potential delay between recieving a response and firing event hooks

[blampe]

I'm skeptical of the events we're seeing reported. Right now response logic looks like this:

response_future = connection.send_request(req)

with timeout(response_future, req.ttl):
    try:
        response = yield response_future
    except TChannelError as error:
        # event: after_receive_error
        self.tchannel.event_emitter.fire(
            EventType.after_receive_error, req, error,
        )
        raise
# event: after_receive_response
self.tchannel.event_emitter.fire(
    EventType.after_receive_response, req, response,
)

I'm pretty sure this is wrong because the yield statement isn't guaranteed to resume immediately after the future resolves, so there's potential for delay between receiving a response and firing the after_receive_response event for it.

More correct (I think) would be this:

response_future = connection.send_request(req)

def fire_events(f):
    if f.exception():
        self.tchannel.event_emitter.fire(
            EventType.after_receive_error, req, error,
        )
        raise f.exception()
    self.tchannel.event_emitter.fire(
        EventType.after_receive_response, req, response,
    )

response_future.add_done_callback(fire_events)

with timeout(response_future, req.ttl):
        yield response_future

because add_done_callback is preemptive and will execute as soon as the future resolves.

[jc-fireball]

It's true the sequence of callback is add_done_callback > add_future > yield