feat(crawler): coordinated browser restart gate in BrowserManager

Adds a Healthy/Draining/Restarting lifecycle gate. `acquire()` parks while
a restart is in progress; `coordinated_restart(deadline)` blocks new
acquires, drains in-flight leases (bounded, then forces), closes +
relaunches Chromium (re-running on_launch → re-inject session + probe),
then resumes parked acquirers. Concurrent restart requests collapse into
one relaunch; the phase always returns to Healthy so a failed relaunch
never wedges acquisition.

The metadata pass cooperates via is_restart_pending() at its per-manga
checkpoint, yielding its long-lived lease (recovery sweep next tick)
instead of stalling the drain.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

backend/src/crawler/browser_manager.rs

@@ -13,7 +13,7 @@
 //! until [`BrowserManager::shutdown`].
 
 use std::ops::Deref;
-use std::sync::atomic::{AtomicUsize, Ordering};
+use std::sync::atomic::{AtomicU8, AtomicUsize, Ordering};
 use std::sync::Arc;
 use std::time::Duration;
 
@@ -71,12 +71,38 @@ impl ActiveTracker {
     }
 }
 
+/// Lifecycle gate for a coordinated browser restart. `acquire()` parks
+/// while not [`RestartPhase::Healthy`] so no new navigation starts mid-
+/// restart; long-lived lease holders (the metadata pass) cooperate by
+/// checking [`BrowserManager::is_restart_pending`] at safe boundaries.
+#[derive(Clone, Copy, PartialEq, Eq, Debug)]
+pub enum RestartPhase {
+    /// Normal operation — acquires proceed.
+    Healthy,
+    /// Restart requested; new acquires park, waiting for in-flight leases
+    /// to drain.
+    Draining,
+    /// Chromium is being closed + relaunched.
+    Restarting,
+}
+
+const PHASE_HEALTHY: u8 = 0;
+const PHASE_DRAINING: u8 = 1;
+const PHASE_RESTARTING: u8 = 2;
+
 pub struct BrowserManager {
     inner: Mutex<Inner>,
     active: Arc<ActiveTracker>,
     launch_opts: LaunchOptions,
     idle_timeout: Duration,
     on_launch: OnLaunch,
+    /// Coarse lifecycle phase (one of the `PHASE_*` constants).
+    phase: AtomicU8,
+    /// Woken when the phase returns to `Healthy` so parked acquires resume.
+    resume: Notify,
+    /// Serialises coordinated restarts so concurrent requests collapse into
+    /// a single relaunch.
+    restart_lock: Mutex<()>,
 }
 
 struct Inner {
@@ -99,28 +125,71 @@ impl BrowserManager {
             launch_opts,
             idle_timeout,
             on_launch,
+            phase: AtomicU8::new(PHASE_HEALTHY),
+            resume: Notify::new(),
+            restart_lock: Mutex::new(()),
         })
     }
 
+    /// Current restart phase.
+    pub fn phase(&self) -> RestartPhase {
+        match self.phase.load(Ordering::Acquire) {
+            PHASE_DRAINING => RestartPhase::Draining,
+            PHASE_RESTARTING => RestartPhase::Restarting,
+            _ => RestartPhase::Healthy,
+        }
+    }
+
+    fn set_phase(&self, phase: RestartPhase) {
+        let v = match phase {
+            RestartPhase::Healthy => PHASE_HEALTHY,
+            RestartPhase::Draining => PHASE_DRAINING,
+            RestartPhase::Restarting => PHASE_RESTARTING,
+        };
+        self.phase.store(v, Ordering::Release);
+    }
+
+    /// Whether a coordinated restart is in progress. Long-lived lease
+    /// holders poll this at safe boundaries and yield their lease so the
+    /// drain can complete promptly.
+    pub fn is_restart_pending(&self) -> bool {
+        self.phase() != RestartPhase::Healthy
+    }
+
+    /// Launch Chromium into `guard`, running the `on_launch` hook before
+    /// publishing the handle so a probe failure doesn't leave a half-
+    /// initialised browser behind.
+    async fn launch_into(&self, guard: &mut Inner) -> anyhow::Result<()> {
+        let handle = browser::launch(self.launch_opts.clone())
+            .await
+            .context("BrowserManager: launch chromium")?;
+        let shared = handle.shared();
+        if let Err(e) = (self.on_launch)(Arc::clone(&shared)).await {
+            let _ = handle.close().await;
+            return Err(e.context("BrowserManager: on_launch hook failed"));
+        }
+        guard.handle = Some(handle);
+        guard.shared = Some(shared);
+        Ok(())
+    }
+
     /// Acquire a shared browser lease. The first acquire after a teardown
     /// launches a fresh Chromium (and runs `on_launch`); subsequent acquires
     /// while a process is alive just bump the counter and clone the `Arc`.
     pub async fn acquire(&self) -> anyhow::Result<BrowserLease> {
+        // Park while a coordinated restart is draining/relaunching so no new
+        // navigation starts against a browser that's about to be torn down.
+        // The short sleep fallback guarantees liveness even if a `resume`
+        // notification is missed (classic Notify lost-wakeup).
+        while self.phase() != RestartPhase::Healthy {
+            tokio::select! {
+                _ = self.resume.notified() => {}
+                _ = tokio::time::sleep(Duration::from_millis(100)) => {}
+            }
+        }
         let mut guard = self.inner.lock().await;
         if guard.handle.is_none() {
-            let handle = browser::launch(self.launch_opts.clone())
-                .await
-                .context("BrowserManager: launch chromium")?;
-            let shared = handle.shared();
-            // Run the on-launch hook before publishing the handle so a session
-            // probe failure doesn't leave a half-initialized browser behind.
-            if let Err(e) = (self.on_launch)(Arc::clone(&shared)).await {
-                // Close the just-launched browser since we won't be using it.
-                let _ = handle.close().await;
-                return Err(e.context("BrowserManager: on_launch hook failed"));
-            }
-            guard.handle = Some(handle);
-            guard.shared = Some(shared);
+            self.launch_into(&mut guard).await?;
         }
         let browser = guard
             .shared
@@ -134,6 +203,45 @@ impl BrowserManager {
         })
     }
 
+    /// Coordinated restart: block new acquires, wait for in-flight leases
+    /// to drain (up to `drain_deadline`, then force), close + relaunch
+    /// Chromium (re-running `on_launch` → re-inject session + probe), then
+    /// resume parked acquirers. Concurrent calls collapse into one
+    /// relaunch. The phase is always returned to `Healthy` — even if the
+    /// relaunch errors — so a failed restart never permanently wedges
+    /// acquisition (the next acquire retries the launch lazily).
+    pub async fn coordinated_restart(&self, drain_deadline: Duration) -> anyhow::Result<()> {
+        // Dedup: if a restart is already running, wait for it and return.
+        let _restart_guard = match self.restart_lock.try_lock() {
+            Ok(g) => g,
+            Err(_) => {
+                let _ = self.restart_lock.lock().await;
+                return Ok(());
+            }
+        };
+
+        self.set_phase(RestartPhase::Draining);
+        await_drain(&self.active, drain_deadline).await;
+
+        self.set_phase(RestartPhase::Restarting);
+        let relaunch = {
+            let mut guard = self.inner.lock().await;
+            guard.shared = None;
+            if let Some(handle) = guard.handle.take() {
+                let _ = handle.close().await;
+            }
+            self.launch_into(&mut guard).await
+        };
+
+        self.set_phase(RestartPhase::Healthy);
+        self.resume.notify_waiters();
+        match &relaunch {
+            Ok(()) => tracing::info!("BrowserManager: coordinated restart complete"),
+            Err(e) => tracing::error!(error = ?e, "BrowserManager: coordinated restart relaunch failed"),
+        }
+        relaunch.context("coordinated_restart: relaunch")
+    }
+
     /// Forcefully close the cached browser regardless of active count.
     /// Used on daemon shutdown. After this returns the next acquire will
     /// re-launch from scratch.
@@ -176,6 +284,29 @@ impl BrowserManager {
     }
 }
 
+/// Wait for the active-lease count to reach zero, up to `deadline`. Wakes
+/// on the tracker's idle signal and re-checks on a short poll so a missed
+/// signal can't strand the drain. Returns when drained or when the
+/// deadline elapses (the caller then force-restarts). Extracted as a free
+/// fn so the timing logic is unit-testable without launching Chromium.
+async fn await_drain(active: &Arc<ActiveTracker>, deadline: Duration) {
+    let start = tokio::time::Instant::now();
+    while active.current() > 0 {
+        let Some(remaining) = deadline.checked_sub(start.elapsed()) else {
+            tracing::warn!(
+                active = active.current(),
+                "coordinated_restart: drain deadline exceeded — forcing relaunch"
+            );
+            return;
+        };
+        let nap = remaining.min(Duration::from_millis(250));
+        tokio::select! {
+            _ = active.idle_signal().notified() => {}
+            _ = tokio::time::sleep(nap) => {}
+        }
+    }
+}
+
 /// Background reaper. Returns immediately when `idle_timeout == 0`.
 /// Otherwise spawns a task that:
 ///   1. Waits on `idle_signal` (woken when active hits zero).
@@ -270,6 +401,63 @@ mod tests {
         mgr.invalidate().await;
     }
 
+    #[tokio::test]
+    async fn await_drain_returns_immediately_when_already_idle() {
+        let active = ActiveTracker::new();
+        let start = tokio::time::Instant::now();
+        await_drain(&active, Duration::from_secs(5)).await;
+        assert!(start.elapsed() < Duration::from_millis(200), "no wait when idle");
+    }
+
+    #[tokio::test]
+    async fn await_drain_completes_when_lease_released() {
+        let active = ActiveTracker::new();
+        active.acquire();
+        let bg = {
+            let a = Arc::clone(&active);
+            tokio::spawn(async move {
+                tokio::time::sleep(Duration::from_millis(100)).await;
+                a.release();
+            })
+        };
+        // Generous deadline; should return shortly after the release, not
+        // at the deadline.
+        let start = tokio::time::Instant::now();
+        await_drain(&active, Duration::from_secs(5)).await;
+        assert!(start.elapsed() < Duration::from_secs(2), "drained on release");
+        assert_eq!(active.current(), 0);
+        bg.await.unwrap();
+    }
+
+    #[tokio::test]
+    async fn await_drain_force_returns_after_deadline_when_stuck() {
+        let active = ActiveTracker::new();
+        active.acquire(); // never released
+        let start = tokio::time::Instant::now();
+        await_drain(&active, Duration::from_millis(300)).await;
+        let elapsed = start.elapsed();
+        assert!(elapsed >= Duration::from_millis(250), "waited ~deadline: {elapsed:?}");
+        assert!(elapsed < Duration::from_secs(2), "but not forever: {elapsed:?}");
+        assert_eq!(active.current(), 1, "still held — caller force-restarts");
+    }
+
+    #[test]
+    fn phase_transitions_reflect_is_restart_pending() {
+        let mgr = BrowserManager::new(
+            crate::crawler::browser::LaunchOptions::default(),
+            Duration::ZERO,
+            noop_on_launch(),
+        );
+        assert_eq!(mgr.phase(), RestartPhase::Healthy);
+        assert!(!mgr.is_restart_pending());
+        mgr.set_phase(RestartPhase::Draining);
+        assert!(mgr.is_restart_pending());
+        mgr.set_phase(RestartPhase::Restarting);
+        assert!(mgr.is_restart_pending());
+        mgr.set_phase(RestartPhase::Healthy);
+        assert!(!mgr.is_restart_pending());
+    }
+
     #[tokio::test]
     async fn active_tracker_signals_idle_only_on_zero_transition() {
         let tracker = ActiveTracker::new();

backend/src/crawler/pipeline.rs

@@ -192,6 +192,17 @@ pub async fn run_metadata_pass(
             }
         };
         for r in batch {
+            // Cooperative checkpoint: if a coordinated browser restart is
+            // pending, yield our (long-lived) lease so the drain can
+            // proceed instead of stalling for the rest of the walk. The
+            // pass exits un-clean, so the next tick recovery-sweeps the
+            // tail we didn't reach.
+            if browser_manager.is_restart_pending() {
+                tracing::info!(
+                    "metadata pass: browser restart pending — yielding (recovery sweep next tick)"
+                );
+                break 'outer;
+            }
             if max_refs.map(|m| stats.discovered >= m).unwrap_or(false) {
                 hit_limit = true;
                 tracing::info!(cap = ?max_refs, "max_results reached; halting walk");