previous sequence:
examine & propagate new => examine old => propagate old
new sequence:
examine new => examine old => propagate new => propagate old
this alone does not buy us much ...
we can bump the internal variable whereever convenient, but we cannot
log it until we know that all messages were copied, as otherwise we
could miss some new messages after an interruption. with the new
approach, interruption would merely cause some additonal traffic.
less code duplication, more logical order of issued driver commands
(especially after the next commit), and the "side effect" of letting the
message expiration code see those deletions if they are asynchronous.
the delay optimized the corner case of previously important but now
expired messages on the slave disappearing, either through an external
expunge or after a journal replay. no point in pessimizing the common
case.
the removed code would only ever trigger if a) we were after a journal
replay or b) something external expunged the expired messages - both are
corner cases not worth the extra code.
however, this means that the syncing code further down now needs to take
care of these zombies.
in the end, the normal cleanup will take care of all expired entries,
new and old.
that is, don't count them towards the total only below the cut-off
point. making them extend the working set even though they are inside it
is counterintuitive.
while maildir has a clearly defined meaning of "recent" and for example
mutt handles it graciously, IMAP's definition is fubared to the point
that some servers (for example gmail) simply refuse to support it.
for symmetry reasons it is best to pretend that it doesn't exist at all.
it doesn't seem too useful anyway (the user can simply mark the messages
as read to allow pruning).
and last but not least, the man page of mbsync says nothing about
"recent", only "unread". unlike the isync man page, though.
even if we are not propagating new messages, the appearance of new
messages on the slave can lead to expiring older messages. for that, we
need to know their importance, and thus flags.
the alternative would be not doing an expiration run when not fetching
new messages, but that would mean more conditionals all over the place.
as the decision is somewhat arbitrary, just do the simpler thing.
the header is not space-critical, so use proper name-value pairs.
this has the additional advantage that subsequent format changes can be
done much easier.
otherwise we would propagate phantom deletions.
this affected only sync runs after an interruption while storing
messages, so it went (mostly?) unnoticed.
the warning suppression pragma within function scope is apparently a new
thing.
as i don't want to disable the check for the entire function (even if
this currently would make no difference), just use a wrapper function
to suppress the format string check.
amends 9c86ec344.
S_FIND was for the sync record status field. it has no business in the
sync vars status fields. its value coincided with ST_SELECTED, which
luckily only means that we always tried to match up TUIDs even if there
was nothing to do.
the need for TUID matching arises in two mostly independent
circumstances, so add two separate flags ST_FIND_{OLD,NEW}.
