[Dovecot] POP (dot) locks

Christian Balzer chibi at gol.com
Sat May 22 06:46:40 EEST 2004

Timo wrote (hmm, given the usual programmers life style I'd say VERY late
at nigth :) :

>There's no possibility for corruption, as long as all software accessing
>mbox uses compatible locking. So if you don't have mbox_read_dotlock
>set, make sure everyone uses also fcntl (or flock) locking.
Oh, my worries are not about the only other bit accessing the boxes, 
which is exim and that uses fcntl. My question was about this, not 
at all theoretical scenario:
User has a client at home to auto-download (but leave mails on server)
and is using a slow link, so those message downloads take a quite 
measurable time. Now he logs into webmail (which of course also 
ultimately uses dovecot to access the mailbox) and deletes a message 
which is currently in transit to his home machine.
Will the result be:
a) The "read" of that message has triggered an internal dovecot lock and
   the "write" of the delete will have to wait until this is released.
b) The delete happens immediately, but thanks to buffering the read and 
   delivery of the message is always successfully finished.
c) The delete of the message happens immediately, the state of the
   message in transit is indeterminable and it might be truncated.

>Maildir can't be locked and so it has the same problem that message
>might get lost. That's pretty much the reason why I didn't bother to
>make pop3 lock mboxes either. Both mbox and pop3 are of smaller priority
>to me than maildir+imap, and making some mbox+pop3-specific locking
>kludge there wasn't very attractive.
Totally understood and agreed with. The solution would of course be to
totally lock out any other logins (imap or pop) while a pop3 session is
active, like qpopper does. Alas that would have the aforementioned 
unpleasant side effects. So as long as the answer to the above question
is a) or b) I see no reason to add either mbox or session locking. 


Christian Balzer
Christian Balzer        Network/Systems Engineer                NOC
chibi at gol.com   	Global OnLine Japan/Fusion Network Services

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