Mbox Locking
============

The only standard way to lock an mbox is using a method called "dotlock". This
means that a file named '<mailbox-name>.lock' is created in the same directory
as the mailbox being locked. This works pretty well when the mbox is locked for
writing, but for reading it's very inefficient. That's why other locking
methods have been used.

*It's important that all software that's reading or writing to mboxes use the
same locking settings.* If they use different methods, they might read/write to
an mbox while another process is modifying it, and see corrupted mails. If they
use the same methods but in a different order, they can both end up in a
deadlock.

If you want to know more details about locking, see <the mbox format page>
[MailboxFormat.mbox.txt].

For Dovecot you can configure locking using the 'mbox_read_locks' and
'mbox_write_locks' settings. The defaults are:

---%<-------------------------------------------------------------------------
mbox_read_locks = fcntl
mbox_write_locks = dotlock fcntl
---%<-------------------------------------------------------------------------

Here's a list of how to find out the locking settings for other software:

Procmail
--------

---%<-------------------------------------------------------------------------
# procmail -v 2>&1|grep Locking
Locking strategies:     dotlocking, fcntl()
---%<-------------------------------------------------------------------------

Postfix
-------

Postfix has two different ways to deliver to mboxes. One is the "mailbox"
transport and another one is the "virtual" transport.

---%<-------------------------------------------------------------------------
# postconf mailbox_delivery_lock
mailbox_delivery_lock = fcntl, dotlock
# postconf virtual_mailbox_lock
virtual_mailbox_lock = fcntl
---%<-------------------------------------------------------------------------

In the above case, if you used the "mailbox" transport, you'd have to change
Dovecot's configuration to 'mbox_write_locks = fcntl dotlock' or vice versa for
Postfix.

If you used the "virtual" transport, it doesn't really matter if the "dotlock"
is missing, since the "fcntl" is common with Dovecot and Postfix.

mutt
----

---%<-------------------------------------------------------------------------
mutt -v|grep -i lock
---%<-------------------------------------------------------------------------

Debian
------

Debian's policy specifies that all software should use "fcntl and then dotlock"
locking, but this probably applies only to most commonly used software.

(This file was created from the wiki on 2019-06-19 12:42)
