File locking

[Chuck Swiger]

Jason Miller wrote:
[ ... ]
> Is there a portable (i.e. *nix, win32, mac) way to lock files in python? 
> From what I know about these platforms it doesn't seem likely, but I
> figured I'd ask.

Alex Martelli provided a reference for Python code which invokes 
underlying OS primitives like lockf() or fcntl().  Regardless of what 
you do in your Python code, however, the underlying OS mechanisms for 
locking don't always work well together: there exist platforms where 
lockf(), fcntl(), and flock() don't know about each other's locks.

Sure, better platforms handle the interactions properly, but mixing 
types of locking *will* bite you or someone else down the line.  Oh, 
yes, and you really don't want to try locking files on anything but a 
local filesystem: rpc.lockd (used by NFS) and Samba's Windows-style 
locking have had a history of frequent locking problems. [1]

Also note that if you are working with remote filesystems, or anything 
having to do with email, Usenet news, or similar you should use explicit 
.lock files as well.  SysV Unices-- ie, Solaris-- wants you to use 
something called maillock().  And Rich $alz of INN also wrote a nice 
program called shlock in ~200 lines of C for doing locking for UUCP and 
Usenet news. [2]

--
-Chuck

[1]: The Samba Team has done an amazing job and have been improving 
Samba's locking quite steadily, and I don't especially want to criticise 
their efforts.  However, I wouldn't write a trading system or a medical 
software system which tried to lock via a Samba-based fileshare.

(The less I say about rpc.lockd, the better. :-)

[2]: The important idea here is that your lockfile should contain the 
PID of the process which holds the lock, so that another process trying 
to acquire the lock can get a lock held by a process which died, or be 
able to break the deadlock.  This type of locking has been adapted by 
system startup scripts which keep the PID of a process (like a Unix 
daemon) under /var/run.

I need to start hacking Python somewhere, would a Python version of 
shlock be of interest to anyone?