First draft of logging functionality.

[Rob Browning]

david at tethera.net writes:

> The patches following this message are my first attempt at
> implementing atomic logging for notmuch.  The idea is that such logs
> could be useful in synchronizing notmuch instances.

For this to be completely safe, I suspect it may need to be adjusted to
do write ahead logging or something similar.  Otherwise operations could
be lost.

i.e. notmuch would atomically and safely write the intended (tag)
operation to the log, then perform the tag.  On startup, notmuch would
need to scan the log to detect and apply operations that hadn't been
fully completed (presumably due to a crash).

More generally, while thinking about sync/logging a few days ago, I
wondered about using sqlite.  That would help with atomicity, rollback,
synchronizing multiple readers/writers, etc.  It might also make
operations more efficient once we implement all the features we want.

For example, with the log information in sqlite, a separate notmuch sync
to another machine could be reading from the log (and with limitations,
writing) in parallel with normal notmuch operations.  Depending on how
we decide to handle sync with multiple peers, the log may also need to
track which peers have seen what, prune appropriately, etc.

Of course sqlite may not be appropriate, and would require performance
testing, etc., but we should probably think about the features we'll
eventually want, and consider how much work they're likely to require
with any given approach, regardless.

Hope this helps
-- 
Rob Browning
rlb @defaultvalue.org and @debian.org
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