fix(watch): prevent watcher resubscribe crashes

[jschwxrz]

• treats dropped fsevents notifications as a resync request instead of replacing a healthy subscription
• serializes genuine watcher restarts and ignores stale callbacks during recovery
• coalesces repeated TreeResource resync requests into one active refresh and one trailing refresh
• updates the resync contract to cover recoverable event gaps

[jschwxrz]

@greptileai

[greptile-apps]

Greptile Summary

This PR makes filesystem watcher recovery resilient to event gaps and restart races. The main changes are:

• Treats dropped FSEvents notifications as resync requests.
• Serializes watcher restarts and ignores stale callbacks.
• Keeps the current subscription active when unsubscribe fails.
• Coalesces repeated tree resync requests.
• Adds focused recovery and disposal tests.

Confidence Score: 5/5

This looks safe to merge.

• Failed unsubscribe attempts keep the existing watcher active and schedule another restart.
• Restart and disposal paths guard against stale callbacks and overlapping work.
• No blocking issues were found in the changed code.

Important Files Changed

Filename	Overview
packages/core/src/services/fs-watch/impl/native-watch.ts	Adds serialized restart handling, generation-based callback filtering, and recovery after unsubscribe failure.
packages/core/src/services/fs-watch/impl/native-watch.test.ts	Covers event gaps, restart retries, stale callbacks, unsubscribe failures, and disposal races.
packages/core/src/files/tree/file-tree.ts	Coalesces repeated resync requests and waits for active resync work during disposal.
packages/core/src/files/tree/file-tree.test.ts	Updates the manual watcher and verifies active-plus-trailing resync behavior.
packages/core/src/services/fs-watch/api/models.ts	Expands the resync callback contract to include native event gaps.

Flowchart

%%{init: {'theme': 'neutral'}}%%
flowchart TD
  E[Watcher callback] --> G{Current generation?}
  G -- No --> I[Ignore stale callback]
  G -- Yes --> R{Recoverable event gap?}
  R -- Yes --> C[Schedule resync]
  R -- No --> T[Schedule serialized restart]
  T --> U{Unsubscribe succeeds?}
  U -- No --> K[Restore current subscription]
  K --> C
  K --> T
  U -- Yes --> S{Replacement subscribes?}
  S -- No --> T
  S -- Yes --> N[Activate new generation]
  N --> C
  C --> F[Refresh file tree]
  F --> Q{Trailing request pending?}
  Q -- Yes --> F
  Q -- No --> D[Complete]

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flowchart TD
  E[Watcher callback] --> G{Current generation?}
  G -- No --> I[Ignore stale callback]
  G -- Yes --> R{Recoverable event gap?}
  R -- Yes --> C[Schedule resync]
  R -- No --> T[Schedule serialized restart]
  T --> U{Unsubscribe succeeds?}
  U -- No --> K[Restore current subscription]
  K --> C
  K --> T
  U -- Yes --> S{Replacement subscribes?}
  S -- No --> T
  S -- Yes --> N[Activate new generation]
  N --> C
  C --> F[Refresh file tree]
  F --> Q{Trailing request pending?}
  Q -- Yes --> F
  Q -- No --> D[Complete]

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_{Reviews (2): Last reviewed commit: "fix: retry recovery after unsubscribe fa..." | Re-trigger Greptile}

[arnestrickmann]

Thanks @jschwxrz