One of the features I would like to see from notmuch is an easier
ability to synchronize tags across machines.  At the very least, I
would need either incremental dump and restore, or some way to
communicate arbitrary tags to a local imap server that shares
notmuch's maildir (much as notmuch currently syncs the standard tags),
so that I synchronize two maildirs with a tool like offlineimap.

As Carl pointed out to me in private email, there has been some
previous discussion in the following thread:

        notmuch show

Based on that thread, there seems to be some desire for notmuch to
keep track of a per-message timestamp when the flags were last
updated.  This would allow much easier expiration for people who want
the deleted tag.  It would also allow incremental dump and restore of
tags, which is exactly what I need to sync tags across servers with
reasonable amounts of bandwidth.

Metadata timestamps are one of those things that probably have a lot
of different applications, so since Carl is considering a new database
format for the next release anyway, perhaps it also makes sense to add
a metadata change time for each messages.

The timestamp would be included in "dump" output, and you could
request a dump of changes since a particular time.  On restore, you
might have several options:

  - overwrite: always set the new tags and timestamp in the database
    to the value in the restore data.

  - update: always set the tags, but update the to the current time.

  - conditional T: update only if the message metadata has not been
    updated since time T.

To sync flags, then you just need to keep track of the last time you
synced with a particular server--call this time T.  Do a dump since
time T, upload to server, do a conditional restore for time T on
server.  Finally do a partial dump from time T on the server and an
overwrite import on the client.  (This policy makes changes on the
server always override conflicting ones on the client--perhaps people
want other policies, like union of the tags, etc.)

Second, there seems to be some desire in that thread to sync with IMAP
flags.  This would be particularly great, but the easies way to do it
is probably *not* to try to implement IMAP, but rather to use an
existing IMAP server and just modify the maildir so that the IMAP
server will pick up the flags.

In the case of dovecot, the arbitrary tag format is very simple.  Each
maildir has a file called dovecot-keywords mapping numbers 0, 1,
... to keywords.  Then mail file names contain lower-case letters for
the flags they are marked with--0 => a, 1 => b, etc.--allowing up to
26 arbitrary tags for each maildir.  One could probably sync to
dovecot's maildir format relatively easily in a script given
incremental dump and restore of tags.  Or possibly notmuch could
natively support dovecot as one of multiple back-end tag storage

Having a static tag mapping in the .notmuch-config file would be much
better than hard-coding flag2tag.  However, I'm not sure it's
sufficient.  The reason is that if you ever completely delete a tag
(e.g., you have "todo", and "meeting" tags and periodically have no
messages in either categories in a given mail folder), then an IMAP
server like dovecot might end up re-allocating the letters
corresponding to those tags in a different order.  Also, at least for
dovecot, the flag mappings are per-folder, which you kind of want
since you are limited to 26 non-standard tags, so global values might
not work.

I'm curious to hear people's thoughts/reactions?

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