[tpop3d-discuss] Maildir locking, to prevent more than 1 simultaneous access?

[Chris Lightfoot]

On Wed, Jan 22, 2003 at 06:04:50PM -0800, Rich, WhidbeyNet NOC wrote:
> >From the documentation, tpop3d relies on "the atomicity of certain
> filesystem operations" for managing Maildir locks. And that apparently
> works; no two clients can actually modify a file at once. However,
> according to RFC 1939, tpop3d should be preventing two or more clients
> from authenticating at all:
> 
> "Once the POP3 server has determined through the use of any
> authentication command that the client should be given access to the
> appropriate maildrop, the POP3 server then acquires an exclusive-access
> lock on the maildrop, as necessary to prevent messages from being
> modified or removed before the session enters the UPDATE state."
    [...]
> Keep in mind we have 5 POP3 machines, who each access Maildirs over NFS.
> So the only authentication lock I can think of would be with a dot-file.
> In the future, could tpop3d be told to "maildir-auth-lock =
> $home/Maildir/tpop3d.lock"? Or is there some way around this issue?

That's certainly feasible, though it's worth remarking
that you may run into problems if you have other clients
which don't obey whatever locking semantics tpop3d
implements.

A better alternative may be to return an error in the case
where the message has been deleted. This requires a few
small changes to the code. So in your example you would
see

    RETR num
    -ERR message already deleted by another instance

-- I don't know in practice which of these would work
better. The latter is probably neater, but as you observe
locking the mailbox exclusively is what the spec demands.
Does anybody know what qpop3d does here?

    http://www.ornl.gov/cts/archives/mailing-lists/qmail/1999/10/msg00168.html

-- doesn't lock, apparently.

I'll put in an option to lock the maildir, and make tpop3d
error if the message has already been deleted.

-- 
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