Three compounding bugs froze the UI when dragging the playhead after
playback:
1. Self-deadlock on preview_windows: supply_preview_window /
cancel_preview_windows / cancel_preview_window called
cancel_preview_sequence / cancel_preview_frame while HOLDING the
preview_windows mutex; those calls fire completions synchronously and
the completion locks preview_windows again. Caught by sampling the
hung process: UI thread in cancel_preview_sequence -> TicketSlot::
finish -> completion -> Mutex::lock. Cancels/releases are now
collected under the lock and fired after it is dropped.
2. Seek starvation by shard pinning: a Seek request's scheduler frame
is its ticket id, pinning it to worker (id mod W). The playback
window fills every worker's slots (window slots are only released by
UI-thread consumption), so the seek's pinned worker could have zero
free slots while the UI thread blocked on the seek — permanent
starvation. Seeks (interactive frame / real-time audio) are now
claimable by ANY worker; the no-stealing shard rule stays for
Playback frames (adjacent frames finish together).
3. No per-worker reserve: the global preview_window_capacity reserve is
pool-wide accounting, but exhaustion happens per worker. Playback /
Background claims now leave one credit unused per worker; Seek
claims may use the last slot (they complete on the worker without
UI involvement).
Also: RealEngine::drop cancels the preview windows — ShmFrameRef has no
self-release, so every dropped engine leaked its window's slots from
the shared pool, starving later windows (surfaced as the full-suite
playback_window_supplies_playhead_frames failure once the new probe
test shifted the test schedule). new_sequence_has_default_two_video_
two_audio_tracks now takes the engine test lock (it asserts on the
global undo stack; running lock-free raced parallel undo histories).
New regression probe interactive_seek_renders_without_hanging: play 30
ticks (window fills and holds shm slots), pause, seek, synchronously
render — must not hang. Scheduler tests updated for the reserve and
seek-any-worker contract. OAK_DEBUG_DISPATCH=1 enables the dispatcher
starvation/pool diagnostics used to track this down.
Mike-Solar
30ef02803d