Code review comment for lp:~vanvugt/mir/fix-1319765

looks good, and what the code is accomplishing makes sense to me. Suggestion and a question:

composite_on_demand_never_deadlocks_with_2_buffers is a short test to run, but wouldn't the same lines be hit if we just ran once or twice? Its hard as someone looking at the code for the first time to tease out what the expectations are.

Could having a smarter callback system also avert the problem? (I think the composition scheduling system has some improvements that are needed, curious if the root cause is related)