Re: [Dovecot] locking question

[Timo Sirainen]

On 29.3.2007, at 21.30, Stewart Dean wrote:

There are three applications that have their mitts on files on my mail server, which is running AIXV5.3 and UWIMAP and mbox format. The mail folders and INBOXES are native to that machine, but also are NFS exported to a login server and a mailing list server. All three machines are running the lockd daemon.

Everybody wants to lock differently

2) UWIMAP which according to wiki.dovecot.org/Migration/UW uses:
mbox_read_locks = flock
mbox_write_locks = dotlock flock

1) procmail (delivering for sendmail), which seems to want to use dotlocking, fcntl and lockf locking; for whatever reasons, the compile time tests seem to disallow flock. 3) I want to run dovecot in the same environment as I switch over, for which the locking strategy is supposed to be (according to http://wiki.dovecot.org/MboxLocking)

mbox_read_locks = fcntl
mbox_write_locks = dotlock fcntl

Since that same page in the wiki says. "*It's important that all software that's reading or writing to mboxes use the same locking settings.",* I had recompiled procmail so it only usedotlocking and fcntl (thus removing lockf in addition to the disallowed flock)...but now I bumped into the Migration/UW page and there looks to be a conflict.

It doesn't actually hurt to use more locking mechanisms, as long as they all use at least one common and they're not in different order.

I want to be able to run dovecot (for testing) and UWIMAP (for all users/production) at the same time (but never at the same time with the same mailbox), but UWIMAP is doc'ed to use flock and Dovecot doesn't It's conceivable that the different locking strategies of UWIMAP and Dovecot don't matter because they would never be going after the same mail files (right?) but there is procmail, which has to interact with both. It appears to not use flock OK, even though the docs say it should since UWIMAP expects it, and I've disabled lockf as well. But will there be a conflict with procmail and dovecot's use of fcntl, which UWIMAP doesn't use?

You could use:

mbox_read_locks = fcntl flock
mbox_write_locks = dotlock fcntl flock

Both UW-IMAP and Procmail use dotlock as the first write lock and reading uses only a single lock method, so there won't be any deadlocks. Hmm. I think when delivering new mails only the write lock matters. At least with UW-IMAP. Looks like Dovecot also at least usually write locks the mbox if it has changed. So the read lock is mostly useful in situations like where one process is reading the mails while another process expunges messages. UW-IMAP doesn't allow two processes to access the same mbox simultaneously anyway.

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