Review (critico) caught a regression: the prod agent image ships a BtbN GPL
ffmpeg with libplacebo COMPILED IN but no Vulkan runtime (debian-slim, no
libvulkan1/mesa-vulkan-drivers/nvidia ICD). The presence probe (ffmpeg
-filters) would flip HasLibplacebo on, the filter's Vulkan device creation
would fail at runtime, and HDR sources that previously tonemapped via zscale
would break.
- FFmpegSupportsLibplacebo now RUNS the filter on one synthetic frame and
requires a clean exit (forces Vulkan device init + filtergraph negotiation),
so it is honest about THIS host: works on Vulkan-capable hosts, falls back to
zscale where Vulkan is absent. Logs the real ffmpeg error on failure.
- Warm the libplacebo (Vulkan init ~1.7s) + zscale caches in a background
goroutine at startup so the first stream session doesn't pay the probe and
risk its setup timeout.
- Benchmark: margin 1.5x -> 2.0x (the probe measures encode only; real decode
of HEVC/10-bit + busier content needs more headroom), per-probe timeout
12s -> 6s + overall 45s -> 20s (it blocks registration on software hosts),
and a 'no rung measured' case (missing lavfi/wedged ffmpeg) now keeps the
1080 default instead of flooring at 480 — an infra failure isn't a slow host.
Verified e2e on the fixed binary: LOTR Two Towers (HEVC 3840x1608 10-bit
HDR10/PQ, 12GB) on desktop-Chrome caps -> hls, ffmpeg runs h264_nvenc with
-vf ...,libplacebo=...:format=yuv420p:tonemapping=bt.2390 (zscale chain
replaced), 45 fMP4 segments, ffprobe confirms output h264 yuv420p bt709
(tonemapped from bt2020/smpte2084), no ffmpeg errors.
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