__del__ pattern?

[BranoZ]

Bryan Olson wrote:
>  > Use file that is writeable by A and B in a directory that is
>  > writeable only by root.
>
> Is that portable?

I have the feeling that you are asking if it works on Windows.
No idea! I have only user experience with Windows.

On UNIX it is as portable as 'flock', which means all modern
Unices (be careful about NFS).

> What's the sequence the program should try?

1.
open a file, which name was previously agreed on
(like /var/tmp/<prog-name>-<user-name>)

If it fails, report error and exit. System error or
somebody has created unaccessible file by the same name.

2.
Try to aquire a flock on the descriptor from step 1.

If it fails, some running process already has the lock, exit

3.
lock will be released and lockfile closed automaticaly by OS
on process exit.

import sys, fcntl

try:
  lockfile=open('/var/tmp/test1', 'w')
  fcntl.flock(lockfile.fileno(),
    fcntl.LOCK_EX | fcntl.LOCK_NB)
except IOError:
  print sys.exc_info()[1]
  sys.exit(-1)

You can flock any open file, no matter if it is read/write/append.

BranoZ