A 10-bit/HDR HEVC source (e.g. an HDR10 x265 file) died at scale_vaapi=format=nv12
on the UHD 630 (Gen 9.5 VPP: "VAProfile not supported"); ffmpeg produced 0 segments
and the item never played, with no fallback — a retry just re-picked the failing HW
path. An 8-bit HEVC and the same file on libx264 both work.
Two defenses:
- `_want_hw` auto mode now also gates on an 8-bit-SDR source via a new
`_probe_video_hwsafe` ffprobe (bit depth from pix_fmt/bits_per_raw_sample, HDR from
color_transfer) — a 10-bit/HDR source takes the software libx264 path, which
downconverts to yuv420p cleanly. The Plex mirror stores no bit-depth signal, so this
is probed at session start. "on" still trusts the admin and skips the gate.
- `start_session` adds a runtime HW→SW fallback: a HW session that emits no first
segment respawns on libx264 and reports hw=False (the route/stats overlay honestly
show CPU). `wait_for` gained an optional `proc` so a dead ffmpeg fails fast instead
of burning the 25s budget before the fallback fires.
Tests: TestProbeVideoHwsafe (parsing + probe-failure-assumes-safe), the 10-bit auto
gate + "on" skip in TestWantHw, and the no-segment HW→SW respawn in
TestTranscodeStartSession. Backend lane green.
peter
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