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Jason Baldwin
8166185a0f
LibraryLock freed the library by letting its File close. On Unix the
flock lock belongs to the open file description and lasts while any
descriptor on it is open. When another thread spawns a process,
posix_spawn copies the whole descriptor table into the child and
O_CLOEXEC only closes the copies at execve, so a LibraryLock dropped in
that window stayed locked until the child exec'd. Re-acquiring the
library then failed with LockError::InUse(None) ("already open in
another LightCraft program") although nobody had it open. That is the
flake in tests_lock::second_opener_is_refused_until_the_first_lets_go,
whose neighbour lock_held_by_another_process_is_released_when_it_dies
spawns the test binary on a parallel libtest thread.
Drop now removes the owner note while still holding the lock (so it
can't delete the next holder's), then calls File::unlock(): LOCK_UN
through our descriptor frees the lock whatever copies a child still
holds. An unlock error is logged, never a panic; the handle still closes
right after, which releases the lock once every copy is gone, as before.
Tests: the_library_is_free_once_dropped_even_if_a_child_holds_a_copy
holds a try_clone of the lock's descriptor across the drop (the same
open file description a child gets between fork and exec) and
re-acquires; dropping_the_lock_frees_the_library_while_another_thread_
spawns_processes runs a thread spawning /bin/true against two seconds of
acquire/drop cycles. Both fail every run without the fix (the first
deterministically) and pass with it; the flaky pair went from 3/100 to
0/100 failed runs here. Both are cfg(unix): Windows handles aren't
inherited by CreateProcess.