[Dovecot] Writing a new mailbox access module

[Tom Alsberg]

Having finished a few other projects, I'm contemplating trying to
write a new mailbox module which could be useful for us here.  I would
appreciate any guidance.

Basically, I need (or rather, could use) a module that provides a
merge of two mailboxes (which are themselves accessed by other
modules), cascading them, so that:

* The user sees one mailbox M, which contains all the messages in the
  mailboxes A and B.

* A and B are both already accessible by existing Dovecot mailbox
  access modules, but possibly by different modules (e.g. A is an
  mbox, B is a Maildir).

* Any write operation (creation of a new message, etc.) is eventually
  performed on B.

* As soon as a message that was in A is accessed (either for read or
  for write), it is transferred to B.

I would use such a module to combine the spool file in /var/mail with
some INBOX in the user's home directory.  The incentive is the
following:

* Traditionally, users receive their mail in the spool, commonly in
  /var/spool.  Mailers, like mail, elm, pine, mutt, etc. move each
  read message to some record mailbox (commonly ~/mbox, but with the
  more advanced mailers it can be configured to be some other mailbox,
  even a Maildir).

* Users keep working this way.  IMAP will not substitute the regular
  mail access when inside the network here.  But it should be as
  convenient as possible to use it from the outside (e.g. at home, or
  from an Internet Café, or using some Webmail interface).

* Mainly for administrative reasons, it is desirable that mail keeps
  being delivered to the spool, and not directly to the user's home
  directory.  One of the reasons is that mail never fails to be
  delivered as long as there is space in the spool.  When the user is
  out of quota on his home directory, the mail does not get rejected,
  and a failure only occurs when the user initiates some operation
  (like reading the message).  Another reason is simply to be
  compatible with the way traditional mailers and users work.

* IMAP, and IMAP clients (e.g. Thunderbird, the most common one here),
  on the other hand, usually do not recognize the concept of a spool.
  There is an INBOX, where all mail arrives, and this is where all new
  messages should arrive, and remain, unless they are deleted moved
  elsewhere.

* It would of course not be desirable to just move messages
  automatically after the user reads them, because it would confuse
  users when a message suddenly disappears from a mailbox all by
  itself.

* The best way I can think of to be consistent with both the
  traditional way mailers work (moving messages from spool to ~/mbox
  after they are read) and the way IMAP is commonly used (INBOX
  contains all received messages not deleted or moved, including
  messages that just arrived), would be using such a module as I
  suggested, with (using the previous terminology) e.g. M=INBOX,
  A=spool, and B=~/mbox.

I have just skimmed again through the mailbox code (which I last
looked at over a year ago, back when it was in alpha releases), not
going too much into details.  From past experience and current
skimming, working with that code doesn't seem too straightforward,
though.

I can think of how to implement the logic I described on its own
(i.e. as a library with a few functions), but not sure how difficult
it will be to interface it with Dovecot.  Is there any
developer/module writer's guide somewhere that explains the mailbox
access module interface?

I'd appreciate some basic guidance to start with.  If I get to working
on it seriously, expect lots of questions from me about the code :-)

  Regards,
  -- Tom

-- 
  Tom Alsberg - hacker (being the best description fitting this space)
  Web page:	http://www.cs.huji.ac.il/~alsbergt/
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