filelock: SoftFileLock evicts a fresh lock file whose watched age another local user's pre-dated file passed on through inode reuse

SoftFileLock and MarkerSoftFileLock evict an unparsable marker (the lock file at ``, whose presence says a process holds the lock) once a waiting contender has watched it unchanged for two seconds. SoftFileLease expires a lease the same way. The watch tells two looks apart by the marker's modification time (mtime) and inode number (the kernel's identity for a file, independent of its name) alone.

Another local user (the other user below) who can create files in the lock's directory, a sticky /tmp included (the sticky bit is a directory flag that lets only a file's owner remove it), parks an empty file of its own at the lock path. It dates that file to a future tick of the kernel's coarse clock (a clock that advances in steps of 4 ms), lets the waiters of the user running the lock (the lock user below) watch it past two seconds, and unlinks it in that tick. On a filesystem that hands a freed inode number straight back (ext4, overlayfs on ext4) the lock user's next marker repeats both values. It inherits the time spent watching the other user's file, and a second waiter removes it while its owner holds the lock.

Two of the lock user's processes then run the critical section together with no error. A SoftFileLease waiter takes a live lease that its owner wrote moments before, within milliseconds instead of after lease_duration. Affected versions: filelock >= 4.1.0 and