[messaging] Message order in group chat (attempt at summary)

[Tom Ritter]

I think that heartbeats are going to be important. With the partial
ordering UI I outlined, it's possible to reflow the conversation N
seconds into the past, where N is whatever window is deemed
acceptable. The way I expect it to work is that a user is not able to
send a message that reflows the conversation too far in the past -
that is, we just don't believe Bob if he claims he hasn't seen
messages sent 5 seconds ago. If Bob's client attempts to do that, we
reject it.  If a heartbeat is sent out every N seconds, and
acknowledged, you cannot pretend to not have seen messages prior to
the heartbeat.  If your client hasn't seen a heartbeat message in the
past N seconds, it should lock up and prevent interaction, grey it out
and saying 'reconnecting' and then unlock it when you've gotten the
current state of the chat.

I agree that canonicalizing partial order into linear order is worth
doing as well.  But with heartbeats the transcript can be 'locked'
prior to the last heartbeat, and everyone will know the state of the
conversation prior to that (or know something is wrong.)  Perhaps, to
get around the 'master is malicious' problem, the master says a
heartbeat with the current transcript state, the client signs it, and
the master distributes it in the next heartbeat.  So at any point in
time you have a master-signed heartbeat value verifying the transcript
as of N seconds ago, and an individual-group-member-signed heartbeat
value verifying the transcript N*2 second ago.  At this point, maybe
Carol didn't sign the heartbeat, and you see Carol is 'reconnecting'
and you know that they're not necessarily seeing everything said.  But
anyway, the the transcript is locked N or N*2 seconds in the past, and
you should be able to a) prevent reflows and b) create a linear
ordering that is at least consistent between members. (Indentation
seems like a good compromise instead of columnular at the expense of
discarding some specific details about who saw what).

-tom