lock: avoid unintentional lock acquisition at failure of readlock Acquiring lock by vfs.makelock() and getting lock holder (aka "locker") information by vfs.readlock() aren't atomic action. Therefore, failure of the former doesn't ensure success of the latter. Before this patch, lock is unintentionally acquired, if ENOENT causes failure of vfs.readlock() while 5 times retrying, because lock._trylock() returns to caller silently after retrying, and lock.lock() assumes that lock._trylock() returns successfully only if lock is acquired. In this case, lock symlink (or file) isn't created, even though lock is treated as acquired in memory. To avoid this issue, this patch makes lock._trylock() raise LockHeld(EAGAIN) at the end of it, if lock isn't acquired while retrying. An empty "locker" meaning "busy for frequent lock/unlock by many processes" might appear in an abortion message, if lock acquisition fails. Therefore, this patch also does: - use '%r' to increase visibility of "locker", even if it is empty - show hint message to explain what empty "locker" means