I hope this will be easier to reason about, and less buggy. It was
certianly easier to write!
An immediate benefit is that with a traversable queue of push requests to
select from, the threads can be a lot fairer about choosing which client to
service next.
This will avoid losing any messages received from 1 client when a push
involving another client is running.
Additionally, the handling of push initiation is improved,
it's no longer allowed to run multiples of the same type of push to
the same client.
Still stalls sometimes :(
Observed: With 2 xmpp clients, one would sometimes stop responding
to CanPush messages. Often it was in the middle of a receive-pack
of its own (or was waiting for a failed one to time out).
Now these are always immediately responded to, which is fine; the point
of CanPush is to find out if there's another client out there that's
interested in our push.
Also, in queueNetPushMessage, queue push initiation messages when
we're already running the side of the push they would initiate.
Before, these messages were sent into the netMessagesPush channel,
which was wrong. The xmpp send-pack and receive-pack code discarded
such messages.
This still doesn't make XMPP push 100% robust. In testing, I am seeing
it sometimes try to run two send-packs, or two receive-packs at once
to the same client (probably because the client sent two requests).
Also, I'm seeing rather a lot of cases where it stalls out until it
runs into the 120 second timeout and cancels a push.
And finally, there seems to be a bug in runPush. I have logs that
show it running its setup action, but never its cleanup action.
How is this possible given its use of E.bracket? Either some exception
is finding its way through, or the action somehow stalls forever.
When this happens, one of the 2 clients stops syncing.
Noticed that, At startup or network reconnect, git push messages were sent,
often before presence info has been gathered, so were not sent to any
buddies.
To fix this, keep track of which buddies have seen such messages,
and when new presence is received from a buddy that has not yet seen it,
resend.
This is done only for push initiation messages, so very little data needs
to be stored.
Noticed that when pairing, sometimes both sides start to push, and the other
side sends a PushRequest, and the two deadlock, neither doing anything.
(Timeout eventually breaks this.) So, let both run at the same time.
It might even work, although nothing yet triggers XMPP pushes.
Also added a set of deferred push messages. Only one push can run at a
time, and unrelated push messages get deferred. The set will never grow
very large, because it only puts two types of messages in there, that
can only vary in the client doing the push.
