Get rid of use-after-free in async registry #21779
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Use-after-free happened in nightly alubsan run.
The mark for deletion of a node after all its references are gone was actually much too complicated: I wanted to make sure that if there is a hierarchy of parents where each parent has only one child and the lowest child is marked for deletion like this:
all parents in the hierarchy are directly marked for deletion as well and we don't have to run the garbage collection several times to do that (A node has a shared pointer to its parent: when the shared pointer to the parent is destroyed - normally happening in the garbage collection - the node is marked for deletion). In the old behaviour, we cascaded through the full hierarchy and marked parents for deletion as well. Then at some point the shared pointer to the parent is also destroyed, leading to another try to access the parent node. If a garbage collection ran in between, the node was already deleted, leading to a use-after-free.
This easy solution: Get rid of the parent reference when marking the node for deletion. Then the shared pointer to the parent is potentially destroyed earlier, leading to a mark for deletion of the parent automatically. Don't know why I did not thought about this earlier...