[Tutor] lockfiles and timestamps and nested if's!

[Michael P. Reilly]

kromag@nsacom.net wrote
> 
> I am attempting a silly thing. I wish to make a script that will:
> 
> 1. Check for the existence of a lockfile.
> 2. Check the age of the lockfile.
>  
> Then either:
> Delete the lockfile if it is too old and continue, 
> or
> exit gracefully.
> 
> I have managed to get myself into a dither here. The script for some reason 
> always writes a new lock file and updates the timestamp. Can someone clue me 
> in? My brain is full.
> 
> import glob
> import time
> import string
> import os
> import socket
> 
> now=time.time()
> mybox=socket.gethostname()+'.txt'
> whoa=glob.glob('\tmp\*')
> if whoa==['\tmp\'+mybox]:
> 	lockfile=open('\tmp\'+mybox, 'r')
> 	timestamp=lockfile.readline()
> 	print timestamp
> 	lockfile.close()
> 	if timestamp > time.time()-10:
> 		print 'old lockfile ok',
> 	else:
> 		os.remove('\tmp\'+mybox)
> 		
> else:
> 	lockfile=open('\tmp\'+mybox, 'w')
> 	lockfile.write(`now`)
> 	print 'new lockfile'
> 	print 'and contiue with the rest of this balderdash!

Off hand, I would say (ignoring the quoting problem), that \tmp\ has
more than just that lockfile in it.  I would check with os.path.exists
instead of glob.glob.

Also beware of race conditions; your form of locking isn't necessarily
"safe".  On Win32, I believe that os.mkdir() is safe in terms of
guaranteeing who owns the lock.

  -Arcege

-- 
+----------------------------------+-----------------------------------+
| Michael P. Reilly                | arcege@speakeasy.net              |