fix(fork): an access-time update no longer discards a fresh fork validation

[teamleaderleo]

Summary

The sharedForkProbe* executable-watch tests in WorkspaceForkConversationContextMenuTests fail intermittently: on #14785, and on main in run 36197594615. The failures:

• ...InvalidatesWhenExecutableChanges
• ...InvalidatesWhenPathDirectorySymlinkRetargets
• ...SharesExecutableWatchAcrossPanelsWithSameExecutable
• ...InvalidatesWhenEarlierPathExecutableAppearsForPiAndOmp

In each one, prepareForkAvailabilityProbe returns false right after refreshForkAvailabilityNow.

Root cause

applyPendingForkValidations does two things around the probe:

1. Before running the fork capability probe, it installs a vnode watch on the executable and its directories.
2. It stores the validation only if the watch generation is unchanged afterwards. The watch also drops the validation on any later event.

The probe's first run of a freshly written or updated executable updates the file's access time, and Darwin reports that as NOTE_ATTRIB. I reproduced it with a standalone kqueue script. Over 5 fresh files each, the first exec fired NOTE_ATTRIB 5/5 times. When the file had already been executed once before the watch was installed, it fired 0/5. The probe was invalidating its own result. Whether the test saw it depended on when the main-actor invalidation ran relative to the check, which is why it flakes. Real users hit the same thing once after installing or updating an agent binary: fork availability is dropped and has to be re-probed.

The file-descriptor headroom theory is ruled out. When the watch budget is exhausted, the validation is still stored with refreshBeforeReuse, and prepareForkAvailabilityProbe returns true (see sharedForkProbeValidationRefreshesBeforeReuseWhenWatchPathBudgetIsExceeded). The app also raises RLIMIT_NOFILE to 65536 at startup.

Change

An attrib-only event now invalidates only when the inode's ctime changed. A real chmod, chown or xattr change moves ctime; an access-time-only update does not. Writes, deletes, renames and link changes invalidate exactly as before.

New regression test: sharedForkProbeValidationSurvivesTheProbeRunningAFreshExecutable. It probes a just-written pi, waits 300 ms for the watch event, then asserts the validation is still there.

Test plan

• CI app-host changed suites: WorkspaceForkConversationContextMenuTests green

🤖 Generated with Claude Code

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Summary by cubic

Fixes a flaky fork-availability validation that was being discarded by the probe that created it. The first run of a freshly written or updated executable updates its access time, which Darwin reports as NOTE_ATTRIB; the executable watch treated that as a real change and invalidated the validation it had just produced. Real changes (chmod, chown, xattr) still work as before.

• An attrib-only event now invalidates only when the inode's ctime changed; an access-time-only update leaves ctime untouched and no longer drops the validation.
• Adds sharedForkProbeValidationSurvivesTheProbeRunningAFreshExecutable to cover the regression.

^{Written for commit c74ec13. Summary will update on new commits.}

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Reviewing files that changed from the base of the PR and between 6f3af0a and c74ec13.

📒 Files selected for processing (2)

• Sources/SharedLiveAgentIndex.swift
• cmuxTests/WorkspaceForkConversationContextMenuTests.swift

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Merge receipt for c74ec13b2d: every check was green at merge (16 verified; 14 skipped by policy). Full suite runs on main after merge.