[Dovecot] Virtual mailbox plans

[Timo Sirainen]

Configuration
-------------

There could be global and user-specific configuration files, similar to
how ACLs work. I think the global virtual mailboxes should be only
defaults though, so that users could delete them and create a new
mailbox (virtual or non-virtual) with the same name.

The global virtual mailboxes could be described in a single file, such
as:

Trash
deleted

Work/My Unseen
(mailboxes "box1" "box2" "box2/*")
  or to "tss at iki.fi" to "timo.sirainen at iki.fi"
  unseen

So the format would be <virtual mailbox name> LF <search condition,
allowed to wrap to new line> LF LF <next virtual mailbox, etc.>

Per-user virtual mailboxes could also be listed in a similar file. If
the search condition is empty, a globally defined virtual mailbox has
been deleted.

Another way would be to have per-mailbox dovecot-virtual files, but I'm
not sure how global virtual mailboxes could then be easily deleted. Also
having all of them in a single file makes listing the mailboxes pretty
fast.

The search condition would be a standard IMAP SEARCH reply, with the
exception of the mailboxes parameter to specify what mailboxes are used.

Virtual mailboxes would be implemented as a new mail storage backend, so
their namespace configuration would be like:

namespace private {
  prefix = "virtual/"
  location = virtual:~/virtual:GLOBAL=/etc/dovecot/virtual-globals.conf
}

Syncing
-------

~/virtual directory would contain index files for the virtual mailboxes.
If the index files don't exist, a full resync for the virtual mailbox is
done. This means opening all the real mailboxes configured for the vbox,
running the SEARCH command and adding the returned UIDs to the vbox's
indexes.

The index would contain at least mailbox ID and message UID numbers.
Probably would be a good idea to also save UIDVALIDITY numbers just to
be safe, although that's a bit of a waste of space and it could be
handled in other ways too, such as changing the mailbox ID whenever its
UIDVALIDITY changes..

Mailbox IDs would be mapped to names in mailbox list index. This index
already exists, although there still seems to be some bugs in it. At
least creating a new list index has a race condition. I should look into
making the list index a bit more realiable. Also the list indexes work
only with Maildir++ layout currently.

After the vbox has initially been synced, it needs to also handle
updates. I think optionally the updates could be done the same way as
full sync (opening all mailboxes to see if there are changes), but a
faster way would be to update the virtual mailboxes immediately when the
real mailboxes are being modified.

So IMAP, POP3 and deliver all need to be aware of the virtual mailboxes
and what real mailboxes are mapped to what virtual ones. Whenever doing
a change that causes the message to either be added to or removed from a
virtual mailbox according to their rules, the virtual mailbox's index is
updated immediately. Being able to determine this change quickly might
be the biggest change required to Dovecot's base code.

Changes
-------

Virtual mailboxes would share their state with the real mailbox, so any
flag changes done in the vbox would be immediately changed in the real
mailbox as well. The same goes for expunges.

Virtual mailboxes' transaction logs would use different records for
expunges and removals. Expunges would be handled normally, but removals
wouldn't be removed from the in-memory view of the mailbox until EXPUNGE
command was given (although they would be removed from dovecot.index
immediately, so new sessions wouldn't see them). This makes it possible
to have a vbox with only unseen messages, but without the message
getting expunged immediately after a client opens the mail. So the
EXPUNGE command could act as a "session synchronization" command. And of
course it would also expunge the messages that had been marked as
\Deleted.

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