weakref'd signal receivers may cause deadlock when gc'd

[john@…]

By default when a receiver is connected to a Signal it is wrapped in a saferef has a call back to Signal._remove_reciever when gc'd.

Inside, _remove_receiver it acquires a lock on itself before proceeding to delete the reference of the receiver from the signal.
To find the receiver, the receiver list is reversed. This generates a new listreverseiterator to be created. In turn, this can may cause a GC to run. Which clears all unreachable weakrefs, one of which may be another receiver connected to the same signal. The gc calls the callback on this receiver (_remove_receiver) and hits a deadlock on trying to acquire the Lock again.

[Jacob]

Do you have some code that can reproduce this?

[john@…]

I've been trying to come up with some code to reliable reproduce this, so far I've been unsuccessful. I will continue to try though.

[Anssi Kääriäinen]

I am marking this as duplicate of #21952, that ticket contains a patch for this issue.