avoid script running twice

[Robin Becker]

Nick Craig-Wood wrote:
> Tim Williams <tim at tdw.net> wrote:
>>  On 18/06/07, Nick Craig-Wood <nick at craig-wood.com> wrote:
>>  On Windows the open-a-file-for-writing method works well, but as *nix
>>  doesn't work the same way then maybe the socket solution is the best
>>  cross-platform option.
> 
> Actually you could combine your solution and Jeff McNeil's solution to
> make something which should work on both windows and unix and is 100%
> guaranteed to release the lock on process exit.
> 
> import sys
> 
> try:
>     # use fcntl lock if we can
>     from fcntl import lockf, LOCK_EX, LOCK_NB
>     from errno import EAGAIN
>     locking = True
> except ImportError:
>     # otherwise platform mustn't open a file twice for write
>     if sys.platform != "win32":
>         raise AssertionError("Unsupported platform for locking")
>     locking = False
> 
> try:
>     fhandle = file("ourlockfile.txt", "w")
>     if locking:
>         lockf(fhandle.fileno(), LOCK_EX|LOCK_NB)
> except IOError, e:
>     if locking and e.errno != EAGAIN:
>         raise
>     print >>sys.stderr, "exiting, another copy currently running"
> 
> import time
> time.sleep(2)
> 
> (I tested it on linux only!)
> 

many interesting suggestions, unfortunately I'm not exactly sure about the 
filesystem to be used. I think it might be some kind of NFS which might impact 
some of these solutions.

-- 
Robin Becker