Accessing an object after it has been unlinked by the cycle collector is a
certain way to cause trouble. Usually, that trouble is just in the form of
null pointer crashes, but it can potentially be much worse. And the most
common cause of those types of issues comes from accessing a weak pointer for
an object after it has been unlinked, but before it has been finalized. The
solution to that entire class of problem is to always clear weak references to
an object when it is unlinked.
Bug 1535617 adds some helper macros for this (NS_IMPL_CYCLE_COLLECTION_UNLINK_WEAK_REFERENCE
and NS_IMPL_CYCLE_COLLECTION_UNLINK_WEAK_PTR, along with some other
convenience macros along the lines of NS_IMPL_CYCLE_COLLECTION(...)), and fixes
most existing classes which implement both cycle collection and weak
references. Any new such classes will need to do the same.
Thanks,
Kris
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