I think this is a reasonable way to move on.Some of the locks can probably be removed but this needs careful investigation and in the mean time, it hinders everyone. Thanks victor for the fast fix up! 

To me, an interesting observation is that the failure seems to only happen when the executor is in broken state. If we can find a way to adapt the behavior to be more conservative on these states (which are not impacting perf) that would be nice.

I will try to look at it today and see if I can remove some of the locks while still not failing with Victor's patch. We can probably remove the lock on `clear` safely. For the others, it might be more complex.