Race condition during Parcel Coalescing Handler creation

[sithhell]

The message handler creation functions currently unlock the lock when creating a new message handler. This might lead to a race condition when constructing coalescing handlers concurrently due to multiple insertions of the same counter functions as well as when adding the callbacks for the runtime config

[hkaiser]

@sithhell I thought this was handled by this code: https://github.com/STEllAR-GROUP/hpx/blob/master/src/runtime/parcelset/parcelhandler.cpp#L708-L718. Did you have something else in mind?

[sithhell]

Unfortunately not. This is not about the insertion into the map. The problem is here:
https://github.com/STEllAR-GROUP/hpx/blob/master/src/runtime/parcelset/parcelhandler.cpp#L702
In the case of parcel coalescing, we eventually end up calling this: https://github.com/STEllAR-GROUP/hpx/blob/master/plugins/parcel/coalescing/coalescing_counter_registry.cpp#L45-L77
which is then not protected by any lock.

[sithhell]

In addition, even if the code shown was protected by a lock, we might end up with stale callbacks if the message handler was constructed twice due to a race.

[hkaiser]

Right, good catch. I think I know how to fix this.