fix(me-pool): resolve 0-writer blackouts with zero-allocation constraints

- Converts adaptive floor logic from proactive idle drops to reactive
  global capacity constraints, fixing sudden drops to 0 active writers.
- Implements `base_req` override gateway via `can_open_writer_for_contour`,
  retaining critical connections for starved datacenters during bursts.
- Applies zero-allocation performance optimization via direct inner lock iter,
  avoiding `HashSet` generation and deep `RwLock` checks in writer validation paths.
- Scrubs now-dead variables/evaluations (`adaptive_idle_since`,
  `adaptive_recover_until`) to fulfill strict memory & hot-path constraints.

src/transport/middle_proxy/health.rs

@@ -82,8 +82,6 @@ pub async fn me_health_monitor(pool: Arc<MePool>, rng: Arc<SecureRandom>, _min_c
     let mut single_endpoint_outage: HashSet<(i32, IpFamily)> = HashSet::new();
     let mut shadow_rotate_deadline: HashMap<(i32, IpFamily), Instant> = HashMap::new();
     let mut idle_refresh_next_attempt: HashMap<(i32, IpFamily), Instant> = HashMap::new();
-    let mut adaptive_idle_since: HashMap<(i32, IpFamily), Instant> = HashMap::new();
-    let mut adaptive_recover_until: HashMap<(i32, IpFamily), Instant> = HashMap::new();
     let mut floor_warn_next_allowed: HashMap<(i32, IpFamily), Instant> = HashMap::new();
     let mut drain_warn_next_allowed: HashMap<u64, Instant> = HashMap::new();
     let mut degraded_interval = true;
@@ -109,8 +107,6 @@ pub async fn me_health_monitor(pool: Arc<MePool>, rng: Arc<SecureRandom>, _min_c
             &mut single_endpoint_outage,
             &mut shadow_rotate_deadline,
             &mut idle_refresh_next_attempt,
-            &mut adaptive_idle_since,
-            &mut adaptive_recover_until,
             &mut floor_warn_next_allowed,
         )
         .await;
@@ -126,8 +122,6 @@ pub async fn me_health_monitor(pool: Arc<MePool>, rng: Arc<SecureRandom>, _min_c
             &mut single_endpoint_outage,
             &mut shadow_rotate_deadline,
             &mut idle_refresh_next_attempt,
-            &mut adaptive_idle_since,
-            &mut adaptive_recover_until,
             &mut floor_warn_next_allowed,
         )
         .await;
@@ -360,8 +354,6 @@ async fn check_family(
     single_endpoint_outage: &mut HashSet<(i32, IpFamily)>,
     shadow_rotate_deadline: &mut HashMap<(i32, IpFamily), Instant>,
     idle_refresh_next_attempt: &mut HashMap<(i32, IpFamily), Instant>,
-    adaptive_idle_since: &mut HashMap<(i32, IpFamily), Instant>,
-    adaptive_recover_until: &mut HashMap<(i32, IpFamily), Instant>,
     floor_warn_next_allowed: &mut HashMap<(i32, IpFamily), Instant>,
 ) -> bool {
     let enabled = match family {
@@ -394,8 +386,6 @@ async fn check_family(
     let reconnect_sem = Arc::new(Semaphore::new(reconnect_budget));
 
     if pool.floor_mode() == MeFloorMode::Static {
-        adaptive_idle_since.clear();
-        adaptive_recover_until.clear();
     }
 
     let mut live_addr_counts = HashMap::<(i32, SocketAddr), usize>::new();
@@ -435,8 +425,6 @@ async fn check_family(
         &live_addr_counts,
         &live_writer_ids_by_addr,
         &bound_clients_by_writer,
-        adaptive_idle_since,
-        adaptive_recover_until,
     )
     .await;
     pool.set_adaptive_floor_runtime_caps(
@@ -503,8 +491,6 @@ async fn check_family(
             outage_next_attempt.remove(&key);
             shadow_rotate_deadline.remove(&key);
             idle_refresh_next_attempt.remove(&key);
-            adaptive_idle_since.remove(&key);
-            adaptive_recover_until.remove(&key);
             info!(
                 dc = %dc,
                 ?family,
@@ -632,21 +618,25 @@ async fn check_family(
                         restored += 1;
                         continue;
                     }
-                    pool_for_reconnect
-                        .stats
-                        .increment_me_floor_cap_block_total();
-                    pool_for_reconnect
-                        .stats
-                        .increment_me_floor_swap_idle_failed_total();
-                    debug!(
-                        dc = %dc,
-                        ?family,
-                        alive,
-                        required,
-                        active_cap_effective_total,
-                        "Adaptive floor cap reached, reconnect attempt blocked"
-                    );
-                    break;
+
+                    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
+                            .increment_me_floor_cap_block_total();
+                        pool_for_reconnect
+                            .stats
+                            .increment_me_floor_swap_idle_failed_total();
+                        debug!(
+                            dc = %dc,
+                            ?family,
+                            alive,
+                            required,
+                            active_cap_effective_total,
+                            "Adaptive floor cap reached, reconnect attempt blocked"
+                        );
+                        break;
+                    }
                 }
                 let res = tokio::time::timeout(
                     pool_for_reconnect.reconnect_runtime.me_one_timeout,
@@ -904,8 +894,6 @@ async fn build_family_floor_plan(
     live_addr_counts: &HashMap<(i32, SocketAddr), usize>,
     live_writer_ids_by_addr: &HashMap<(i32, SocketAddr), Vec<u64>>,
     bound_clients_by_writer: &HashMap<u64, usize>,
-    adaptive_idle_since: &mut HashMap<(i32, IpFamily), Instant>,
-    adaptive_recover_until: &mut HashMap<(i32, IpFamily), Instant>,
 ) -> FamilyFloorPlan {
     let mut entries = Vec::<DcFloorPlanEntry>::new();
     let mut by_dc = HashMap::<i32, DcFloorPlanEntry>::new();
@@ -921,18 +909,7 @@ async fn build_family_floor_plan(
         if endpoints.is_empty() {
             continue;
         }
-        let key = (*dc, family);
-        let reduce_for_idle = should_reduce_floor_for_idle(
-            pool,
-            key,
-            *dc,
-            endpoints,
-            live_writer_ids_by_addr,
-            bound_clients_by_writer,
-            adaptive_idle_since,
-            adaptive_recover_until,
-        )
-        .await;
+        let _key = (*dc, family);
         let base_required = pool.required_writers_for_dc(endpoints.len()).max(1);
         let min_required = if is_adaptive {
             adaptive_floor_class_min(pool, endpoints.len(), base_required)
@@ -947,11 +924,11 @@ async fn build_family_floor_plan(
         if max_required < min_required {
             max_required = min_required;
         }
-        let desired_raw = if is_adaptive && reduce_for_idle {
-            min_required
-        } else {
-            base_required
-        };
+        // We initialize target_required at base_required to prevent 0-writer blackouts
+        // caused by proactively dropping an idle DC to a single fragile connection.
+        // The Adaptive Floor constraint loop below will gracefully compress idle DCs
+        // (prioritized via has_bound_clients = false) to min_required only when global capacity is reached.
+        let desired_raw = base_required;
         let target_required = desired_raw.clamp(min_required, max_required);
         let alive = endpoints
             .iter()
@@ -1278,43 +1255,6 @@ async fn maybe_refresh_idle_writer_for_dc(
     );
 }
 
-async fn should_reduce_floor_for_idle(
-    pool: &Arc<MePool>,
-    key: (i32, IpFamily),
-    dc: i32,
-    endpoints: &[SocketAddr],
-    live_writer_ids_by_addr: &HashMap<(i32, SocketAddr), Vec<u64>>,
-    bound_clients_by_writer: &HashMap<u64, usize>,
-    adaptive_idle_since: &mut HashMap<(i32, IpFamily), Instant>,
-    adaptive_recover_until: &mut HashMap<(i32, IpFamily), Instant>,
-) -> bool {
-    if pool.floor_mode() != MeFloorMode::Adaptive {
-        adaptive_idle_since.remove(&key);
-        adaptive_recover_until.remove(&key);
-        return false;
-    }
-
-    let now = Instant::now();
-    let writer_ids = list_writer_ids_for_endpoints(dc, endpoints, live_writer_ids_by_addr);
-    let has_bound_clients = has_bound_clients_on_endpoint(&writer_ids, bound_clients_by_writer);
-    if has_bound_clients {
-        adaptive_idle_since.remove(&key);
-        adaptive_recover_until.insert(key, now + pool.adaptive_floor_recover_grace_duration());
-        return false;
-    }
-
-    if let Some(recover_until) = adaptive_recover_until.get(&key)
-        && now < *recover_until
-    {
-        adaptive_idle_since.remove(&key);
-        return false;
-    }
-    adaptive_recover_until.remove(&key);
-
-    let idle_since = adaptive_idle_since.entry(key).or_insert(now);
-    now.saturating_duration_since(*idle_since) >= pool.adaptive_floor_idle_duration()
-}
-
 fn has_bound_clients_on_endpoint(
     writer_ids: &[u64],
     bound_clients_by_writer: &HashMap<u64, usize>,

src/transport/middle_proxy/pool.rs

@@ -1422,22 +1422,6 @@ impl MePool {
         MeFloorMode::from_u8(self.floor_runtime.me_floor_mode.load(Ordering::Relaxed))
     }
 
-    pub(super) fn adaptive_floor_idle_duration(&self) -> Duration {
-        Duration::from_secs(
-            self.floor_runtime
-                .me_adaptive_floor_idle_secs
-                .load(Ordering::Relaxed),
-        )
-    }
-
-    pub(super) fn adaptive_floor_recover_grace_duration(&self) -> Duration {
-        Duration::from_secs(
-            self.floor_runtime
-                .me_adaptive_floor_recover_grace_secs
-                .load(Ordering::Relaxed),
-        )
-    }
-
     pub(super) fn adaptive_floor_min_writers_multi_endpoint(&self) -> usize {
         (self
             .floor_runtime
@@ -1659,6 +1643,7 @@ impl MePool {
         &self,
         contour: WriterContour,
         allow_coverage_override: bool,
+        writer_dc: i32,
     ) -> bool {
         let (active_writers, warm_writers, _) = self.non_draining_writer_counts_by_contour().await;
         match contour {
@@ -1670,6 +1655,33 @@ impl MePool {
                 if !allow_coverage_override {
                     return false;
                 }
+
+                let mut endpoints_len = 0;
+                let now_epoch = Self::now_epoch_secs();
+                if self.family_enabled_for_drain_coverage(IpFamily::V4, now_epoch) {
+                    if let Some(addrs) = self.proxy_map_v4.read().await.get(&writer_dc) {
+                        endpoints_len += addrs.len();
+                    }
+                }
+                if self.family_enabled_for_drain_coverage(IpFamily::V6, now_epoch) {
+                    if let Some(addrs) = self.proxy_map_v6.read().await.get(&writer_dc) {
+                        endpoints_len += addrs.len();
+                    }
+                }
+
+                if endpoints_len > 0 {
+                    let base_req = self.required_writers_for_dc_with_floor_mode(endpoints_len, false);
+                    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)
+                            .count()
+                    };
+                    if active_for_dc < base_req {
+                        return true;
+                    }
+                }
+
                 let coverage_required = self.active_coverage_required_total().await;
                 active_writers < coverage_required
             }

src/transport/middle_proxy/pool_writer.rs

@@ -342,7 +342,7 @@ impl MePool {
         allow_coverage_override: bool,
     ) -> Result<()> {
         if !self
-            .can_open_writer_for_contour(contour, allow_coverage_override)
+            .can_open_writer_for_contour(contour, allow_coverage_override, writer_dc)
             .await
         {
             return Err(ProxyError::Proxy(format!(