Re: locking files on Linux

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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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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

Thread

locking files on Linux andrea crotti <andrea.crotti.0@gmail.com> - 2012-10-18 14:14 +0100
  Re: locking files on Linux Grant Edwards <invalid@invalid.invalid> - 2012-10-18 13:27 +0000
    Re: locking files on Linux andrea crotti <andrea.crotti.0@gmail.com> - 2012-10-18 14:44 +0100
      Re: locking files on Linux Grant Edwards <invalid@invalid.invalid> - 2012-10-18 13:57 +0000
        Re: locking files on Linux andrea crotti <andrea.crotti.0@gmail.com> - 2012-10-18 15:49 +0100
        Re: locking files on Linux Oscar Benjamin <oscar.j.benjamin@gmail.com> - 2012-10-18 16:04 +0100
        Re: locking files on Linux andrea crotti <andrea.crotti.0@gmail.com> - 2012-10-18 16:08 +0100
        Re: locking files on Linux Oscar Benjamin <oscar.j.benjamin@gmail.com> - 2012-10-18 16:31 +0100
        Re: locking files on Linux andrea crotti <andrea.crotti.0@gmail.com> - 2012-10-19 13:15 +0100
      Re: locking files on Linux Nobody <nobody@nowhere.com> - 2012-10-19 15:36 +0100
    Re: locking files on Linux Oscar Benjamin <oscar.j.benjamin@gmail.com> - 2012-10-18 14:58 +0100

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