This came up in #330; the Character destructor caused something to call lock() on a weak pointer to that character, which would induce a ref() and unref() pair, but since the refcount was 0, this would call the destructor and thereby create infinite recursion.
I considered instead calling mark_deleted() inside unref() so that the callbacks get to run before the object is actually deleted, and was_deleted() will become true as soon as unref() reaches 0. However, this would require grabbing the lock in unref() to be fully thread-safe, since we would need to bring the refcount to 0 and mark the object as deleted in one atomic operation, so this would be an unacceptable general performance penalty.
Instead, WeakPointerTo::lock() now atomically increments the reference count if it is not already zero, and returns null otherwise. This should be safe because the object cannot be deleted while the WeakReferenceList lock is held.
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