6863eefc3d handled this situation by using
an atomic variable to express the state for which the wakeup is caused
by AO control, and the dispatch queue is only processed at this state.
However, this can cause permanent lockup of the player core when the
following happens:
- AO control sets the thread state to WASAPI_THREAD_DISPATCH, and
sets the wakeup handle.
- WASAPI thread reads the WASAPI_THREAD_DISPATCH state and processes
the dispatch queue.
- Another AO control happens. A dispatch item is enqueued, and the
state stays at WASAPI_THREAD_DISPATCH.
- WASAPI thread resets the thread state to WASAPI_THREAD_FEED since
the state has not changed.
- WaitForSingleObject() returns in the WASAPI thread, sees this state,
and does not process the dispatch queue.
- The player core locks permanently because it is waiting for the dispatch
to be processed.
This has been experimentally verified on a system under high contention:
The easiest way to trigger this lockup is to continuously hold down "i",
which rapidly issues AO get volume/mute controls.
To properly handle this, use separate handles for system and user wakeup
requests. Only feed audio when woke up by system and only process the
dispatch queue when woke up by user.
Fixes: 6863eefc3d
51e01e9772