[Dovecot] mailbox lock

[Timo Sirainen]

On Thu, 2008-08-28 at 18:43 -0300, Thiago Monaco Papageorgiou wrote:
> Ok, but we must lock the mailbox because we often move groups of mailbox 
> from a file system to another. While the mailboxes are being moved, our 
> LDA shouldn't delivery a new message at the old mailbox, and isn't able 
> to delivery at the new one. The mailbox is unlocked after the exchange 
> of the file system , if the dovecot locks the index files and then the 
> mailbox exchanges of file system, what will unlock the index files of 
> the dovecot?
> The dovecot process that perform the lock, will unlock the old index 
> files. This old index files will be erased as well as the whole old 
> mailbox, the new mailbox, that was moved to an another file system, will 
> remains locked.
> 
> We have almost 10M of mailboxes, if the chance that it happens is 
> something like 1/1M, it will happens 10 times per day!

I guess the main problem is that you need to lock user's all mailboxes
instead of just one. A single mailbox can be easily locked using
v1.1.2's new maildirlock utility, but I don't think this will help you,
at least not reliably.

I've actually a related item in my TODO list:

 - convert plugin: Create a r/w lock for a file. It's read-locked if
   conversion isn't wanted and released when process dies. If conversion is
   wanted and write-lock succeeds, conversion is done, if write-lock doesn't
   succeed it fallbacks to using the old storage. When process is exiting it
   again tries to write-lock and do the conversion. Add a parameter that
   specifies if conversion should be done.

So the idea would be that both the deliver and imap/pop3 would load the
convert plugin and use that user-global lock file for figuring out when
it's safe to do the conversion. With IMAP processes perhaps it should
try the conversion also while the process is running, otherwise for
long-running connections it could take a long time to get the mailbox
converted.
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