for conditional storing and quick resynchronisation (not sure if mail clients are actually using it that).
each message now has a "modseq". it is increased for each change. with
condstore, imap clients can request changes since a certain modseq. that
already allows quickly finding changes since a previous connection. condstore
also allows storing (e.g. setting new message flags) only when the modseq of a
message hasn't changed.
qresync should make it fast for clients to get a full list of changed messages
for a mailbox, including removals.
we now also keep basic metadata of messages that have been removed (expunged).
just enough (uid, modseq) to tell client that the messages have been removed.
this does mean we have to be careful when querying messages from the database.
we must now often filter the expunged messages out.
we also keep "createseq", the modseq when a message was created. this will be
useful for the jmap implementation.
when broadcasting a change, we would try to send the changes on a channel,
non-blocking. if we couldn't send (because there was no pending blocked
receive), we would wait until the potential receiver would explicitly request
the changes. however, the imap idle handler would not explicitly request the
changes, but do a receive on the changes channel. since there was no pending
blocked send on the channel, that receive would block. only when another event
would come in, would both the pending and the new changes be sent.
we now use a channel only for signaling there are pending changes. the channel
is buffered, so when broadcasting we can just set the signal by a non-blocking
send and continue with the next listener. the receiver will get the buffered
signal. it can then get the changes directly, but lock-protected.
found when looking at a missing/delayed new message notification in thunderbird
when two messages arrive immediately after each other. this doesn't fix that
problem though: it seems thunderbird just ignores imap untagged "exists"
messages (indicating a new message arrived) during the "uid fetch" command that
it issued after notifications from an "idle" command.
the mailbox select/examine responses now return all flags used in a mailbox in
the FLAGS response. and indicate in the PERMANENTFLAGS response that clients
can set new keywords. we store these values on the new Message.Keywords field.
system/well-known flags are still in Message.Flags, so we're recognizing those
and handling them separately.
the imap store command handles the new flags. as does the append command, and
the search command.
we store keywords in a mailbox when a message in that mailbox gets the keyword.
we don't automatically remove the keywords from a mailbox. there is currently
no way at all to remove a keyword from a mailbox.
the import commands now handle non-system/well-known keywords too, when
importing from mbox/maildir.
jmap requires keyword support, so best to get it out of the way now.
