use lstat when checking lifetime so a symlinked lock file is not followed#550
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dxbjavid wants to merge 1 commit into
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use lstat when checking lifetime so a symlinked lock file is not followed#550dxbjavid wants to merge 1 commit into
dxbjavid wants to merge 1 commit into
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Jun 6, 2026
…#551) A process sharing the lock directory on the same host could swap a held soft lock file for a symlink pointing at an old file, so the lifetime check saw the target's stale mtime, a waiter broke the still-live lock, and two processes held it at once. 🔒 This builds on @dxbjavid's #550, which switched the lifetime check to `os.lstat` so it reads the symlink's own mtime, and closes the remaining gaps in the same stale-breaking path. Breaking a stale lock now claims the file by inode before removing it. A shared `break_lock_file` helper renames the lock to a process-private name, re-checks the modification time, and unlinks only when it still matches the value seen at the stale decision. A newer time means a peer recreated the lock in the gap, so the helper leaves the file in place rather than deleting it out from under a live holder. Both the soft lock and the lifetime path use this helper, replacing the duplicated rename logic. Malformed lock files self-heal more reliably too: a non-integer PID or creation time now counts as unparseable, so a two or three line garbage file gets evicted once it ages past the safety window instead of wedging acquirers forever. `AsyncReadWriteLock` gained a destructor that shuts down the single-thread executor it owns, so forgetting to call `close()` no longer leaks the worker thread, while a caller-supplied executor stays untouched. Reviewers should know that releasing a soft lock may now remove the lock file as part of self-healing, and that filelock evicts malformed files automatically after a brief safety window. Supersedes #550. 🙏 --------- Co-authored-by: dxbjavid <dxbjavid@gmail.com>
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reading the code I noticed _try_break_expired_lock uses Path(self.lock_file).stat() to decide if a lock is expired, which follows symlinks. a same-host process with write access to the lock directory can swap a held lock file for a symlink onto an old file, so stat sees the stale target mtime and a waiter breaks the live lock and acquires it, so two holders coexist. lstat reads the symlink's own mtime instead, matching the O_NOFOLLOW lock-file reads already in _soft.py and _soft_rw.