SyncSourceResolver::join waits for the Resolver's _state member to transition to kComplete. This transition is performed by the final '_finishCallback()' step, performed by the last callback scheduled by SyncSourceResolver. One of callback in the chain, _rbidRequestCallback, schedules a callback, takes a mutex on the SyncSourceResolver, and saves the handle into a member. However, nothing prevents the scheduled callback from executing, calling _finishCallback(), and allowing SyncSourceResolver's deconstructor to finish. If the handle is saved after the destructor calls, its own destructor will never be called, which prevents its shared_ptr's reference count from decrementing, causing a leak.