Runtime guardrails

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

src/config/defaults.rs

@@ -36,12 +36,16 @@ const DEFAULT_ME_HEALTH_INTERVAL_MS_UNHEALTHY: u64 = 1000;
 const DEFAULT_ME_HEALTH_INTERVAL_MS_HEALTHY: u64 = 3000;
 const DEFAULT_ME_ADMISSION_POLL_MS: u64 = 1000;
 const DEFAULT_ME_WARN_RATE_LIMIT_MS: u64 = 5000;
+const DEFAULT_ME_ROUTE_HYBRID_MAX_WAIT_MS: u64 = 3000;
+const DEFAULT_ME_ROUTE_BLOCKING_SEND_TIMEOUT_MS: u64 = 250;
+const DEFAULT_ME_C2ME_SEND_TIMEOUT_MS: u64 = 4000;
 const DEFAULT_ME_POOL_DRAIN_SOFT_EVICT_ENABLED: bool = true;
 const DEFAULT_ME_POOL_DRAIN_SOFT_EVICT_GRACE_SECS: u64 = 30;
 const DEFAULT_ME_POOL_DRAIN_SOFT_EVICT_PER_WRITER: u8 = 1;
 const DEFAULT_ME_POOL_DRAIN_SOFT_EVICT_BUDGET_PER_CORE: u16 = 8;
 const DEFAULT_ME_POOL_DRAIN_SOFT_EVICT_COOLDOWN_MS: u64 = 5000;
 const DEFAULT_USER_MAX_UNIQUE_IPS_WINDOW_SECS: u64 = 30;
+const DEFAULT_ACCEPT_PERMIT_TIMEOUT_MS: u64 = 250;
 const DEFAULT_UPSTREAM_CONNECT_RETRY_ATTEMPTS: u32 = 2;
 const DEFAULT_UPSTREAM_UNHEALTHY_FAIL_THRESHOLD: u32 = 5;
 const DEFAULT_UPSTREAM_CONNECT_BUDGET_MS: u64 = 3000;
@@ -156,6 +160,10 @@ pub(crate) fn default_server_max_connections() -> u32 {
     10_000
 }
 
+pub(crate) fn default_accept_permit_timeout_ms() -> u64 {
+    DEFAULT_ACCEPT_PERMIT_TIMEOUT_MS
+}
+
 pub(crate) fn default_prefer_4() -> u8 {
     4
 }
@@ -380,6 +388,18 @@ pub(crate) fn default_me_warn_rate_limit_ms() -> u64 {
     DEFAULT_ME_WARN_RATE_LIMIT_MS
 }
 
+pub(crate) fn default_me_route_hybrid_max_wait_ms() -> u64 {
+    DEFAULT_ME_ROUTE_HYBRID_MAX_WAIT_MS
+}
+
+pub(crate) fn default_me_route_blocking_send_timeout_ms() -> u64 {
+    DEFAULT_ME_ROUTE_BLOCKING_SEND_TIMEOUT_MS
+}
+
+pub(crate) fn default_me_c2me_send_timeout_ms() -> u64 {
+    DEFAULT_ME_C2ME_SEND_TIMEOUT_MS
+}
+
 pub(crate) fn default_upstream_connect_retry_attempts() -> u32 {
     DEFAULT_UPSTREAM_CONNECT_RETRY_ATTEMPTS
 }

src/config/hot_reload.rs

@@ -612,6 +612,8 @@ fn warn_non_hot_changes(old: &ProxyConfig, new: &ProxyConfig, non_hot_changed: b
         || old.server.listen_tcp != new.server.listen_tcp
         || old.server.listen_unix_sock != new.server.listen_unix_sock
         || old.server.listen_unix_sock_perm != new.server.listen_unix_sock_perm
+        || old.server.max_connections != new.server.max_connections
+        || old.server.accept_permit_timeout_ms != new.server.accept_permit_timeout_ms
     {
         warned = true;
         warn!("config reload: server listener settings changed; restart required");
@@ -671,6 +673,9 @@ fn warn_non_hot_changes(old: &ProxyConfig, new: &ProxyConfig, non_hot_changed: b
     }
     if old.general.me_route_no_writer_mode != new.general.me_route_no_writer_mode
         || old.general.me_route_no_writer_wait_ms != new.general.me_route_no_writer_wait_ms
+        || old.general.me_route_hybrid_max_wait_ms != new.general.me_route_hybrid_max_wait_ms
+        || old.general.me_route_blocking_send_timeout_ms
+            != new.general.me_route_blocking_send_timeout_ms
         || old.general.me_route_inline_recovery_attempts
             != new.general.me_route_inline_recovery_attempts
         || old.general.me_route_inline_recovery_wait_ms
@@ -679,6 +684,10 @@ fn warn_non_hot_changes(old: &ProxyConfig, new: &ProxyConfig, non_hot_changed: b
         warned = true;
         warn!("config reload: general.me_route_no_writer_* changed; restart required");
     }
+    if old.general.me_c2me_send_timeout_ms != new.general.me_c2me_send_timeout_ms {
+        warned = true;
+        warn!("config reload: general.me_c2me_send_timeout_ms changed; restart required");
+    }
     if old.general.unknown_dc_log_path != new.general.unknown_dc_log_path
         || old.general.unknown_dc_file_log_enabled != new.general.unknown_dc_file_log_enabled
     {

src/config/load.rs

@@ -346,6 +346,12 @@ impl ProxyConfig {
             ));
         }
 
+        if config.general.me_c2me_send_timeout_ms > 60_000 {
+            return Err(ProxyError::Config(
+                "general.me_c2me_send_timeout_ms must be within [0, 60000]".to_string(),
+            ));
+        }
+
         if config.general.me_reader_route_data_wait_ms > 20 {
             return Err(ProxyError::Config(
                 "general.me_reader_route_data_wait_ms must be within [0, 20]".to_string(),
@@ -627,6 +633,18 @@ impl ProxyConfig {
             ));
         }
 
+        if !(50..=60_000).contains(&config.general.me_route_hybrid_max_wait_ms) {
+            return Err(ProxyError::Config(
+                "general.me_route_hybrid_max_wait_ms must be within [50, 60000]".to_string(),
+            ));
+        }
+
+        if config.general.me_route_blocking_send_timeout_ms > 5000 {
+            return Err(ProxyError::Config(
+                "general.me_route_blocking_send_timeout_ms must be within [0, 5000]".to_string(),
+            ));
+        }
+
         if !(2..=4).contains(&config.general.me_writer_pick_sample_size) {
             return Err(ProxyError::Config(
                 "general.me_writer_pick_sample_size must be within [2, 4]".to_string(),
@@ -687,6 +705,12 @@ impl ProxyConfig {
             ));
         }
 
+        if config.server.accept_permit_timeout_ms > 60_000 {
+            return Err(ProxyError::Config(
+                "server.accept_permit_timeout_ms must be within [0, 60000]".to_string(),
+            ));
+        }
+
         if config.general.effective_me_pool_force_close_secs() > 0
             && config.general.effective_me_pool_force_close_secs()
                 < config.general.me_pool_drain_ttl_secs

src/config/types.rs

@@ -462,6 +462,11 @@ pub struct GeneralConfig {
     #[serde(default = "default_me_c2me_channel_capacity")]
     pub me_c2me_channel_capacity: usize,
 
+    /// Maximum wait in milliseconds for enqueueing C2ME commands when the queue is full.
+    /// `0` keeps legacy unbounded wait behavior.
+    #[serde(default = "default_me_c2me_send_timeout_ms")]
+    pub me_c2me_send_timeout_ms: u64,
+
     /// Bounded wait in milliseconds for routing ME DATA to per-connection queue.
     /// `0` keeps legacy no-wait behavior.
     #[serde(default = "default_me_reader_route_data_wait_ms")]
@@ -716,6 +721,15 @@ pub struct GeneralConfig {
     #[serde(default = "default_me_route_no_writer_wait_ms")]
     pub me_route_no_writer_wait_ms: u64,
 
+    /// Maximum cumulative wait in milliseconds for hybrid no-writer mode before failfast.
+    #[serde(default = "default_me_route_hybrid_max_wait_ms")]
+    pub me_route_hybrid_max_wait_ms: u64,
+
+    /// Maximum wait in milliseconds for blocking ME writer channel send fallback.
+    /// `0` keeps legacy unbounded wait behavior.
+    #[serde(default = "default_me_route_blocking_send_timeout_ms")]
+    pub me_route_blocking_send_timeout_ms: u64,
+
     /// Number of inline recovery attempts in legacy mode.
     #[serde(default = "default_me_route_inline_recovery_attempts")]
     pub me_route_inline_recovery_attempts: u32,
@@ -921,6 +935,7 @@ impl Default for GeneralConfig {
             me_writer_cmd_channel_capacity: default_me_writer_cmd_channel_capacity(),
             me_route_channel_capacity: default_me_route_channel_capacity(),
             me_c2me_channel_capacity: default_me_c2me_channel_capacity(),
+            me_c2me_send_timeout_ms: default_me_c2me_send_timeout_ms(),
             me_reader_route_data_wait_ms: default_me_reader_route_data_wait_ms(),
             me_d2c_flush_batch_max_frames: default_me_d2c_flush_batch_max_frames(),
             me_d2c_flush_batch_max_bytes: default_me_d2c_flush_batch_max_bytes(),
@@ -975,6 +990,8 @@ impl Default for GeneralConfig {
             me_warn_rate_limit_ms: default_me_warn_rate_limit_ms(),
             me_route_no_writer_mode: MeRouteNoWriterMode::default(),
             me_route_no_writer_wait_ms: default_me_route_no_writer_wait_ms(),
+            me_route_hybrid_max_wait_ms: default_me_route_hybrid_max_wait_ms(),
+            me_route_blocking_send_timeout_ms: default_me_route_blocking_send_timeout_ms(),
             me_route_inline_recovery_attempts: default_me_route_inline_recovery_attempts(),
             me_route_inline_recovery_wait_ms: default_me_route_inline_recovery_wait_ms(),
             links: LinksConfig::default(),
@@ -1207,6 +1224,11 @@ pub struct ServerConfig {
     /// 0 means unlimited.
     #[serde(default = "default_server_max_connections")]
     pub max_connections: u32,
+
+    /// Maximum wait in milliseconds while acquiring a connection slot permit.
+    /// `0` keeps legacy unbounded wait behavior.
+    #[serde(default = "default_accept_permit_timeout_ms")]
+    pub accept_permit_timeout_ms: u64,
 }
 
 impl Default for ServerConfig {
@@ -1226,6 +1248,7 @@ impl Default for ServerConfig {
             api: ApiConfig::default(),
             listeners: Vec::new(),
             max_connections: default_server_max_connections(),
+            accept_permit_timeout_ms: default_accept_permit_timeout_ms(),
         }
     }
 }

src/maestro/helpers.rs

@@ -205,6 +205,7 @@ pub(crate) fn format_uptime(total_secs: u64) -> String {
     format!("{} / {} seconds", parts.join(", "), total_secs)
 }
 
+#[allow(dead_code)]
 pub(crate) async fn wait_until_admission_open(admission_rx: &mut watch::Receiver<bool>) -> bool {
     loop {
         if *admission_rx.borrow() {

src/maestro/listeners.rs

@@ -24,7 +24,7 @@ use crate::transport::{
     ListenOptions, UpstreamManager, create_listener, find_listener_processes,
 };
 
-use super::helpers::{is_expected_handshake_eof, print_proxy_links, wait_until_admission_open};
+use super::helpers::{is_expected_handshake_eof, print_proxy_links};
 
 pub(crate) struct BoundListeners {
     pub(crate) listeners: Vec<(TcpListener, bool)>,
@@ -195,7 +195,7 @@ pub(crate) async fn bind_listeners(
         has_unix_listener = true;
 
         let mut config_rx_unix: watch::Receiver<Arc<ProxyConfig>> = config_rx.clone();
-        let mut admission_rx_unix = admission_rx.clone();
+        let admission_rx_unix = admission_rx.clone();
         let stats = stats.clone();
         let upstream_manager = upstream_manager.clone();
         let replay_checker = replay_checker.clone();
@@ -212,17 +212,44 @@ pub(crate) async fn bind_listeners(
             let unix_conn_counter = Arc::new(std::sync::atomic::AtomicU64::new(1));
 
             loop {
-                if !wait_until_admission_open(&mut admission_rx_unix).await {
-                    warn!("Conditional-admission gate channel closed for unix listener");
-                    break;
-                }
                 match unix_listener.accept().await {
                     Ok((stream, _)) => {
-                        let permit = match max_connections_unix.clone().acquire_owned().await {
+                        if !*admission_rx_unix.borrow() {
-                            Ok(permit) => permit,
+                            drop(stream);
-                            Err(_) => {
+                            continue;
-                                error!("Connection limiter is closed");
+                        }
-                                break;
+                        let accept_permit_timeout_ms = config_rx_unix
+                            .borrow()
+                            .server
+                            .accept_permit_timeout_ms;
+                        let permit = if accept_permit_timeout_ms == 0 {
+                            match max_connections_unix.clone().acquire_owned().await {
+                                Ok(permit) => permit,
+                                Err(_) => {
+                                    error!("Connection limiter is closed");
+                                    break;
+                                }
+                            }
+                        } else {
+                            match tokio::time::timeout(
+                                Duration::from_millis(accept_permit_timeout_ms),
+                                max_connections_unix.clone().acquire_owned(),
+                            )
+                            .await
+                            {
+                                Ok(Ok(permit)) => permit,
+                                Ok(Err(_)) => {
+                                    error!("Connection limiter is closed");
+                                    break;
+                                }
+                                Err(_) => {
+                                    debug!(
+                                        timeout_ms = accept_permit_timeout_ms,
+                                        "Dropping accepted unix connection: permit wait timeout"
+                                    );
+                                    drop(stream);
+                                    continue;
+                                }
                             }
                         };
                         let conn_id =
@@ -312,7 +339,7 @@ pub(crate) fn spawn_tcp_accept_loops(
 ) {
     for (listener, listener_proxy_protocol) in listeners {
         let mut config_rx: watch::Receiver<Arc<ProxyConfig>> = config_rx.clone();
-        let mut admission_rx_tcp = admission_rx.clone();
+        let admission_rx_tcp = admission_rx.clone();
         let stats = stats.clone();
         let upstream_manager = upstream_manager.clone();
         let replay_checker = replay_checker.clone();
@@ -327,17 +354,46 @@ pub(crate) fn spawn_tcp_accept_loops(
 
         tokio::spawn(async move {
             loop {
-                if !wait_until_admission_open(&mut admission_rx_tcp).await {
-                    warn!("Conditional-admission gate channel closed for tcp listener");
-                    break;
-                }
                 match listener.accept().await {
                     Ok((stream, peer_addr)) => {
-                        let permit = match max_connections_tcp.clone().acquire_owned().await {
+                        if !*admission_rx_tcp.borrow() {
-                            Ok(permit) => permit,
+                            debug!(peer = %peer_addr, "Admission gate closed, dropping connection");
-                            Err(_) => {
+                            drop(stream);
-                                error!("Connection limiter is closed");
+                            continue;
-                                break;
+                        }
+                        let accept_permit_timeout_ms = config_rx
+                            .borrow()
+                            .server
+                            .accept_permit_timeout_ms;
+                        let permit = if accept_permit_timeout_ms == 0 {
+                            match max_connections_tcp.clone().acquire_owned().await {
+                                Ok(permit) => permit,
+                                Err(_) => {
+                                    error!("Connection limiter is closed");
+                                    break;
+                                }
+                            }
+                        } else {
+                            match tokio::time::timeout(
+                                Duration::from_millis(accept_permit_timeout_ms),
+                                max_connections_tcp.clone().acquire_owned(),
+                            )
+                            .await
+                            {
+                                Ok(Ok(permit)) => permit,
+                                Ok(Err(_)) => {
+                                    error!("Connection limiter is closed");
+                                    break;
+                                }
+                                Err(_) => {
+                                    debug!(
+                                        peer = %peer_addr,
+                                        timeout_ms = accept_permit_timeout_ms,
+                                        "Dropping accepted connection: permit wait timeout"
+                                    );
+                                    drop(stream);
+                                    continue;
+                                }
                             }
                         };
                         let config = config_rx.borrow_and_update().clone();

src/maestro/me_startup.rs

@@ -267,6 +267,8 @@ pub(crate) async fn initialize_me_pool(
                     config.general.me_warn_rate_limit_ms,
                     config.general.me_route_no_writer_mode,
                     config.general.me_route_no_writer_wait_ms,
+                    config.general.me_route_hybrid_max_wait_ms,
+                    config.general.me_route_blocking_send_timeout_ms,
                     config.general.me_route_inline_recovery_attempts,
                     config.general.me_route_inline_recovery_wait_ms,
                 );

src/proxy/middle_relay.rs

@@ -222,6 +222,7 @@ fn should_yield_c2me_sender(sent_since_yield: usize, has_backlog: bool) -> bool
 async fn enqueue_c2me_command(
     tx: &mpsc::Sender<C2MeCommand>,
     cmd: C2MeCommand,
+    send_timeout: Duration,
 ) -> std::result::Result<(), mpsc::error::SendError<C2MeCommand>> {
     match tx.try_send(cmd) {
         Ok(()) => Ok(()),
@@ -231,7 +232,17 @@ async fn enqueue_c2me_command(
             if tx.capacity() <= C2ME_SOFT_PRESSURE_MIN_FREE_SLOTS {
                 tokio::task::yield_now().await;
             }
-            tx.send(cmd).await
+            if send_timeout.is_zero() {
+                return tx.send(cmd).await;
+            }
+            match tokio::time::timeout(send_timeout, tx.reserve()).await {
+                Ok(Ok(permit)) => {
+                    permit.send(cmd);
+                    Ok(())
+                }
+                Ok(Err(_)) => Err(mpsc::error::SendError(cmd)),
+                Err(_) => Err(mpsc::error::SendError(cmd)),
+            }
         }
     }
 }
@@ -355,6 +366,7 @@ where
         .general
         .me_c2me_channel_capacity
         .max(C2ME_CHANNEL_CAPACITY_FALLBACK);
+    let c2me_send_timeout = Duration::from_millis(config.general.me_c2me_send_timeout_ms);
     let (c2me_tx, mut c2me_rx) = mpsc::channel::<C2MeCommand>(c2me_channel_capacity);
     let me_pool_c2me = me_pool.clone();
     let effective_tag = effective_tag;
@@ -363,15 +375,42 @@ where
         while let Some(cmd) = c2me_rx.recv().await {
             match cmd {
                 C2MeCommand::Data { payload, flags } => {
-                    me_pool_c2me.send_proxy_req(
+                    if c2me_send_timeout.is_zero() {
-                        conn_id,
+                        me_pool_c2me
-                        success.dc_idx,
+                            .send_proxy_req(
-                        peer,
+                                conn_id,
-                        translated_local_addr,
+                                success.dc_idx,
-                        payload.as_ref(),
+                                peer,
-                        flags,
+                                translated_local_addr,
-                        effective_tag.as_deref(),
+                                payload.as_ref(),
-                    ).await?;
+                                flags,
+                                effective_tag.as_deref(),
+                            )
+                            .await?;
+                    } else {
+                        match tokio::time::timeout(
+                            c2me_send_timeout,
+                            me_pool_c2me.send_proxy_req(
+                                conn_id,
+                                success.dc_idx,
+                                peer,
+                                translated_local_addr,
+                                payload.as_ref(),
+                                flags,
+                                effective_tag.as_deref(),
+                            ),
+                        )
+                        .await
+                        {
+                            Ok(send_result) => send_result?,
+                            Err(_) => {
+                                return Err(ProxyError::Proxy(format!(
+                                    "ME send timeout after {}ms",
+                                    c2me_send_timeout.as_millis()
+                                )));
+                            }
+                        }
+                    }
                     sent_since_yield = sent_since_yield.saturating_add(1);
                     if should_yield_c2me_sender(sent_since_yield, !c2me_rx.is_empty()) {
                         sent_since_yield = 0;
@@ -555,7 +594,7 @@ where
     loop {
         if session_lease.is_stale() {
             stats.increment_reconnect_stale_close_total();
-            let _ = enqueue_c2me_command(&c2me_tx, C2MeCommand::Close).await;
+            let _ = enqueue_c2me_command(&c2me_tx, C2MeCommand::Close, c2me_send_timeout).await;
             main_result = Err(ProxyError::Proxy("Session evicted by reconnect".to_string()));
             break;
         }
@@ -573,7 +612,7 @@ where
                 "Cutover affected middle session, closing client connection"
             );
             tokio::time::sleep(delay).await;
-            let _ = enqueue_c2me_command(&c2me_tx, C2MeCommand::Close).await;
+            let _ = enqueue_c2me_command(&c2me_tx, C2MeCommand::Close, c2me_send_timeout).await;
             main_result = Err(ProxyError::Proxy(ROUTE_SWITCH_ERROR_MSG.to_string()));
             break;
         }
@@ -607,9 +646,13 @@ where
                             flags |= RPC_FLAG_NOT_ENCRYPTED;
                         }
                         // Keep client read loop lightweight: route heavy ME send path via a dedicated task.
-                        if enqueue_c2me_command(&c2me_tx, C2MeCommand::Data { payload, flags })
+                        if enqueue_c2me_command(
-                            .await
+                            &c2me_tx,
-                            .is_err()
+                            C2MeCommand::Data { payload, flags },
+                            c2me_send_timeout,
+                        )
+                        .await
+                        .is_err()
                         {
                             main_result = Err(ProxyError::Proxy("ME sender channel closed".into()));
                             break;
@@ -618,7 +661,12 @@ where
                     Ok(None) => {
                         debug!(conn_id, "Client EOF");
                         client_closed = true;
-                        let _ = enqueue_c2me_command(&c2me_tx, C2MeCommand::Close).await;
+                        let _ = enqueue_c2me_command(
+                            &c2me_tx,
+                            C2MeCommand::Close,
+                            c2me_send_timeout,
+                        )
+                        .await;
                         break;
                     }
                     Err(e) => {
@@ -993,6 +1041,7 @@ mod tests {
                 payload: Bytes::from_static(&[1, 2, 3]),
                 flags: 0,
             },
+            TokioDuration::from_millis(50),
         )
         .await
         .unwrap();
@@ -1028,6 +1077,7 @@ mod tests {
                     payload: Bytes::from_static(&[7, 7]),
                     flags: 7,
                 },
+                TokioDuration::from_millis(100),
             )
             .await
             .unwrap();

src/transport/middle_proxy/health.rs

@@ -1574,6 +1574,8 @@ mod tests {
             general.me_warn_rate_limit_ms,
             MeRouteNoWriterMode::default(),
             general.me_route_no_writer_wait_ms,
+            general.me_route_hybrid_max_wait_ms,
+            general.me_route_blocking_send_timeout_ms,
             general.me_route_inline_recovery_attempts,
             general.me_route_inline_recovery_wait_ms,
         )

src/transport/middle_proxy/health_adversarial_tests.rs

@@ -111,6 +111,8 @@ async fn make_pool(
         general.me_warn_rate_limit_ms,
         MeRouteNoWriterMode::default(),
         general.me_route_no_writer_wait_ms,
+        general.me_route_hybrid_max_wait_ms,
+        general.me_route_blocking_send_timeout_ms,
         general.me_route_inline_recovery_attempts,
         general.me_route_inline_recovery_wait_ms,
     );

src/transport/middle_proxy/health_integration_tests.rs

@@ -110,6 +110,8 @@ async fn make_pool(
         general.me_warn_rate_limit_ms,
         MeRouteNoWriterMode::default(),
         general.me_route_no_writer_wait_ms,
+        general.me_route_hybrid_max_wait_ms,
+        general.me_route_blocking_send_timeout_ms,
         general.me_route_inline_recovery_attempts,
         general.me_route_inline_recovery_wait_ms,
     );

src/transport/middle_proxy/health_regression_tests.rs

@@ -103,6 +103,8 @@ async fn make_pool(me_pool_drain_threshold: u64) -> Arc<MePool> {
         general.me_warn_rate_limit_ms,
         MeRouteNoWriterMode::default(),
         general.me_route_no_writer_wait_ms,
+        general.me_route_hybrid_max_wait_ms,
+        general.me_route_blocking_send_timeout_ms,
         general.me_route_inline_recovery_attempts,
         general.me_route_inline_recovery_wait_ms,
     )

src/transport/middle_proxy/pool.rs

@@ -193,6 +193,8 @@ pub struct MePool {
     pub(super) me_reader_route_data_wait_ms: Arc<AtomicU64>,
     pub(super) me_route_no_writer_mode: AtomicU8,
     pub(super) me_route_no_writer_wait: Duration,
+    pub(super) me_route_hybrid_max_wait: Duration,
+    pub(super) me_route_blocking_send_timeout: Duration,
     pub(super) me_route_inline_recovery_attempts: u32,
     pub(super) me_route_inline_recovery_wait: Duration,
     pub(super) me_health_interval_ms_unhealthy: AtomicU64,
@@ -307,6 +309,8 @@ impl MePool {
         me_warn_rate_limit_ms: u64,
         me_route_no_writer_mode: MeRouteNoWriterMode,
         me_route_no_writer_wait_ms: u64,
+        me_route_hybrid_max_wait_ms: u64,
+        me_route_blocking_send_timeout_ms: u64,
         me_route_inline_recovery_attempts: u32,
         me_route_inline_recovery_wait_ms: u64,
     ) -> Arc<Self> {
@@ -490,6 +494,10 @@ impl MePool {
             me_reader_route_data_wait_ms: Arc::new(AtomicU64::new(me_reader_route_data_wait_ms)),
             me_route_no_writer_mode: AtomicU8::new(me_route_no_writer_mode.as_u8()),
             me_route_no_writer_wait: Duration::from_millis(me_route_no_writer_wait_ms),
+            me_route_hybrid_max_wait: Duration::from_millis(me_route_hybrid_max_wait_ms),
+            me_route_blocking_send_timeout: Duration::from_millis(
+                me_route_blocking_send_timeout_ms,
+            ),
             me_route_inline_recovery_attempts,
             me_route_inline_recovery_wait: Duration::from_millis(me_route_inline_recovery_wait_ms),
             me_health_interval_ms_unhealthy: AtomicU64::new(me_health_interval_ms_unhealthy.max(1)),

src/transport/middle_proxy/pool_writer.rs

@@ -312,41 +312,28 @@ impl MePool {
                 let mut p = Vec::with_capacity(12);
                 p.extend_from_slice(&RPC_PING_U32.to_le_bytes());
                 p.extend_from_slice(&sent_id.to_le_bytes());
-                {
+                let now_epoch_ms = std::time::SystemTime::now()
-                    let mut tracker = ping_tracker_ping.lock().await;
+                    .duration_since(std::time::UNIX_EPOCH)
-                    let now_epoch_ms = std::time::SystemTime::now()
+                    .unwrap_or_default()
-                        .duration_since(std::time::UNIX_EPOCH)
+                    .as_millis() as u64;
-                        .unwrap_or_default()
+                let mut run_cleanup = false;
-                        .as_millis() as u64;
+                if let Some(pool) = pool_ping.upgrade() {
-                    let mut run_cleanup = false;
+                    let last_cleanup_ms = pool
-                    if let Some(pool) = pool_ping.upgrade() {
+                        .ping_tracker_last_cleanup_epoch_ms
-                        let last_cleanup_ms = pool
+                        .load(Ordering::Relaxed);
+                    if now_epoch_ms.saturating_sub(last_cleanup_ms) >= 30_000
+                        && pool
                             .ping_tracker_last_cleanup_epoch_ms
-                            .load(Ordering::Relaxed);
+                            .compare_exchange(
-                        if now_epoch_ms.saturating_sub(last_cleanup_ms) >= 30_000
+                                last_cleanup_ms,
-                            && pool
+                                now_epoch_ms,
-                                .ping_tracker_last_cleanup_epoch_ms
+                                Ordering::AcqRel,
-                                .compare_exchange(
+                                Ordering::Relaxed,
-                                    last_cleanup_ms,
+                            )
-                                    now_epoch_ms,
+                            .is_ok()
-                                    Ordering::AcqRel,
+                    {
-                                    Ordering::Relaxed,
+                        run_cleanup = true;
-                                )
-                                .is_ok()
-                        {
-                            run_cleanup = true;
-                        }
                     }
-
-                    if run_cleanup {
-                        let before = tracker.len();
-                        tracker.retain(|_, (ts, _)| ts.elapsed() < Duration::from_secs(120));
-                        let expired = before.saturating_sub(tracker.len());
-                        if expired > 0 {
-                            stats_ping.increment_me_keepalive_timeout_by(expired as u64);
-                        }
-                    }
-                    tracker.insert(sent_id, (std::time::Instant::now(), writer_id));
                 }
                 ping_id = ping_id.wrapping_add(1);
                 stats_ping.increment_me_keepalive_sent();
@@ -367,6 +354,16 @@ impl MePool {
                     }
                     break;
                 }
+                let mut tracker = ping_tracker_ping.lock().await;
+                if run_cleanup {
+                    let before = tracker.len();
+                    tracker.retain(|_, (ts, _)| ts.elapsed() < Duration::from_secs(120));
+                    let expired = before.saturating_sub(tracker.len());
+                    if expired > 0 {
+                        stats_ping.increment_me_keepalive_timeout_by(expired as u64);
+                    }
+                }
+                tracker.insert(sent_id, (std::time::Instant::now(), writer_id));
             }
         });
 

src/transport/middle_proxy/send.rs

@@ -6,6 +6,7 @@ use std::sync::atomic::Ordering;
 use std::time::{Duration, Instant};
 
 use bytes::Bytes;
+use tokio::sync::mpsc;
 use tokio::sync::mpsc::error::TrySendError;
 use tracing::{debug, warn};
 
@@ -29,6 +30,29 @@ const PICK_PENALTY_DRAINING: u64 = 600;
 const PICK_PENALTY_STALE: u64 = 300;
 const PICK_PENALTY_DEGRADED: u64 = 250;
 
+enum TimedSendError<T> {
+    Closed(T),
+    Timeout(T),
+}
+
+async fn send_writer_command_with_timeout(
+    tx: &mpsc::Sender<WriterCommand>,
+    cmd: WriterCommand,
+    timeout: Duration,
+) -> std::result::Result<(), TimedSendError<WriterCommand>> {
+    if timeout.is_zero() {
+        return tx.send(cmd).await.map_err(|err| TimedSendError::Closed(err.0));
+    }
+    match tokio::time::timeout(timeout, tx.reserve()).await {
+        Ok(Ok(permit)) => {
+            permit.send(cmd);
+            Ok(())
+        }
+        Ok(Err(_)) => Err(TimedSendError::Closed(cmd)),
+        Err(_) => Err(TimedSendError::Timeout(cmd)),
+    }
+}
+
 impl MePool {
     /// Send RPC_PROXY_REQ. `tag_override`: per-user ad_tag (from access.user_ad_tags); if None, uses pool default.
     pub async fn send_proxy_req(
@@ -78,8 +102,18 @@ impl MePool {
         let mut hybrid_last_recovery_at: Option<Instant> = None;
         let hybrid_wait_step = self.me_route_no_writer_wait.max(Duration::from_millis(50));
         let mut hybrid_wait_current = hybrid_wait_step;
+        let hybrid_deadline = Instant::now() + self.me_route_hybrid_max_wait;
 
         loop {
+            if matches!(no_writer_mode, MeRouteNoWriterMode::HybridAsyncPersistent)
+                && Instant::now() >= hybrid_deadline
+            {
+                self.stats.increment_me_no_writer_failfast_total();
+                return Err(ProxyError::Proxy(
+                    "No ME writer available in hybrid wait window".into(),
+                ));
+            }
+            let mut skip_writer_id: Option<u64> = None;
             let current_meta = self
                 .registry
                 .get_meta(conn_id)
@@ -90,12 +124,30 @@ impl MePool {
                 match current.tx.try_send(WriterCommand::Data(current_payload.clone())) {
                     Ok(()) => return Ok(()),
                     Err(TrySendError::Full(cmd)) => {
-                        if current.tx.send(cmd).await.is_ok() {
+                        match send_writer_command_with_timeout(
-                            return Ok(());
+                            &current.tx,
+                            cmd,
+                            self.me_route_blocking_send_timeout,
+                        )
+                        .await
+                        {
+                            Ok(()) => return Ok(()),
+                            Err(TimedSendError::Closed(_)) => {
+                                warn!(writer_id = current.writer_id, "ME writer channel closed");
+                                self.remove_writer_and_close_clients(current.writer_id).await;
+                                continue;
+                            }
+                            Err(TimedSendError::Timeout(_)) => {
+                                debug!(
+                                    conn_id,
+                                    writer_id = current.writer_id,
+                                    timeout_ms = self.me_route_blocking_send_timeout.as_millis()
+                                        as u64,
+                                    "ME writer send timed out for bound writer, trying reroute"
+                                );
+                                skip_writer_id = Some(current.writer_id);
+                            }
                         }
-                        warn!(writer_id = current.writer_id, "ME writer channel closed");
-                        self.remove_writer_and_close_clients(current.writer_id).await;
-                        continue;
                     }
                     Err(TrySendError::Closed(_)) => {
                         warn!(writer_id = current.writer_id, "ME writer channel closed");
@@ -200,6 +252,9 @@ impl MePool {
                     .candidate_indices_for_dc(&writers_snapshot, routed_dc, true)
                     .await;
             }
+            if let Some(skip_writer_id) = skip_writer_id {
+                candidate_indices.retain(|idx| writers_snapshot[*idx].id != skip_writer_id);
+            }
             if candidate_indices.is_empty() {
                 let pick_mode = self.writer_pick_mode();
                 match no_writer_mode {
@@ -422,7 +477,13 @@ 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);
-            match w.tx.send(WriterCommand::Data(payload.clone())).await {
+            match send_writer_command_with_timeout(
+                &w.tx,
+                WriterCommand::Data(payload.clone()),
+                self.me_route_blocking_send_timeout,
+            )
+            .await
+            {
                 Ok(()) => {
                     self.stats
                         .increment_me_writer_pick_success_fallback_total(pick_mode);
@@ -439,11 +500,20 @@ impl MePool {
                     }
                     return Ok(());
                 }
-                Err(_) => {
+                Err(TimedSendError::Closed(_)) => {
                     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(_)) => {
+                    self.stats.increment_me_writer_pick_full_total(pick_mode);
+                    debug!(
+                        conn_id,
+                        writer_id = w.id,
+                        timeout_ms = self.me_route_blocking_send_timeout.as_millis() as u64,
+                        "ME writer blocking fallback send timed out"
+                    );
+                }
             }
         }
     }