The previous attempt (8531135) dropped zero-copy on Mutter+NVIDIA for a sticky
CPU/SHM fallback that (a) still listed SPA_DATA_DmaBuf in its buffer types, so
Mutter kept handing dmabufs that got mmap-read UNsynced — making the flashing
worse, not better — and (b) hinged on producer explicit sync, which Mutter+NVIDIA
cannot do (`error alloc buffers` / no cogl sync_fd, confirmed in worker-3 logs).
Revert the capture restructure to the original zero-copy dmabuf path, and fix the
NVIDIA stale-frame race the RIGHT way for a producer that can't do explicit sync:
the consumer snapshots the dmabuf's implicit fence (DMA_BUF_IOCTL_EXPORT_SYNC_FILE)
and waits the producer's render before sampling (new dmabuf_fence module, ioctl
number unit-tested). Covers the GPU import and the CPU mmap read. Logs once whether
a render was actually in flight (waited=true → the driver fences and the race is
closed; false → no implicit fence, so we learn zero-copy still needs SHM here).
drm_sync (the explicit-sync primitive) is kept and verified but marked unused —
no targeted compositor produces a usable sync_fd today; ready to wire in when one
does. The Bug-2 input fix (held-key release on disconnect) from 8531135 is kept.
Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>
