordering::Relaxed))
+                .filter(|w| !w.draining.load(Ordering::Acquire))
                 .filter(|w| w.generation == generation)
                 .map(|w| (w.writer_dc, w.addr))
                 .collect();
@@ -441,7 +441,7 @@ impl MePool {
 
         let stale_writer_ids: Vec<u64> = writers
             .iter()
-            .filter(|w| !w.draining.load(Ordering::Relaxed))
+            .filter(|w| !w.draining.load(Ordering::Acquire))
             .filter(|w| {
                 if hardswap {
                     w.generation < generation

src/transport/middle_proxy/pool_status.rs

@@ -166,10 +166,10 @@ impl MePool {
             return false;
         }
 
-        let writers = self.writers.read().await.clone();
+        let writers = self.writers.load_full();
         let mut live_writers_by_dc = HashMap::<i16, usize>::new();
-        for writer in writers {
-            if writer.draining.load(Ordering::Relaxed) {
+        for writer in writers.iter() {
+            if writer.draining.load(Ordering::Acquire) {
                 continue;
             }
             if let Ok(dc) = i16::try_from(writer.writer_dc) {
@@ -203,10 +203,10 @@ impl MePool {
             return false;
         }
 
-        let writers = self.writers.read().await.clone();
+        let writers = self.writers.load_full();
         let mut live_writers_by_dc = HashMap::<i16, usize>::new();
-        for writer in writers {
-            if writer.draining.load(Ordering::Relaxed) {
+        for writer in writers.iter() {
+            if writer.draining.load(Ordering::Acquire) {
                 continue;
             }
             if let Ok(dc) = i16::try_from(writer.writer_dc) {
@@ -255,7 +255,7 @@ impl MePool {
         let idle_since = self.registry.writer_idle_since_snapshot().await;
         let activity = self.registry.writer_activity_snapshot().await;
         let rtt = self.rtt_stats.lock().await.clone();
-        let writers = self.writers.read().await.clone();
+        let writers = self.writers.load_full();
 
         let mut live_writers_by_dc_endpoint = HashMap::<(i16, SocketAddr), usize>::new();
         let mut live_writers_by_dc = HashMap::<i16, usize>::new();
@@ -263,10 +263,10 @@ impl MePool {
         let mut dc_rtt_agg = HashMap::<i16, (f64, u64)>::new();
         let mut writer_rows = Vec::<MeApiWriterStatusSnapshot>::with_capacity(writers.len());
 
-        for writer in writers {
+        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
@@ -296,7 +296,7 @@ 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 state = match WriterContour::from_u8(writer.contour.load(Ordering::Acquire)) {
                 WriterContour::Warm => "warm",
                 WriterContour::Active => "active",
                 WriterContour::Draining => "draining",
@@ -501,7 +501,7 @@ impl MePool {
         }
 
         MeApiRuntimeSnapshot {
-            active_generation: self.active_generation.load(Ordering::Relaxed),
+            active_generation: self.active_generation.load(Ordering::Acquire),
             warm_generation: self.warm_generation.load(Ordering::Relaxed),
             pending_hardswap_generation: self.pending_hardswap_generation.load(Ordering::Relaxed),
             pending_hardswap_age_secs,

src/transport/middle_proxy/pool_writer.rs

@@ -34,10 +34,13 @@ fn is_me_peer_closed_error(error: &ProxyError) -> bool {
 
 impl MePool {
     pub(crate) async fn prune_closed_writers(self: &Arc<Self>) {
-        let closed_writer_ids: Vec<u64> = {
-            let ws = self.writers.read().await;
-            ws.iter().filter(|w| w.tx.is_closed()).map(|w| w.id).collect()
-        };
+        let closed_writer_ids: Vec<u64> = self
+            .writers
+            .load_full()
+            .iter()
+            .filter(|w| w.tx.is_closed())
+            .map(|w| w.id)
+            .collect();
         if closed_writer_ids.is_empty() {
             return;
         }
@@ -178,7 +181,9 @@ impl MePool {
             drain_deadline_epoch_secs: drain_deadline_epoch_secs.clone(),
             allow_drain_fallback: allow_drain_fallback.clone(),
         };
-        self.writers.write().await.push(writer.clone());
+        let mut new_writers = (*self.writers.load_full()).clone();
+        new_writers.push(writer.clone());
+        self.writers.store(Arc::new(new_writers));
         self.registry.register_writer(writer_id, tx.clone()).await;
         self.registry.mark_writer_idle(writer_id).await;
         self.conn_count.fetch_add(1, Ordering::Relaxed);
@@ -254,8 +259,9 @@ impl MePool {
                     warn!(error = %e, "ME reader ended");
                 }
             }
-            let mut ws = writers_arc.write().await;
+            let mut ws = (*writers_arc.load_full()).clone();
             ws.retain(|w| w.id != writer_id);
+            writers_arc.store(Arc::new(ws.clone()));
             info!(remaining = ws.len(), "Dead ME writer removed from pool");
         });
 
@@ -503,11 +509,16 @@ impl MePool {
         let mut removed_dc: Option<i32> = None;
         let mut removed_uptime: Option<Duration> = None;
         let mut trigger_refill = false;
+        if let Some(pos) = self
+            .writers
+            .load_full()
+            .iter()
+            .position(|w| w.id == writer_id)
         {
-            let mut ws = self.writers.write().await;
-            if let Some(pos) = ws.iter().position(|w| w.id == writer_id) {
+            let mut ws = (*self.writers.load_full()).clone();
             let w = ws.remove(pos);
-                let was_draining = w.draining.load(Ordering::Relaxed);
+            self.writers.store(Arc::new(ws));
+            let was_draining = w.draining.load(Ordering::Acquire);
             if was_draining {
                 self.stats.decrement_pool_drain_active();
             }
@@ -523,7 +534,6 @@ impl MePool {
             close_tx = Some(w.tx.clone());
             self.conn_count.fetch_sub(1, Ordering::Relaxed);
         }
-        }
         // State invariant:
         // - writer is removed from `self.writers` (pool visibility),
         // - writer is removed from registry routing/binding maps via `writer_lost`.
@@ -576,25 +586,27 @@ impl MePool {
     ) {
         let timeout = timeout.filter(|d| !d.is_zero());
         let found = {
-            let mut ws = self.writers.write().await;
+            let current = self.writers.load_full();
+            let mut ws = (*current).clone();
             if let Some(w) = ws.iter_mut().find(|w| w.id == writer_id) {
-                let already_draining = w.draining.swap(true, Ordering::Relaxed);
+                let already_draining = w.draining.swap(true, Ordering::Acquire);
                 w.allow_drain_fallback
-                    .store(allow_drain_fallback, Ordering::Relaxed);
+                    .store(allow_drain_fallback, Ordering::Release);
                 let now_epoch_secs = Self::now_epoch_secs();
                 w.draining_started_at_epoch_secs
-                    .store(now_epoch_secs, Ordering::Relaxed);
+                    .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);
                 w.drain_deadline_epoch_secs
-                    .store(drain_deadline_epoch_secs, Ordering::Relaxed);
+                    .store(drain_deadline_epoch_secs, Ordering::Release);
                 if !already_draining {
                     self.stats.increment_pool_drain_active();
                 }
                 w.contour
-                    .store(WriterContour::Draining.as_u8(), Ordering::Relaxed);
-                w.draining.store(true, Ordering::Relaxed);
+                    .store(WriterContour::Draining.as_u8(), Ordering::Release);
+                w.draining.store(true, Ordering::Release);
+                self.writers.store(Arc::new(ws));
                 true
             } else {
                 false
@@ -620,10 +632,10 @@ impl MePool {
     }
 
     pub(super) fn writer_accepts_new_binding(&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) {
+        if !writer.allow_drain_fallback.load(Ordering::Acquire) {
             return false;
         }
 

src/transport/middle_proxy/send.rs

@@ -158,9 +158,8 @@ impl MePool {
             }
 
             let mut writers_snapshot = {
-                let ws = self.writers.read().await;
+                let ws = self.writers.load_full();
                 if ws.is_empty() {
-                    drop(ws);
                     match no_writer_mode {
                         MeRouteNoWriterMode::AsyncRecoveryFailfast => {
                             let deadline = *no_writer_deadline.get_or_insert_with(|| {
@@ -200,19 +199,19 @@ impl MePool {
                                             }
                                         }
                                     }
-                                    if !self.writers.read().await.is_empty() {
+                                    if !self.writers.load_full().is_empty() {
                                         break;
                                     }
                                 }
                             }
 
-                            if !self.writers.read().await.is_empty() {
+                            if !self.writers.load_full().is_empty() {
                                 continue;
                             }
                             let deadline = *no_writer_deadline
                                 .get_or_insert_with(|| Instant::now() + self.me_route_inline_recovery_wait);
                             if !self.wait_for_writer_until(deadline).await {
-                                if !self.writers.read().await.is_empty() {
+                                if !self.writers.load_full().is_empty() {
                                     continue;
                                 }
                                 self.stats.increment_me_no_writer_failfast_total();
@@ -241,15 +240,15 @@ impl MePool {
                         }
                     }
                 }
-                ws.clone()
+                ws
             };
 
             let mut candidate_indices = self
-                .candidate_indices_for_dc(&writers_snapshot, routed_dc, false)
+                .candidate_indices_for_dc(writers_snapshot.as_ref(), routed_dc, false)
                 .await;
             if candidate_indices.is_empty() {
                 candidate_indices = self
-                    .candidate_indices_for_dc(&writers_snapshot, routed_dc, true)
+                    .candidate_indices_for_dc(writers_snapshot.as_ref(), routed_dc, true)
                     .await;
             }
             if let Some(skip_writer_id) = skip_writer_id {
@@ -304,15 +303,13 @@ impl MePool {
                             }
                         }
                         tokio::time::sleep(Duration::from_millis(100 * emergency_attempts as u64)).await;
-                        let ws2 = self.writers.read().await;
-                        writers_snapshot = ws2.clone();
-                        drop(ws2);
+                        writers_snapshot = self.writers.load_full();
                         candidate_indices = self
-                            .candidate_indices_for_dc(&writers_snapshot, routed_dc, false)
+                            .candidate_indices_for_dc(writers_snapshot.as_ref(), routed_dc, false)
                             .await;
                         if candidate_indices.is_empty() {
                             candidate_indices = self
-                                .candidate_indices_for_dc(&writers_snapshot, routed_dc, true)
+                                .candidate_indices_for_dc(writers_snapshot.as_ref(), routed_dc, true)
                                 .await;
                         }
                         if candidate_indices.is_empty() {
@@ -354,7 +351,7 @@ impl MePool {
             let ordered_candidate_indices = if pick_mode == MeWriterPickMode::P2c {
                 self.p2c_ordered_candidate_indices(
                     &candidate_indices,
-                    &writers_snapshot,
+                    writers_snapshot.as_ref(),
                     &writer_idle_since,
                     now_epoch_secs,
                     start,
@@ -427,10 +424,7 @@ impl MePool {
                 }
                 let effective_our_addr = SocketAddr::new(w.source_ip, our_addr.port());
                 let (payload, meta) = build_routed_payload(effective_our_addr);
-                match w.tx.try_send(WriterCommand::Data(payload.clone())) {
-                    Ok(()) => {
-                        self.stats.increment_me_writer_pick_success_try_total(pick_mode);
-                        if !self.registry.bind_writer(conn_id, w.id, meta).await {
+                if !self.registry.bind_writer(conn_id, w.id, meta.clone()).await {
                     debug!(
                         conn_id,
                         writer_id = w.id,
@@ -438,6 +432,9 @@ impl MePool {
                     );
                     continue;
                 }
+                match w.tx.try_send(WriterCommand::Data(payload.clone())) {
+                    Ok(()) => {
+                        self.stats.increment_me_writer_pick_success_try_total(pick_mode);
                         if w.generation < self.current_generation() {
                             self.stats.increment_pool_stale_pick_total();
                             debug!(
@@ -451,11 +448,19 @@ impl MePool {
                         return Ok(());
                     }
                     Err(TrySendError::Full(_)) => {
+                        let _ = self
+                            .registry
+                            .evict_bound_conn_if_writer(conn_id, w.id)
+                            .await;
                         if fallback_blocking_idx.is_none() {
                             fallback_blocking_idx = Some(idx);
                         }
                     }
                     Err(TrySendError::Closed(_)) => {
+                        let _ = self
+                            .registry
+                            .evict_bound_conn_if_writer(conn_id, w.id)
+                            .await;
                         self.stats.increment_me_writer_pick_closed_total(pick_mode);
                         warn!(writer_id = w.id, "ME writer channel closed");
                         self.remove_writer_and_close_clients(w.id).await;
@@ -477,6 +482,14 @@ impl MePool {
             self.stats.increment_me_writer_pick_blocking_fallback_total();
             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.clone()).await {
+                debug!(
+                    conn_id,
+                    writer_id = w.id,
+                    "ME writer disappeared before fallback bind commit, retrying"
+                );
+                continue;
+            }
             match send_writer_command_with_timeout(
                 &w.tx,
                 WriterCommand::Data(payload.clone()),
@@ -487,25 +500,25 @@ impl MePool {
                 Ok(()) => {
                     self.stats
                         .increment_me_writer_pick_success_fallback_total(pick_mode);
-                    if !self.registry.bind_writer(conn_id, w.id, meta).await {
-                        debug!(
-                            conn_id,
-                            writer_id = w.id,
-                            "ME writer disappeared before fallback bind commit, retrying"
-                        );
-                        continue;
-                    }
                     if w.generation < self.current_generation() {
                         self.stats.increment_pool_stale_pick_total();
                     }
                     return Ok(());
                 }
                 Err(TimedSendError::Closed(_)) => {
+                    let _ = self
+                        .registry
+                        .evict_bound_conn_if_writer(conn_id, w.id)
+                        .await;
                     self.stats.increment_me_writer_pick_closed_total(pick_mode);
                     warn!(writer_id = w.id, "ME writer channel closed (blocking)");
                     self.remove_writer_and_close_clients(w.id).await;
                 }
                 Err(TimedSendError::Timeout(_)) => {
+                    let _ = self
+                        .registry
+                        .evict_bound_conn_if_writer(conn_id, w.id)
+                        .await;
                     self.stats.increment_me_writer_pick_full_total(pick_mode);
                     debug!(
                         conn_id,
@@ -520,18 +533,18 @@ impl MePool {
 
     async fn wait_for_writer_until(&self, deadline: Instant) -> bool {
         let waiter = self.writer_available.notified();
-        if !self.writers.read().await.is_empty() {
+        if !self.writers.load_full().is_empty() {
             return true;
         }
         let now = Instant::now();
         if now >= deadline {
-            return !self.writers.read().await.is_empty();
+            return !self.writers.load_full().is_empty();
         }
         let timeout = deadline.saturating_duration_since(now);
         if tokio::time::timeout(timeout, waiter).await.is_ok() {
             return true;
         }
-        !self.writers.read().await.is_empty()
+        !self.writers.load_full().is_empty()
     }
 
     async fn wait_for_candidate_until(&self, routed_dc: i32, deadline: Instant) -> bool {
@@ -560,19 +573,16 @@ impl MePool {
     }
 
     async fn has_candidate_for_target_dc(&self, routed_dc: i32) -> bool {
-        let writers_snapshot = {
-            let ws = self.writers.read().await;
-            if ws.is_empty() {
+        let writers_snapshot = self.writers.load_full();
+        if writers_snapshot.is_empty() {
             return false;
         }
-            ws.clone()
-        };
         let mut candidate_indices = self
-            .candidate_indices_for_dc(&writers_snapshot, routed_dc, false)
+            .candidate_indices_for_dc(writers_snapshot.as_ref(), routed_dc, false)
             .await;
         if candidate_indices.is_empty() {
             candidate_indices = self
-                .candidate_indices_for_dc(&writers_snapshot, routed_dc, true)
+                .candidate_indices_for_dc(writers_snapshot.as_ref(), routed_dc, true)
                 .await;
         }
         !candidate_indices.is_empty()
@@ -736,7 +746,7 @@ impl MePool {
             return false;
         }
 
-        match WriterContour::from_u8(writer.contour.load(Ordering::Relaxed)) {
+        match WriterContour::from_u8(writer.contour.load(Ordering::Acquire)) {
             WriterContour::Active => true,
             WriterContour::Warm => include_warm,
             WriterContour::Draining => true,
@@ -744,7 +754,7 @@ impl MePool {
     }
 
     fn writer_contour_rank_for_selection(&self, writer: &super::pool::MeWriter) -> usize {
-        match WriterContour::from_u8(writer.contour.load(Ordering::Relaxed)) {
+        match WriterContour::from_u8(writer.contour.load(Ordering::Acquire)) {
             WriterContour::Active => 0,
             WriterContour::Warm => 1,
             WriterContour::Draining => 2,
@@ -776,7 +786,7 @@ impl MePool {
         idle_since_by_writer: &HashMap<u64, u64>,
         now_epoch_secs: u64,
     ) -> u64 {
-        let contour_penalty = match WriterContour::from_u8(writer.contour.load(Ordering::Relaxed)) {
+        let contour_penalty = match WriterContour::from_u8(writer.contour.load(Ordering::Acquire)) {
             WriterContour::Active => 0,
             WriterContour::Warm => PICK_PENALTY_WARM,
             WriterContour::Draining => PICK_PENALTY_DRAINING,