The controller carries seventeen mutexes, reached from the MPV event loop,
JNI callbacks, sleep and autoplay timers, the session poller and every IPC
command. Nothing prevented two threads taking the same pair in opposite
orders, which deadlocks playback outright — and this subsystem has already
produced one deadlock.
No inversion exists today: the acquisitions really are scoped, and
`previous()` explicitly drops the backend guard before touching the queue.
That is the point. It holds by convention, convention is not checked, and
the failure it guards against is a frozen app with no error anywhere.
`LOCK_ORDER` writes the convention down, following the nesting the code
already relies on — `backend` before `queue` ("what is playing" before
"what is next"), `event_emitter` last because notifying the frontend must
never reach back for player state.
The tripwire only reports acquisitions that actually *overlap*, since two
locks taken one after another, each released before the next, cannot
deadlock. Verified by injecting a real inversion into `seek()`, which the
test located by line and rank.
dtourolle
65d3d912f7