flaky(macOS): spawn-lock post-release re-acquire can transiently report contention (flock close-release lag)

[zackees]

Observed

daemon_ownership::tests::spawn_lock_is_exclusive_within_process failed on macOS CI (PR #1338 run, merge of current main): panicked at crates/fbuild-paths/src/daemon_ownership.rs:226:14: lock must be available after release — the post-release re-acquire through the error-propagating variant returned Ok(None) (kernel reported EWOULDBLOCK) immediately after the holder was dropped.

Analysis

• fbuild_core::file_lock::FileLockGuard releases by dropping the File (close-release). The soldr reference implementation this lock pattern was copied from (soldr-daemon/src/daemon/lifecycle/mod.rs) instead calls file.unlock() explicitly in Drop.
• This test family has macOS flake history: Flaky macOS CI: spawn-lock re-acquire and paused-time download retry test fail intermittently #1222 de-flaked the EINTR path; 5a0bc88 moved the post-release re-acquire assertion onto the error-propagating variant precisely because the swallowing wrapper can't distinguish contention from error.
• The prod contract already treats a transient None as benign (callers poll/retry / fall back to herd-spawn), so the test should assert the eventual property, not a single-shot one.

Fix (in PR #1338)

1. FileLockGuard::Drop performs an explicit unlock() before close, matching the soldr reference.
2. spawn_lock_is_exclusive_within_process re-acquires with a short bounded retry (~1 s deadline, 10 ms poll), still hard-failing on I/O errors — the lock MUST become available, just not necessarily on the first nanosecond after release.

Note

Main is currently red on Check (macos-latest) for this test, which blocks every open PR's macOS check until this lands.

[zackees]

Fixed by #1338: now unlocks explicitly in (matching soldr's lifecycle guard) and the post-release re-acquire assertions use a bounded 1s retry helper instead of asserting the first immediate attempt. Check (macos-latest) went green on the PR's round 3.

[zackees]

Correction — the previous comment lost its code formatting. The change: FileLockGuard now unlocks explicitly in Drop (matching soldr's lifecycle guard), and the post-release re-acquire assertions use a bounded 1s retry helper instead of asserting the first immediate attempt.