[PATCH] Add configurable changed tag to messages that have been changed on disk

[David Mazieres]

Gaute Hope <eg at gaute.vetsj.com> writes:

> A db-tick or a _good_ ctime solution can as far as I can see solve both
> David M's (correct me if I am wrong) and my purposes, as well as
> probably have more use cases in the future. It would even be an
> interesting direct search: show me everything that changed lately,
> sorted.

I could live with a db-tick scheme.  I would prefer a ctime scheme,
since then I can answer questions such as "what has changed in the last
five minutes"?  I mean all kinds of other stuff starts to break if your
clock goes backwards on a mail server machine, not the least of which is
that incremental backups will fail silently, so you risk losing your
mail.

A middle ground might be to use the maximum of two values: 1) the
time-of-day at which notmuch started executing, and 2) the highest ctime
in the database plus 100 microseconds (leaving plenty of slop to store
timestamps as IEEE doubles with 52 significant bits).  Since the values
will be Btree-indexed, computing the max plus one will be cheap.

Incidentally, if you are really this paranoid about time stamps, it
should bother you that notmuch's directory timestamps only have one
second granularity.  It's not that hard to get a new message delivered
in the same second that notmuch new finished running.  In my
synchronizer, I convert st_mtim (a struct timespec) into a double and
keep that plus size in the database to decide if I need to re-hash
files.  But for directories, I'm stuck with NOTMUCH_VALUE_TIMESTAMP,
which are quantized to the second.  (Ironically, I think
Xapian::sortable_serialize converts time_ts to doubles anyway, so
avoiding st_mtim is not really helping performance.)

David