InterProcessLock does not actually lock?

[slhck]

Maybe there is something that I don't understand about locks, but this code allows me to run two instances of the same program concurrently:

lock = fasteners.InterProcessLock('.lock')

with lock:
    # do some work

Whereas if I really want exclusive access, I have to implement something like:

lock = fasteners.InterProcessLock(".lock")
locked = lock.acquire(blocking=False)

if locked:
    try:
        # do some work
    except Exception as exc:
        raise exc
    finally:
        lock.release()
else:
    raise RuntimeError("Could not acquire lock, is another process runnning?")

In other words, what is the purpose of the context manager function if it does not actually enforce the lock?

[psarka]

Hi, could you, please, provide more info, in particular your OS and fasteners version? I cannot reproduce your issue, and if I run two instances of the program you provided I get the expected exclusive behavior. In particular, I modified your example like this and ran in two terminals:

import time
import fasteners

lock=fasteners.InterProcessLock('.lock')

with lock:
    print('hi')
    time.sleep(10)

[slhck]

Oh, I get it now, the difference is that it's blocking by default! In your example it works — it does lock. I will close this. Thanks for your quick response!

Still: is there any way to get the context manager behavior working with a lock that does not block but raises an exception immediately?

[psarka]

No, context managers are not flexible and always block.

I thought about this at some point (context managers raising timeout exceptions), but don't remember now what was the conclusion :roll_eyes:

[slhck]

No problem, I can live with the example I gave above.