On Mon, 25 Jan 2010, martin f krafft wrote:
also sprach Asheesh Laroia <ashe...@asheesh.org> [2010.01.21.1928
+1300]:
I suppose that I never actually considered merges on the IMAP server
side, but obviously the IMAP server has to work off a clone, and that
means it needs to merge.
It's not "merge" that's unsafe; that just builds a tree in the git
index (assuming no conflicts). It's the ensuing process of git writing
a tree to the filesystem that is problematic.
There is no way to make that atomic, I am afraid. As you say.
I could probably actually write a wrapper that locks the Maildir while
git is operating. It would probably be specific to each IMAP server.
Ouch! I'd really rather not go there.
You say "Ouch" but you should know Dovecot *already* does this. I don't
mind interoperating with that.
See http://wiki.dovecot.org/MailboxFormat/Maildir, section "Issues with
the specification", subsection "Locking". I term this the famous readdir()
race. Without this lock, Maildir is fundamentally incompatible with IMAP
-- one Maildir-using process modifying message flags could make a
different Maildir-using process think said message is actually deleted. In
the case of temporary disappearing mails in Mutt locally, that's not the
end of the world. For IMAP, it will make the IMAP daemon (one of the
Maildir-using processes) send a note to IMAP clients saying that the
message has been deleted and expunged.
Note that this mean git is fundamentally incompatible with Maildir, not
just IMAP servers.
We had an idea about using Git to replace IMAP altogether, along with
making notmuch use a bare Git repository as object store. The idea is
that notmuch uses low-level Git commands to access the .git repository
(from which you can still checkout a tree tying the blobs into a
Maildir). The benefit would be compression, lower inode count (due to
packs), and backups using clones/merges.
Sure, that makes sense to me.
You could either have the MDA write to a Git repo on the server side and
use git packs to download mail to a local clone, or one could have e.g.
offlineimap grow a Git storage backend. The interface to notmuch would
be the same.
Yeah, I generally like this.
If we used this, all the rename and delete code would be refactored into
Git and could be removed from notmuch. In addition, notmuch could
actually use Git tree objects to represent the results of searches, and
you could checkout these trees. However, deleting messages from search
results would not have any effect on the message or its existence in
other search results, much like what happens with mairix nowadays.
That's okay with me.
I think we all kinda agreed that the Maildir flags should not be used by
notmuch and that things like Sebastian's notmuchsync should be used if
people wanted flags represented in Maildir filenames.
Aww, I like Maildir flags, but if there's a sync tool, I'm fine with that.
Instead of a Maildir checkout, notmuch could provide an interface to
browse the store contents in a way that could make it accessible to
mutt. The argument is that with 'notmuch {ls,cat,rm,…}', a mutt backend
could be trivially written. I am not sure about that, but it's worth a
try.
Sure.
But there are still good reasons why you'd want to have IMAP capability
too, e.g. Webmail. Given the atomicity problems that come from Git,
maybe an IMAP server reading from the Git store would make sense.
It wouldn't be too hard to write a FUSE filesystem that presented an
interface to a Git repository that didn't allow the contents of files to
be modified. Then Dovecot could think it's interacting with the
filesystem.
However, this all sounds like a lot of NIH and reinvention. It's
a bit like the marriage between the hypothetical Maildir2 and Git,
which is definitely worth pursuing. Before we embark on any of this,
however, we'd need to define the way in which Git stores mail.
Sure. If it were me, I'd just say, "For phase 1 of notmuch, just have git
store Maildir spools." When you need a filesystem interface for e.g.
Dovecot, have a FUSE wrapper.
See how far that can take you, and then see if version 2 is necessary.
(-:
Stewart, you've worked most on this so far. Would you like to share your
thoughts?
I'll listen, too.
Just don't fall into the trap of thinking Maildir is compatible with IMAP.
It's not, because as I understand things, the filesystem doesn't guarantee
that you can actually iterate across a directory's files if another
process is modifying the list of files.
I'm not sure, but maybe it's safe if you refuse to ever modify a
message's flags in the filename.
Anyway, as I see it, further hacks that aren't much worse than Dovecot's
should be considered okay, unless you have a more elegant design up your
sleeve.
If I'm slightly wrong about something, try to give me the benefit of
doubt. It's past midnight. (-:
-- Asheesh.
--
There's no real need to do housework -- after four years it doesn't get
any worse.
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