Code review comment for lp:~vanvugt/mir/fix-1653658

Looks good.

Nonblocking nits:
We should fix it so that MirConnection::channel is always valid. If some tests don't provide a valid channel, we should fix the tests.

It's a bit weird that complete_response searches through pending_calls twice, but I can see why you've done it that way. Doing it better probably requires deeper changes.