crates/irtt-cli/tests/client_multi_target.rs

@@ -425,8 +425,26 @@ fn finite_multi_target_peer_close_is_accepted_as_completion() {
 #[test]
 fn continuous_all_peer_closed_targets_exit_nonzero() {
     let params = test_params(None, Duration::from_millis(10));
-    let a = start_peer_close_server(params.clone());
-    let b = start_peer_close_server(params);
+    let (a_seen_tx, a_seen_rx) = mpsc::channel();
+    let (a_release_tx, a_release_rx) = mpsc::channel();
+    let (b_seen_tx, b_seen_rx) = mpsc::channel();
+    let (b_release_tx, b_release_rx) = mpsc::channel();
+    let a = start_synchronized_peer_close_server(params.clone(), a_seen_tx, a_release_rx);
+    let b = start_synchronized_peer_close_server(params, b_seen_tx, b_release_rx);
+    let coordinator = thread::spawn(move || {
+        a_seen_rx
+            .recv_timeout(Duration::from_secs(2))
+            .expect("timed out waiting for target A's first probe");
+        b_seen_rx
+            .recv_timeout(Duration::from_secs(2))
+            .expect("timed out waiting for target B's first probe");
+        a_release_tx
+            .send(())
+            .expect("target A stopped waiting for peer-close release");
+        b_release_tx
+            .send(())
+            .expect("target B stopped waiting for peer-close release");
+    });
     let mut command = Command::new(env!("CARGO_BIN_EXE_irtt-cli"));
     command.args([
         "--duration",
@@ -446,6 +464,7 @@ fn continuous_all_peer_closed_targets_exit_nonzero() {
     ]);
 
     let output = run_with_timeout(command, Duration::from_secs(3));
+    coordinator.join().unwrap();
     a.join();
     b.join();
 
@@ -860,6 +879,55 @@ fn start_peer_close_server(params: Params) -> FakeServer {
     start_gated_peer_close_server(params, Vec::new())
 }
 
+fn start_synchronized_peer_close_server(
+    params: Params,
+    first_probe_seen_tx: mpsc::Sender<()>,
+    release_peer_close_rx: mpsc::Receiver<()>,
+) -> FakeServer {
+    let socket = UdpSocket::bind("127.0.0.1:0").unwrap();
+    socket
+        .set_read_timeout(Some(Duration::from_secs(2)))
+        .unwrap();
+    let addr = socket.local_addr().unwrap();
+    let done = thread::spawn(move || {
+        let (_, peer) = recv_request(&socket);
+        socket
+            .send_to(&open_reply(FLAG_OPEN | FLAG_REPLY, TOKEN, &params), peer)
+            .unwrap();
+        let (packet, peer) = recv_request(&socket);
+        let flags = packet[3];
+        assert_eq!(
+            flags, 0,
+            "target closed before synchronized first peer-close probe"
+        );
+        assert_eq!(
+            packet.len(),
+            echo_packet_len(false, &params).unwrap(),
+            "expected a complete echo probe before synchronized peer-close reply"
+        );
+        let seq = u32::from_le_bytes(packet[12..16].try_into().unwrap());
+        first_probe_seen_tx
+            .send(())
+            .expect("peer-close coordinator stopped waiting for first probe");
+        release_peer_close_rx
+            .recv_timeout(Duration::from_secs(2))
+            .expect("timed out waiting to release synchronized peer-close reply");
+        socket
+            .send_to(
+                &echo_reply_packet(
+                    TOKEN,
+                    seq,
+                    &params,
+                    &TimestampFields::default(),
+                    FLAG_REPLY | FLAG_CLOSE,
+                ),
+                peer,
+            )
+            .unwrap();
+    });
+    FakeServer { addr, done }
+}
+
 fn start_gated_peer_close_server(
     params: Params,
     release_gates: Vec<mpsc::Receiver<()>>,
@@ -875,10 +943,19 @@ fn start_gated_peer_close_server(
             .send_to(&open_reply(FLAG_OPEN | FLAG_REPLY, TOKEN, &params), peer)
             .unwrap();
         let (packet, peer) = recv_request(&socket);
+        let flags = packet[3];
+        if flags & FLAG_CLOSE != 0 {
+            return;
+        }
         for gate in release_gates {
             gate.recv_timeout(Duration::from_secs(2))
                 .expect("timed out waiting to release peer-close reply");
         }
+        assert_eq!(
+            packet.len(),
+            echo_packet_len(false, &params).unwrap(),
+            "expected a complete echo probe before peer-close reply"
+        );
         let seq = u32::from_le_bytes(packet[12..16].try_into().unwrap());
         socket
             .send_to(

crates/irtt-client/src/async_client.rs

@@ -27,7 +27,7 @@ use crate::{
     receive::try_recv_tokio_datagram,
     session::machine::{
         recv_buffer_size, OpenDatagramDisposition, PreparedOpenAcceptance, PreparedOpenRequest,
-        PreparedProbe, SessionMachine, MAX_OPEN_PACKET_SIZE,
+        PreparedProbe, SessionMachine, TimeoutBatch, MAX_OPEN_PACKET_SIZE,
     },
     socket::{connect_tokio_udp_socket, resolve_remote_tokio, validate_open_timeouts},
     socket_options::{apply_dscp_to_tokio_socket, clear_dscp_on_tokio_socket},
@@ -300,6 +300,14 @@ impl AsyncClient {
         self.machine.poll_timeouts_at(now)
     }
 
+    pub(crate) fn poll_timeouts_bounded_at(
+        &mut self,
+        now: Instant,
+        limit: usize,
+    ) -> Result<TimeoutBatch, ClientError> {
+        self.machine.poll_timeouts_bounded_at(now, limit)
+    }
+
     /// Send the retained close packet and commit local close exactly once.
     ///
     /// DSCP is cleared only after write readiness and immediately before the
@@ -765,6 +773,19 @@ impl AsyncClient {
         self.socket.poll_recv_ready(cx)
     }
 
+    #[cfg(test)]
+    pub(crate) fn remove_pending_for_test(
+        &mut self,
+        wire_seq: u32,
+    ) -> Option<crate::probe::PendingProbe> {
+        self.machine.remove_pending_for_test(wire_seq)
+    }
+
+    #[cfg(test)]
+    pub(crate) fn replace_pending_for_test(&mut self, probe: crate::probe::PendingProbe) {
+        self.machine.replace_pending_for_test(probe);
+    }
+
     pub(crate) fn skip_missed_probe_slots_at(&mut self, now: Instant) -> Result<(), ClientError> {
         let Some(schedule) = self.schedule.as_mut() else {
             return Ok(());

crates/irtt-client/src/async_client/tests.rs

@@ -16,7 +16,10 @@ use irtt_proto::{
 use tokio::runtime::{Builder, Runtime};
 
 use super::*;
-use crate::{socket_options::tokio_socket_traffic_class, Client, RunMode, SocketConfig};
+use crate::{
+    probe::PendingProbe, socket_options::tokio_socket_traffic_class, Client, ClientTimestamp,
+    RunMode, SocketConfig,
+};
 
 const TOKEN: u64 = 0x1234_5678_90ab_cdef;
 
@@ -708,6 +711,35 @@ fn probe_false_readiness_resamples_all_timestamps() {
     assert_eq!(server.finish().len(), 2);
 }
 
+#[test]
+fn public_timeout_polling_remains_exhaustive() {
+    let server = start_open_server(None, 0);
+    runtime().block_on(async {
+        let mut client = opened_client(server.addr, 0).await;
+        let now = Instant::now();
+        for seq in 0..3 {
+            client.replace_pending_for_test(PendingProbe {
+                wire_seq: seq,
+                sent_at: ClientTimestamp {
+                    mono: now - Duration::from_secs(2),
+                    wall: SystemTime::UNIX_EPOCH,
+                },
+                timeout_at: now - Duration::from_secs(1),
+            });
+        }
+
+        assert!(matches!(
+            client.poll_timeouts_at(now).unwrap().as_slice(),
+            [
+                ClientEvent::EchoLoss { seq: 0, .. },
+                ClientEvent::EchoLoss { seq: 1, .. },
+                ClientEvent::EchoLoss { seq: 2, .. },
+            ]
+        ));
+    });
+    assert_eq!(server.finish().len(), 1);
+}
+
 #[test]
 fn successful_short_probe_commits_before_length_error() {
     let server = start_open_server(None, 0);

crates/irtt-client/src/managed/task.rs

@@ -37,6 +37,7 @@ use super::{
 
 const TARGET_WORK_BUDGET: usize = 128;
 const COMMAND_WORK_BUDGET: usize = 32;
+const TIMEOUT_WORK_BUDGET: usize = 128;
 const POST_DEADLINE_RECEIVE_BUDGET: usize = TARGET_WORK_BUDGET;
 // Tokio 1.53 broadcast asserts this exact bound before rounding capacity to a
 // power of two; Tokio does not expose it as a public constant.
@@ -415,6 +416,12 @@ struct TargetRuntime {
     state: TargetState,
 }
 
+#[derive(Clone, Copy, Debug, Eq, PartialEq)]
+struct TimeoutBacklogEntry {
+    index: usize,
+    generation: u64,
+}
+
 struct PlannedTarget {
     target: ManagedTargetConfig,
     client_config: ClientConfig,
@@ -573,6 +580,8 @@ pub struct ManagedClientTask {
     wake: Option<WakeFuture>,
     timer: Option<Pin<Box<Sleep>>>,
     cursor: usize,
+    timeout_cursor: usize,
+    timeout_backlog: VecDeque<TimeoutBacklogEntry>,
     scan_remaining: usize,
     send_cursor: usize,
     burst_remaining: usize,
@@ -589,6 +598,10 @@ pub struct ManagedClientTask {
     stagger_observations: Option<StaggerObservations>,
     #[cfg(test)]
     drain_test_hook: DrainTestHook,
+    #[cfg(test)]
+    timeout_inspections: usize,
+    #[cfg(test)]
+    deadline_inspections: usize,
 }
 
 #[cfg(test)]
@@ -1046,9 +1059,11 @@ impl ManagedClientTask {
         let len = self.targets.len();
         if len == 0 {
             self.cursor = 0;
+            self.timeout_cursor = 0;
             self.send_cursor = 0;
         } else {
             self.cursor %= len;
+            self.timeout_cursor %= len;
             self.send_cursor %= len;
         }
         self.scan_remaining = self.scan_remaining.min(len);
@@ -1205,19 +1220,8 @@ impl ManagedClientTask {
                     .next_probe_timeout_deadline()
                     .is_some_and(|deadline| deadline <= now)
                 {
-                    match client.poll_timeouts_at(now) {
-                        Ok(events) => self.publish_client_events(index, events),
-                        Err(error) => {
-                            return self.begin_open_session_failure(
-                                index,
-                                client,
-                                ManagedTargetFailurePhase::Timing,
-                                error,
-                                now,
-                                OpenSessionFailureCleanup::Close,
-                            );
-                        }
-                    }
+                    self.targets[index].state = TargetState::Active { client };
+                    return true;
                 }
                 let received = match client.poll_recv(cx) {
                     Poll::Pending => false,
@@ -1277,35 +1281,26 @@ impl ManagedClientTask {
                     return false;
                 }
 
+                if client
+                    .next_probe_timeout_deadline()
+                    .is_some_and(|timeout| timeout <= now)
+                {
+                    self.targets[index].state = TargetState::Draining {
+                        client,
+                        drain_started_at,
+                        deadline,
+                        primary_end,
+                        cleanup_failure,
+                        post_deadline_receives_remaining,
+                    };
+                    return true;
+                }
+
                 #[cfg(test)]
                 let injected_receive_failure = self.take_drain_receive_failure();
                 #[cfg(not(test))]
                 let injected_receive_failure = None;
                 let mut retained_state_changed = false;
-                if client
-                    .next_probe_timeout_deadline()
-                    .is_some_and(|timeout| timeout <= now)
-                {
-                    match client.poll_timeouts_at(now) {
-                        Ok(events) => {
-                            self.publish_client_events(index, events);
-                            retained_state_changed = true;
-                        }
-                        Err(error) => {
-                            self.targets[index].counters.packets_sent = client.packets_sent();
-                            cleanup_failure.get_or_insert_with(|| {
-                                classify_client_error(ManagedTargetFailurePhase::Timing, &error)
-                            });
-                            return self.begin_close(
-                                index,
-                                client,
-                                primary_end,
-                                cleanup_failure,
-                                now,
-                            );
-                        }
-                    }
-                }
                 let received = match client.poll_recv(cx) {
                     Poll::Pending => false,
                     Poll::Ready(Ok(events)) => {
@@ -1595,6 +1590,14 @@ impl ManagedClientTask {
             self.targets[index].state = state;
             return SendResult::NotAttempted;
         };
+        if client
+            .next_probe_timeout_deadline()
+            .is_some_and(|deadline| deadline <= now)
+        {
+            self.targets[index].send_waiting = false;
+            self.targets[index].state = TargetState::Active { client };
+            return SendResult::NotAttempted;
+        }
         if client
             .next_send_deadline()
             .is_none_or(|deadline| deadline > now)
@@ -1728,13 +1731,176 @@ impl ManagedClientTask {
         immediate || self.scan_remaining > 0
     }
 
+    fn poll_timeout_pass(&mut self, now: Instant) -> bool {
+        let target_len = self.targets.len();
+        if target_len == 0 {
+            return false;
+        }
+
+        let mut transitions_remaining = TIMEOUT_WORK_BUDGET;
+        for _ in 0..target_len.min(TIMEOUT_WORK_BUDGET) {
+            if transitions_remaining == 0 {
+                break;
+            }
+            let index = self.timeout_cursor;
+            self.timeout_cursor = (self.timeout_cursor + 1) % target_len;
+            #[cfg(test)]
+            {
+                self.timeout_inspections += 1;
+            }
+            if self.target_has_due_timeout(index, now) {
+                let generation = self.targets[index].instance.generation;
+                let step = self.poll_target_timeout(index, now);
+                transitions_remaining -= 1;
+                if step.more_due {
+                    self.enqueue_timeout_backlog(index, generation);
+                }
+            }
+        }
+
+        while transitions_remaining > 0 {
+            let Some(entry) = self.timeout_backlog.pop_front() else {
+                break;
+            };
+            let valid = self
+                .targets
+                .get(entry.index)
+                .is_some_and(|target| target.instance.generation == entry.generation)
+                && self.target_has_due_timeout(entry.index, now);
+            if !valid {
+                continue;
+            }
+
+            let step = self.poll_target_timeout(entry.index, now);
+            transitions_remaining -= 1;
+            if step.more_due {
+                self.enqueue_timeout_backlog(entry.index, entry.generation);
+            }
+        }
+
+        !self.timeout_backlog.is_empty()
+    }
+
+    fn enqueue_timeout_backlog(&mut self, index: usize, generation: u64) {
+        if self.timeout_backlog.len() == TIMEOUT_WORK_BUDGET
+            || self
+                .timeout_backlog
+                .iter()
+                .any(|entry| entry.index == index && entry.generation == generation)
+        {
+            return;
+        }
+        self.timeout_backlog
+            .push_back(TimeoutBacklogEntry { index, generation });
+    }
+
+    fn target_has_due_timeout(&self, index: usize, now: Instant) -> bool {
+        match &self.targets[index].state {
+            TargetState::Active { client } => {
+                self.state != DriverState::Stopping
+                    && self.effective_retirement(index).is_none()
+                    && client
+                        .next_probe_timeout_deadline()
+                        .is_some_and(|deadline| deadline <= now)
+            }
+            TargetState::Draining {
+                client,
+                deadline: _deadline,
+                ..
+            } => {
+                #[cfg(test)]
+                let defer_work = self.drain_test_hook.defer_work_until_deadline && now < *_deadline;
+                #[cfg(not(test))]
+                let defer_work = false;
+                !defer_work
+                    && client
+                        .next_probe_timeout_deadline()
+                        .is_some_and(|timeout| timeout <= now)
+            }
+            _ => false,
+        }
+    }
+
+    fn poll_target_timeout(&mut self, index: usize, now: Instant) -> TimeoutStep {
+        let state = mem::replace(&mut self.targets[index].state, TargetState::Terminal);
+        match state {
+            TargetState::Active { mut client } => match client.poll_timeouts_bounded_at(now, 1) {
+                Ok(batch) => {
+                    self.publish_client_events(index, batch.events);
+                    self.targets[index].state = TargetState::Active { client };
+                    TimeoutStep {
+                        more_due: batch.more_due,
+                    }
+                }
+                Err(error) => {
+                    self.begin_open_session_failure(
+                        index,
+                        client,
+                        ManagedTargetFailurePhase::Timing,
+                        error,
+                        now,
+                        OpenSessionFailureCleanup::Close,
+                    );
+                    TimeoutStep::NONE
+                }
+            },
+            TargetState::Draining {
+                mut client,
+                drain_started_at,
+                deadline,
+                primary_end,
+                mut cleanup_failure,
+                post_deadline_receives_remaining,
+            } => match client.poll_timeouts_bounded_at(now, 1) {
+                Ok(batch) => {
+                    self.publish_client_events(index, batch.events);
+                    let deadline = match self.drain_deadline(&client, drain_started_at) {
+                        Some(candidate) => deadline.min(candidate),
+                        None => {
+                            cleanup_failure.get_or_insert_with(duration_overflow_failure);
+                            self.begin_close(index, client, primary_end, cleanup_failure, now);
+                            return TimeoutStep::NONE;
+                        }
+                    };
+                    self.targets[index].state = TargetState::Draining {
+                        client,
+                        drain_started_at,
+                        deadline,
+                        primary_end,
+                        cleanup_failure,
+                        post_deadline_receives_remaining,
+                    };
+                    TimeoutStep {
+                        more_due: batch.more_due,
+                    }
+                }
+                Err(error) => {
+                    self.targets[index].counters.packets_sent = client.packets_sent();
+                    cleanup_failure.get_or_insert_with(|| {
+                        classify_client_error(ManagedTargetFailurePhase::Timing, &error)
+                    });
+                    self.begin_close(index, client, primary_end, cleanup_failure, now);
+                    TimeoutStep::NONE
+                }
+            },
+            state => {
+                self.targets[index].state = state;
+                TimeoutStep::NONE
+            }
+        }
+    }
+
     fn all_targets_terminal(&self) -> bool {
         self.targets
             .iter()
             .all(|target| matches!(target.state, TargetState::Terminal))
     }
 
-    fn next_deadline(&self) -> Option<Instant> {
+    fn next_deadline(&mut self) -> Option<Instant> {
+        #[cfg(test)]
+        {
+            self.deadline_inspections += self.targets.len();
+        }
         let mut non_send_deadline = None;
         let mut send_deadline = None;
         for target in &self.targets {
@@ -1888,6 +2054,15 @@ enum SendResult {
     Failed { accepted: bool },
 }
 
+#[derive(Clone, Copy)]
+struct TimeoutStep {
+    more_due: bool,
+}
+
+impl TimeoutStep {
+    const NONE: Self = Self { more_due: false };
+}
+
 impl SendResult {
     fn accepted(self) -> bool {
         matches!(
@@ -1922,6 +2097,7 @@ impl Future for ManagedClientTask {
             this.observe_stop();
         }
         let now = Instant::now();
+        immediate |= this.poll_timeout_pass(now);
         immediate |= this.poll_target_pass(cx, now);
         this.prune_undesired_terminal();
         if this.state == DriverState::Running {
@@ -1962,7 +2138,9 @@ impl Future for ManagedClientTask {
         }
 
         immediate |= this.register_stop_wake(cx);
+        if !immediate {
             immediate |= this.register_timer(cx);
+        }
         if immediate {
             cx.waker().wake_by_ref();
         }
@@ -2129,6 +2307,8 @@ fn build_task(
         wake: Some(arm_wake(wake_receiver)),
         timer: None,
         cursor: 0,
+        timeout_cursor: 0,
+        timeout_backlog: VecDeque::with_capacity(TIMEOUT_WORK_BUDGET),
         scan_remaining: 0,
         send_cursor: 0,
         burst_remaining: 0,
@@ -2145,6 +2325,10 @@ fn build_task(
         stagger_observations: None,
         #[cfg(test)]
         drain_test_hook: DrainTestHook::default(),
+        #[cfg(test)]
+        timeout_inspections: 0,
+        #[cfg(test)]
+        deadline_inspections: 0,
     };
     let handle = ManagedClientHandle {
         stop,

crates/irtt-client/src/probe.rs

@@ -12,9 +12,24 @@ pub(crate) struct PendingProbe {
     pub timeout_at: Instant,
 }
 
+#[derive(Debug)]
+struct PendingEntry {
+    probe: PendingProbe,
+    previous: Option<u32>,
+    next: Option<u32>,
+}
+
+#[derive(Debug)]
+pub(crate) struct ExpiredBatch {
+    pub probes: Vec<PendingProbe>,
+    pub more_due: bool,
+}
+
 #[derive(Debug)]
 pub(crate) struct PendingMap {
-    map: HashMap<u32, PendingProbe>,
+    map: HashMap<u32, PendingEntry>,
+    first: Option<u32>,
+    last: Option<u32>,
     max_capacity: usize,
 }
 
@@ -22,6 +37,8 @@ impl PendingMap {
     pub fn new(max_capacity: usize) -> Self {
         Self {
             map: HashMap::new(),
+            first: None,
+            last: None,
             max_capacity,
         }
     }
@@ -42,43 +59,140 @@ impl PendingMap {
     }
 
     pub fn commit_insert(&mut self, probe: PendingProbe) {
-        let replaced = self.map.insert(probe.wire_seq, probe);
+        let wire_seq = probe.wire_seq;
+        let previous = self.last;
+        if let Some(previous) = previous {
+            let previous = self
+                .map
+                .get_mut(&previous)
+                .expect("pending list tail remains present");
+            // probe_timeout is fixed for an open session and committed sends
+            // have nondecreasing monotonic timestamps, so deadlines append.
+            debug_assert!(previous.probe.timeout_at <= probe.timeout_at);
+            debug_assert!(
+                previous.next.is_none(),
+                "pending list tail has no successor"
+            );
+            previous.next = Some(wire_seq);
+        } else {
+            debug_assert!(self.first.is_none(), "empty pending list has no head");
+            self.first = Some(wire_seq);
+        }
+
+        let replaced = self.map.insert(
+            wire_seq,
+            PendingEntry {
+                probe,
+                previous,
+                next: None,
+            },
+        );
         debug_assert!(replaced.is_none(), "preflight rejected pending collision");
+        self.last = Some(wire_seq);
+        self.assert_links_consistent();
     }
 
     pub fn remove(&mut self, wire_seq: u32) -> Option<PendingProbe> {
-        self.map.remove(&wire_seq)
+        let PendingEntry {
+            probe,
+            previous,
+            next,
+        } = self.map.remove(&wire_seq)?;
+
+        if let Some(previous) = previous {
+            let previous = self
+                .map
+                .get_mut(&previous)
+                .expect("pending list predecessor remains present");
+            debug_assert_eq!(previous.next, Some(wire_seq));
+            previous.next = next;
+        } else {
+            debug_assert_eq!(self.first, Some(wire_seq));
+            self.first = next;
+        }
+        if let Some(next) = next {
+            let next = self
+                .map
+                .get_mut(&next)
+                .expect("pending list successor remains present");
+            debug_assert_eq!(next.previous, Some(wire_seq));
+            next.previous = previous;
+        } else {
+            debug_assert_eq!(self.last, Some(wire_seq));
+            self.last = previous;
         }
 
-    pub fn drain_expired(&mut self, now: Instant) -> Vec<PendingProbe> {
-        let expired_keys: Vec<u32> = self
+        self.assert_links_consistent();
+        Some(probe)
+    }
+
+    pub(crate) fn drain_expired_bounded(&mut self, now: Instant, limit: usize) -> ExpiredBatch {
+        let mut probes = Vec::with_capacity(limit.min(self.map.len()));
+        while probes.len() < limit {
+            let Some(wire_seq) = self.first else {
+                break;
+            };
+            let timeout_at = self
                 .map
-            .iter()
-            .filter(|(_, probe)| probe.timeout_at <= now)
-            .map(|(key, _)| *key)
-            .collect();
-        let mut expired = Vec::with_capacity(expired_keys.len());
-        for key in expired_keys {
-            if let Some(probe) = self.map.remove(&key) {
-                expired.push(probe);
+                .get(&wire_seq)
+                .expect("pending list head remains present")
+                .probe
+                .timeout_at;
+            if timeout_at > now {
+                break;
             }
+
+            let probe = self
+                .remove(wire_seq)
+                .expect("pending list head remains present");
+            probes.push(probe);
         }
-        expired.sort_by_key(|p| p.sent_at.mono);
-        expired
+        let more_due = self
+            .first
+            .and_then(|wire_seq| self.map.get(&wire_seq))
+            .is_some_and(|entry| entry.probe.timeout_at <= now);
+        ExpiredBatch { probes, more_due }
     }
 
     pub fn len(&self) -> usize {
         self.map.len()
     }
 
-    #[cfg(feature = "tokio")]
+    #[cfg(any(feature = "tokio", test))]
     pub fn next_timeout_deadline(&self) -> Option<Instant> {
-        self.map.values().map(|probe| probe.timeout_at).min()
+        self.first
+            .and_then(|wire_seq| self.map.get(&wire_seq))
+            .map(|entry| entry.probe.timeout_at)
     }
 
-    #[cfg(feature = "tokio")]
+    #[cfg(any(feature = "tokio", test))]
     pub fn latest_timeout_deadline(&self) -> Option<Instant> {
-        self.map.values().map(|probe| probe.timeout_at).max()
+        self.last
+            .and_then(|wire_seq| self.map.get(&wire_seq))
+            .map(|entry| entry.probe.timeout_at)
+    }
+
+    fn assert_links_consistent(&self) {
+        debug_assert_eq!(self.first.is_none(), self.map.is_empty());
+        debug_assert_eq!(self.last.is_none(), self.map.is_empty());
+        if let Some(first) = self.first {
+            debug_assert_eq!(
+                self.map
+                    .get(&first)
+                    .expect("pending list head remains present")
+                    .previous,
+                None
+            );
+        }
+        if let Some(last) = self.last {
+            debug_assert_eq!(
+                self.map
+                    .get(&last)
+                    .expect("pending list tail remains present")
+                    .next,
+                None
+            );
+        }
     }
 
     #[cfg(test)]
@@ -90,6 +204,18 @@ impl PendingMap {
     pub fn capacity(&self) -> usize {
         self.map.capacity()
     }
+
+    #[cfg(test)]
+    pub fn linkage(&self) -> (Option<u32>, Option<u32>) {
+        (self.first, self.last)
+    }
+
+    #[cfg(test)]
+    pub fn entry_links(&self, wire_seq: u32) -> Option<(Option<u32>, Option<u32>)> {
+        self.map
+            .get(&wire_seq)
+            .map(|entry| (entry.previous, entry.next))
+    }
 }
 
 fn pending_allocation_failed(source: TryReserveError) -> ClientError {
@@ -244,6 +370,11 @@ mod tests {
         }
     }
 
+    fn insert(map: &mut PendingMap, probe: PendingProbe) {
+        map.preflight_insert(probe.wire_seq).unwrap();
+        map.commit_insert(probe);
+    }
+
     #[test]
     fn pending_map_rejects_over_capacity() {
         let mut map = PendingMap::new(2);
@@ -303,6 +434,163 @@ mod tests {
         ));
     }
 
+    #[test]
+    fn pending_map_unlinks_head_middle_and_tail_and_updates_extrema() {
+        let mut map = PendingMap::new(4);
+        let now = Instant::now();
+        insert(&mut map, pending(1, now + Duration::from_secs(1)));
+        insert(&mut map, pending(2, now + Duration::from_secs(2)));
+        insert(&mut map, pending(3, now + Duration::from_secs(3)));
+
+        assert_eq!(
+            map.next_timeout_deadline(),
+            Some(now + Duration::from_secs(1))
+        );
+        assert_eq!(
+            map.latest_timeout_deadline(),
+            Some(now + Duration::from_secs(3))
+        );
+        assert_eq!(map.linkage(), (Some(1), Some(3)));
+        assert_eq!(map.entry_links(1), Some((None, Some(2))));
+        assert_eq!(map.entry_links(2), Some((Some(1), Some(3))));
+        assert_eq!(map.entry_links(3), Some((Some(2), None)));
+
+        assert!(map.remove(2).is_some());
+        assert_eq!(
+            map.next_timeout_deadline(),
+            Some(now + Duration::from_secs(1))
+        );
+        assert_eq!(
+            map.latest_timeout_deadline(),
+            Some(now + Duration::from_secs(3))
+        );
+        assert_eq!(map.linkage(), (Some(1), Some(3)));
+        assert_eq!(map.entry_links(1), Some((None, Some(3))));
+        assert_eq!(map.entry_links(3), Some((Some(1), None)));
+
+        assert!(map.remove(1).is_some());
+        assert_eq!(
+            map.next_timeout_deadline(),
+            Some(now + Duration::from_secs(3))
+        );
+        assert_eq!(
+            map.latest_timeout_deadline(),
+            Some(now + Duration::from_secs(3))
+        );
+        assert_eq!(map.linkage(), (Some(3), Some(3)));
+        assert_eq!(map.entry_links(3), Some((None, None)));
+
+        assert!(map.remove(3).is_some());
+        assert_eq!(map.next_timeout_deadline(), None);
+        assert_eq!(map.latest_timeout_deadline(), None);
+        assert_eq!(map.linkage(), (None, None));
+    }
+
+    #[test]
+    fn pending_map_bounded_expiration_matches_exhaustive_order() {
+        let now = Instant::now();
+        let probes = [
+            pending(1, now + Duration::from_secs(1)),
+            pending(2, now + Duration::from_secs(2)),
+            pending(3, now + Duration::from_secs(3)),
+            pending(4, now + Duration::from_secs(4)),
+        ];
+        let mut bounded = PendingMap::new(probes.len());
+        let mut exhaustive = PendingMap::new(probes.len());
+        for probe in probes.iter().cloned() {
+            insert(&mut bounded, probe.clone());
+            insert(&mut exhaustive, probe);
+        }
+
+        let empty = bounded.drain_expired_bounded(now + Duration::from_secs(4), 0);
+        assert!(empty.probes.is_empty());
+        assert!(empty.more_due);
+        assert_eq!(bounded.linkage(), (Some(1), Some(4)));
+
+        let mut actual = Vec::new();
+        loop {
+            let batch = bounded.drain_expired_bounded(now + Duration::from_secs(4), 2);
+            actual.extend(batch.probes.into_iter().map(|probe| probe.wire_seq));
+            if !batch.more_due {
+                break;
+            }
+        }
+        let expected = exhaustive
+            .drain_expired_bounded(now + Duration::from_secs(4), usize::MAX)
+            .probes
+            .into_iter()
+            .map(|probe| probe.wire_seq)
+            .collect::<Vec<_>>();
+
+        assert_eq!(actual, expected);
+        assert_eq!(actual, vec![1, 2, 3, 4]);
+        assert_eq!(bounded.linkage(), (None, None));
+    }
+
+    #[test]
+    fn pending_map_sequence_reuse_leaves_no_stale_link() {
+        let mut map = PendingMap::new(1);
+        let now = Instant::now();
+        insert(&mut map, pending(7, now));
+        assert!(map.remove(7).is_some());
+        insert(&mut map, pending(7, now + Duration::from_secs(1)));
+
+        assert!(map.drain_expired_bounded(now, usize::MAX).probes.is_empty());
+        assert_eq!(map.linkage(), (Some(7), Some(7)));
+        assert_eq!(map.entry_links(7), Some((None, None)));
+        assert_eq!(
+            map.drain_expired_bounded(now + Duration::from_secs(1), usize::MAX)
+                .probes
+                .into_iter()
+                .map(|probe| probe.wire_seq)
+                .collect::<Vec<_>>(),
+            vec![7]
+        );
+        assert_eq!(map.linkage(), (None, None));
+    }
+
+    #[test]
+    fn pending_map_commit_uses_only_preflighted_map_capacity() {
+        let mut map = PendingMap::new(2);
+        let now = Instant::now();
+        map.preflight_insert(7).unwrap();
+        let capacity = map.capacity();
+
+        map.commit_insert(pending(7, now));
+
+        assert_eq!(map.capacity(), capacity);
+        assert_eq!(map.len(), 1);
+        assert_eq!(map.linkage(), (Some(7), Some(7)));
+    }
+
+    #[test]
+    fn pending_map_mixed_removals_preserve_links() {
+        let mut map = PendingMap::new(4);
+        let now = Instant::now();
+        for seq in 1..=4 {
+            insert(
+                &mut map,
+                pending(seq, now + Duration::from_secs(u64::from(seq))),
+            );
+        }
+
+        assert!(map.remove(2).is_some());
+        assert_eq!(map.entry_links(1), Some((None, Some(3))));
+        assert_eq!(map.entry_links(3), Some((Some(1), Some(4))));
+        let expired = map.drain_expired_bounded(now + Duration::from_secs(3), usize::MAX);
+        assert_eq!(
+            expired
+                .probes
+                .into_iter()
+                .map(|probe| probe.wire_seq)
+                .collect::<Vec<_>>(),
+            vec![1, 3]
+        );
+        assert_eq!(map.linkage(), (Some(4), Some(4)));
+        assert!(map.remove(4).is_some());
+        assert_eq!(map.linkage(), (None, None));
+    }
+
     #[test]
     fn bounded_probe_tracking_evicts_oldest_entries() {
         let mut set = CompletedSet::new(3);

crates/irtt-client/src/session/machine.rs

@@ -157,6 +157,12 @@ pub(crate) struct ProbeCommit {
     next_packets_sent: u64,
 }
 
+#[derive(Debug)]
+pub(crate) struct TimeoutBatch {
+    pub events: Vec<ClientEvent>,
+    pub more_due: bool,
+}
+
 #[derive(Debug, Clone, Copy)]
 pub(crate) struct ProbeSent {
     pub(crate) seq: u32,
@@ -408,11 +414,21 @@ impl SessionMachine {
         &mut self,
         now: Instant,
     ) -> Result<Vec<ClientEvent>, ClientError> {
+        let batch = self.poll_timeouts_bounded_at(now, usize::MAX)?;
+        debug_assert!(!batch.more_due);
+        Ok(batch.events)
+    }
+
+    pub(crate) fn poll_timeouts_bounded_at(
+        &mut self,
+        now: Instant,
+        limit: usize,
+    ) -> Result<TimeoutBatch, ClientError> {
         let session = self.open_session_mut()?;
 
-        let expired = session.pending.drain_expired(now);
-        let mut events = Vec::with_capacity(expired.len());
-        for probe in expired {
+        let expired = session.pending.drain_expired_bounded(now, limit);
+        let mut events = Vec::with_capacity(expired.probes.len());
+        for probe in expired.probes {
             events.push(ClientEvent::EchoLoss {
                 seq: probe.wire_seq,
                 sent_at: probe.sent_at,
@@ -421,7 +437,10 @@ impl SessionMachine {
             session.timed_out.insert(probe);
         }
 
-        Ok(events)
+        Ok(TimeoutBatch {
+            events,
+            more_due: expired.more_due,
+        })
     }
 
     pub(crate) fn prepare_close(&self) -> Result<PreparedClose<'_>, ClientError> {
@@ -1019,6 +1038,25 @@ impl SessionMachine {
     }
 }
 
+#[cfg(all(test, feature = "tokio"))]
+impl SessionMachine {
+    pub(crate) fn remove_pending_for_test(&mut self, wire_seq: u32) -> Option<PendingProbe> {
+        self.open_session_mut()
+            .expect("test pending probes require an open session")
+            .pending
+            .remove(wire_seq)
+    }
+
+    pub(crate) fn replace_pending_for_test(&mut self, probe: PendingProbe) {
+        let session = self
+            .open_session_mut()
+            .expect("test pending probes require an open session");
+        session.pending.remove(probe.wire_seq);
+        session.pending.preflight_insert(probe.wire_seq).unwrap();
+        session.pending.commit_insert(probe);
+    }
+}
+
 #[cfg(test)]
 mod tests {
     use super::*;
@@ -1803,6 +1841,34 @@ mod tests {
         assert_eq!(active(&machine).pending.len(), 0);
     }
 
+    #[test]
+    fn exhaustive_timeout_polling_returns_every_due_loss() {
+        let mut machine = open_machine(4, Duration::from_secs(1));
+        let now = Instant::now();
+        for seq in [2, 1, 0] {
+            let sent_at = timestamp(now - Duration::from_secs(u64::from(seq) + 1));
+            let session = active_mut(&mut machine);
+            session.pending.preflight_insert(seq).unwrap();
+            session.pending.commit_insert(PendingProbe {
+                wire_seq: seq,
+                sent_at,
+                timeout_at: sent_at.mono + Duration::from_secs(1),
+            });
+        }
+
+        let events = machine.poll_timeouts_at(now).unwrap();
+        assert!(matches!(
+            events.as_slice(),
+            [
+                ClientEvent::EchoLoss { seq: 2, .. },
+                ClientEvent::EchoLoss { seq: 1, .. },
+                ClientEvent::EchoLoss { seq: 0, .. },
+            ]
+        ));
+        assert_eq!(active(&machine).pending.len(), 0);
+        assert_eq!(active(&machine).timed_out.len(), 3);
+    }
+
     #[test]
     fn wrapping_sequence_from_max_to_zero_remains_valid() {
         let mut machine = open_machine(2, Duration::from_secs(1));