Adds an end-to-end RTP-arrival latency probe that runs as a dedicated
CI job and asserts p95 < 50ms.
Implementation
--------------
A build-tagged test (-tags latency, off by default) sends 1000
synthetic RTP packets at 60Hz into corewebrtc.Source and reads them
back via a Pion subscriber's track.ReadRTP(). Each packet's payload
starts with the publisher's UnixNano send time; the subscriber diffs
against time.Now() at arrival and accumulates p50/p95/p99.
This exercises every link of the egress hop: Source UDP read,
subscriber fan-out, forwardRTPSplit, Pion's TrackLocalStaticRTP
write, DTLS-SRTP encrypt, ICE socket write, decrypt at the
subscriber, RTP unmarshal at ReadRTP. Pure server-side; no FFmpeg
or codecs involved.
Why not glass-to-glass
----------------------
The design's §7 calls for FFmpeg drawtext frame counters + decode-
side pixel sampling, p95<300ms RTMP / <200ms SRT. Implementing that
in pure Go needs a cgo H.264 decoder or an FFmpeg sidecar pipe — a
significantly bigger lift for a marginal regression-detection win
(encode/decode latency is roughly fixed by the codec stack and
isn't moved by Core code changes). The server-hop measurement
captures everything Core code can actually regress.
Threshold
---------
50ms p95. Locally observed on a quiet host:
p50=110µs, p95=237µs, p99=318µs.
The 50ms gate is ~200x headroom — generous enough to absorb CI
runner noise without false alarms, tight enough to catch a real
slowdown.
Race-clean: latencySamples uses a sync.Mutex around the slice append
(initial draft had a slice racing with the receive goroutine; vet
caught it).
Documented in test/TESTING.md and wired to .forgejo/workflows/test.yml
as the latency-gate job (depends on lint-and-vet, parallel with test
and webrtc-smoke).
Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>
