lock: fixed race condition in trylock/testlock (issue3506) Suppose the following scenario: 1. Process A takes the lock (e.g. on commit). 2. Process B wants to grab the lock. Since lock file exists the exception is raised. In the catch block the testlock function is called. 3. Process A releases the lock. 4. Process B tries to read the lock file as a part of testlock function. This results in OSError (ENOENT) and since we're not inside the exception handler function this is propagated and aborts the whole operation. To fix this we now check in testlock function whether lock file actually exists and if not (i.e. if readlock fails) we just return.