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Date: Thu, 18 Oct 2012 16:04:43 +0100
Subject: Re: locking files on Linux
From: Oscar Benjamin <oscar.j.benjamin@gmail.com>
To: andrea crotti <andrea.crotti.0@gmail.com>
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Xref: csiph.com comp.lang.python:31649

On 18 October 2012 15:49, andrea crotti <andrea.crotti.0@gmail.com> wrote:
> 2012/10/18 Grant Edwards <invalid@invalid.invalid>:
>>
>> If what you're guarding against is multiple instances of your
>> application modifying the file, then either of the advisory file
>> locking schemes or the separate lock file should work fine.
>
> Ok so I tried a small example to see if I can make it fail, but this
> below just works perfectly fine.
>
> Maybe it's too fast and it release the file in time, but I would
> expect it to take some time and fail instead..

Why not come up with a test that actually shows you if it works? Here
are two suggestions:

1) Use time.sleep() so that you know how long the lock is held for.
2) Write different data into the file from each process and see what
you end up with.

>
> import fcntl
>
> from multiprocessing import Process
>
> FILENAME = 'file.txt'
>
>
> def long_text():
>     return ('some text' * (100 * 100))
>
>
> class Locked:
>     def __init__(self, fileobj):
>         self.fileobj = fileobj
>
>     def __enter__(self):
>         # any problems here?
>         fcntl.lockf(self.fileobj, fcntl.LOCK_EX)
>         return self.fileobj
>
>     def __exit__(self, type, value, traceback):
>         fcntl.lockf(self.fileobj, fcntl.LOCK_UN)
>
>
> def write_to_file():
>     with open(FILENAME, 'w') as to_lock:

I don't think it will work if you truncate the file like this. This
will empty the file *before* checking for the lock. Try opening the
file for reading and writing (without truncating).

>         with Locked(to_lock):
>             to_lock.write(long_text())
>
>
> if __name__ == '__main__':
>     Process(target=write_to_file).start()
>     Process(target=write_to_file).start()


Oscar