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Colby MchenryandClaude Opus 5.5
29ce0149cf
Issue #2979: FileLock.acquire() read the lock file, saw a dead PID, unlinked the file and created its own with an exclusive create. Two processes that read the same dead PID could both succeed: the first one replaced the stale lock, then the second one, acting on what it had read a moment earlier, unlinked the first one's live lock and created its own. Both returned from acquire() and wrote to the index as its only writer. The same steal happened in two narrower ways: a lock file read in the instant between its exclusive create and the write of its PID was empty, so it counted as stale, and a PID the process may not signal (EPERM: another user's process, or an elevated one on Windows) counted as dead. The fix keeps the lock file and its format (a bare PID) and changes how it is taken: - A free lock costs one create, as before. The lock file is created with its PID already in it: a private file hard-linked into place, which fails if the lock exists. Where hard links are missing it falls back to the exclusive create, and an empty lock younger than 10 s counts as held, since its creator is still writing its PID. - A stale lock (dead PID, malformed, or empty and old) is replaced only by the process holding <lock>.break, itself a FileLock, after reading the lock again there, the way Mercurial breaks its locks. A break lock a crashed process left is broken the same way, one level up, at most three levels. - EPERM from a signal-0 probe means the holder is alive. Validation: __tests__/filelock-stale-takeover.test.ts runs two real processes with a barrier: the slow one stops right after reading the dead PID and resumes only once the other holds the lock. On main both held it, 5 runs of 5; with the fix the slow one is refused and the lock stays the holder's. The other cases cover the slow one getting in once the lock is free, a live and a dead break lock, the empty-lock window, EPERM (mocked, and for real on Windows with the System process), and no leftover files. Fixes #2979 Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>