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ejc3
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…hang Replace the racy double-rebind approach with a deterministic handshake chain that guarantees the exec server's AsyncFd epoll is re-registered before the host starts health-checking. Reduces restore-to-healthy from ~61s to ~0.5s. ## The Problem After snapshot restore, Firecracker's vsock transport reset (VIRTIO_VSOCK_EVENT_TRANSPORT_RESET) leaves the exec server's AsyncFd epoll registration stale. The previous fix (c15aa6b) removed the duplicate rebind signal from agent.rs but left a timing gap: if the restore-epoch watcher's single signal arrived late, the host's health monitor would start exec calls against a stale listener, hanging for ~60s until the kernel's vsock cleanup expired the stale connections. ## Trace Evidence (the smoking gun) From the vsock muxer log of a failing run (vm-ba97c): T+0.009s Exec call #1 → WORKS (167+144+176+123+71+27 bytes response) T+0.076s Exec call #2 → WORKS T+0.520s Exec call #3 → guest ACKs, receives request, sends NOTHING → 5s timeout T+5.5-55s Exec calls #4-#9 → same pattern: kernel accepts, app never processes T+60.5s Guest sends RST for ALL stale connections simultaneously T+60.5s Exec call #10 → WORKS → "container running status running=true" The container started at T+0.28s. The exec server was broken for 60 more seconds because the duplicate re_register() from agent.rs corrupted the edge-triggered epoll: the old AsyncFd consumed the edge notification, and the new AsyncFd never received events for pending connections. ## The Fix: Deterministic Handshake Chain exec_rebind_signal → exec_re_register → rebind_done → output.reconnect() ↓ host accepts output connection ↓ health monitor spawns Every transition has an explicit signal. Zero timing dependencies. ### fc-agent side (4 files): - exec.rs: After re_register(), signals rebind_done (AtomicBool + Notify) - restore.rs: Signals exec rebind, waits for rebind_done confirmation (5s timeout), THEN reconnects output vsock - agent.rs: Removed duplicate rebind signal after notify_cache_ready_and_wait; added exec_rebind_done/exec_rebind_done_notify Arcs - mmds.rs: Threads new params through watch_restore_epoch to both handle_clone_restore call sites ### Host side (3 files): - listeners.rs: Added connected_tx oneshot to run_output_listener(), fired on first output connection accept - snapshot.rs: Waits for output_connected_rx (30s timeout) before spawning health monitor; removed stale output_reconnect.notify_one() for startup snapshots - podman/mod.rs: Passes None for connected_tx (non-snapshot path) ## Results Before: restore-to-healthy = ~61s (exec broken, 9 consecutive 5s timeouts) After: restore-to-healthy = ~0.5s (35ms to output connected, 533ms to healthy) Post-restore exec stress test: 10 parallel calls completed in 16.3ms (max single: 15.3ms), zero timeouts. Tested: make test-root FILTER=localhost_rootless_btrfs_snapshot_restore STREAM=1
ejc3
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Mar 2, 2026
Use --merge instead of --squash for PRs
ejc3
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Mar 2, 2026
…hang Replace the racy double-rebind approach with a deterministic handshake chain that guarantees the exec server's AsyncFd epoll is re-registered before the host starts health-checking. Reduces restore-to-healthy from ~61s to ~0.5s. ## The Problem After snapshot restore, Firecracker's vsock transport reset (VIRTIO_VSOCK_EVENT_TRANSPORT_RESET) leaves the exec server's AsyncFd epoll registration stale. The previous fix (c15aa6b) removed the duplicate rebind signal from agent.rs but left a timing gap: if the restore-epoch watcher's single signal arrived late, the host's health monitor would start exec calls against a stale listener, hanging for ~60s until the kernel's vsock cleanup expired the stale connections. ## Trace Evidence (the smoking gun) From the vsock muxer log of a failing run (vm-ba97c): T+0.009s Exec call #1 → WORKS (167+144+176+123+71+27 bytes response) T+0.076s Exec call #2 → WORKS T+0.520s Exec call #3 → guest ACKs, receives request, sends NOTHING → 5s timeout T+5.5-55s Exec calls #4-#9 → same pattern: kernel accepts, app never processes T+60.5s Guest sends RST for ALL stale connections simultaneously T+60.5s Exec call #10 → WORKS → "container running status running=true" The container started at T+0.28s. The exec server was broken for 60 more seconds because the duplicate re_register() from agent.rs corrupted the edge-triggered epoll: the old AsyncFd consumed the edge notification, and the new AsyncFd never received events for pending connections. ## The Fix: Deterministic Handshake Chain exec_rebind_signal → exec_re_register → rebind_done → output.reconnect() ↓ host accepts output connection ↓ health monitor spawns Every transition has an explicit signal. Zero timing dependencies. ### fc-agent side (4 files): - exec.rs: After re_register(), signals rebind_done (AtomicBool + Notify) - restore.rs: Signals exec rebind, waits for rebind_done confirmation (5s timeout), THEN reconnects output vsock - agent.rs: Removed duplicate rebind signal after notify_cache_ready_and_wait; added exec_rebind_done/exec_rebind_done_notify Arcs - mmds.rs: Threads new params through watch_restore_epoch to both handle_clone_restore call sites ### Host side (3 files): - listeners.rs: Added connected_tx oneshot to run_output_listener(), fired on first output connection accept - snapshot.rs: Waits for output_connected_rx (30s timeout) before spawning health monitor; removed stale output_reconnect.notify_one() for startup snapshots - podman/mod.rs: Passes None for connected_tx (non-snapshot path) ## Results Before: restore-to-healthy = ~61s (exec broken, 9 consecutive 5s timeouts) After: restore-to-healthy = ~0.5s (35ms to output connected, 533ms to healthy) Post-restore exec stress test: 10 parallel calls completed in 16.3ms (max single: 15.3ms), zero timeouts. Tested: make test-root FILTER=localhost_rootless_btrfs_snapshot_restore STREAM=1
ejc3
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that referenced
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Mar 2, 2026
Use --merge instead of --squash for PRs
ejc3
added a commit
that referenced
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Mar 2, 2026
…hang Replace the racy double-rebind approach with a deterministic handshake chain that guarantees the exec server's AsyncFd epoll is re-registered before the host starts health-checking. Reduces restore-to-healthy from ~61s to ~0.5s. ## The Problem After snapshot restore, Firecracker's vsock transport reset (VIRTIO_VSOCK_EVENT_TRANSPORT_RESET) leaves the exec server's AsyncFd epoll registration stale. The previous fix (c15aa6b) removed the duplicate rebind signal from agent.rs but left a timing gap: if the restore-epoch watcher's single signal arrived late, the host's health monitor would start exec calls against a stale listener, hanging for ~60s until the kernel's vsock cleanup expired the stale connections. ## Trace Evidence (the smoking gun) From the vsock muxer log of a failing run (vm-ba97c): T+0.009s Exec call #1 → WORKS (167+144+176+123+71+27 bytes response) T+0.076s Exec call #2 → WORKS T+0.520s Exec call #3 → guest ACKs, receives request, sends NOTHING → 5s timeout T+5.5-55s Exec calls #4-#9 → same pattern: kernel accepts, app never processes T+60.5s Guest sends RST for ALL stale connections simultaneously T+60.5s Exec call #10 → WORKS → "container running status running=true" The container started at T+0.28s. The exec server was broken for 60 more seconds because the duplicate re_register() from agent.rs corrupted the edge-triggered epoll: the old AsyncFd consumed the edge notification, and the new AsyncFd never received events for pending connections. ## The Fix: Deterministic Handshake Chain exec_rebind_signal → exec_re_register → rebind_done → output.reconnect() ↓ host accepts output connection ↓ health monitor spawns Every transition has an explicit signal. Zero timing dependencies. ### fc-agent side (4 files): - exec.rs: After re_register(), signals rebind_done (AtomicBool + Notify) - restore.rs: Signals exec rebind, waits for rebind_done confirmation (5s timeout), THEN reconnects output vsock - agent.rs: Removed duplicate rebind signal after notify_cache_ready_and_wait; added exec_rebind_done/exec_rebind_done_notify Arcs - mmds.rs: Threads new params through watch_restore_epoch to both handle_clone_restore call sites ### Host side (3 files): - listeners.rs: Added connected_tx oneshot to run_output_listener(), fired on first output connection accept - snapshot.rs: Waits for output_connected_rx (30s timeout) before spawning health monitor; removed stale output_reconnect.notify_one() for startup snapshots - podman/mod.rs: Passes None for connected_tx (non-snapshot path) ## Results Before: restore-to-healthy = ~61s (exec broken, 9 consecutive 5s timeouts) After: restore-to-healthy = ~0.5s (35ms to output connected, 533ms to healthy) Post-restore exec stress test: 10 parallel calls completed in 16.3ms (max single: 15.3ms), zero timeouts. Tested: make test-root FILTER=localhost_rootless_btrfs_snapshot_restore STREAM=1
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