[Dovecot] Is dovecot locking properly?

[Joseph Tam]

Scott Galambos writes:

> Hello, I'm attempting to move form qpopper 4.1 to Dovecot 2.2.5 on
> Linux.  When a user checks POP mail qpopper seems to make a
> .username.pop temporary file in the same /var/mail directory as the mbox
> INBOX file.  Watching what dovecot does I don't see this happening.

Probably too quick (or perhaps you haven't configured dotlocking).

Qpopper locks the INBOX when it creates a drop copy: this can be a time
consuming operation, during which you can see the .$user.pop lock file.

Dovecot is more intelligent: it doesn't create copies, and goes out of
its way to avoid grautitous I/O.  Most of the operations can be done by
consulting the indices.

However, I did contrive a situation to observe lock files being created.

 	1) Create large INBOX file test user (assuming mbox /inbox/testuser)

 	2a) In one window, run a shell loop monitor
 		while : ; do
 			ls -al /inbox/testuser.lock
 		done

 	2b) In another window, create a POP3 session and do an
 	operation that would require a lock:

 		telnet pop-server 110

 		USER testuser
 		PASS testpassword
 		DELE 1
 			... this still won't need locking -- not until ...
 		QUIT

If you use maildir format, it may still be too quick to catch. in this
situation, you can process trace the pop3 worker process to verify a
lock file is created.

> Is this .pop file a lock file or just a temporary file?

Temporary.  If you have them lying around, qpopper crashed.  Note:
dovecot makes lockfiles of the form "$user.lock".

> If its a temp file does dovecot do the same thing elsewhere? I'm
> concerned dovecot is not locking properly and there will be mailbox
> corruption, so I'm trying to get dovecot to mimic what qpopper does as
> much as possible.

By "mimic" you mean that locking works, but otherwise, you don't want
dovecot to mimic qpopper: the former is much better.  I have not had
problems with users complaining about scrambled files.  As I said, process
trace your pop3 worker process if you need to verify the locking process.

Joseph Tam <jtam.home at gmail.com>