[Dovecot] mbox locking

[Timo Sirainen]

On Tue, 2006-06-27 at 13:57 +0200, Thomas Hummel wrote:
> On Tue, Jun 27, 2006 at 12:31:27PM +0300, Timo Sirainen wrote:
> > On Mon, 2006-06-26 at 18:03 +0200, Thomas Hummel wrote:
> > > Hello, 
> 
> > >   - "mbox_dotlock_change_timeout" directive :
> > > 
> > >     a process seeing that an already locked (by another process) mbox
> > >     he want to access hasn't change for this amount of time, allows
> > >     himself to override the lock ? 
> > 
> > Right. It checks that both the mbox and the dotlock file hasn't changed.
> 
> Thanks for your answers. 
> 
> But still I don't get the use of overriding the dotlock since the
> other process that put that dotlock may well have fcntl'ed the mbox
> too (it could use a "dotlock + fcntl" exclusive locking method, just
> as dovecot does - it could be another dovecot mail process).

The idea is to override dotlock files that have been left around by
proceses that have already died. They don't have fcntl locks or anything
else. If Dovecot didn't ever override them, they'd just keep the mailbox
locked until someone manually goes and deletes them.

> So don't we risk to end up in a situation where the dotlock is ours
> but the mailbox is still fcntl-locked by another process ?

Actually not with Dovecot. When Dovecot is about to override the stale
dotlock, it first tries to do the fcntl locking. If it succeeds, then
the dotlock is overridden, if it doesn't succeed, then it fails with
lock timeout. These checks have some race conditions so they're not
perfect, but good enough to work practically always.
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