[haffi96]

EncodedVideoSource takes Annex-B access units, but hardware encoder bridges can commonly deliver 4-byte length-prefixed NAL units (avc3 / hev1). This adds a parser that turns such a stream, in any chunking (e.g. straight from a TCP socket), into ready-to-capture EncodedVideoSource::Frames.

• Converts to Annex-B in place during the single copy out of its buffer (~2 µs per 1080p access unit).
• Sets is_keyframe with the same rules as livekit-capture (H.265 needs VPS/SPS/PPS with the IDR).
• single_slice_flush completes an access unit as soon as its slice arrives instead of when the next one begins, which otherwise holds every picture for a frame interval.
• frame_metadata_from_sei fills Frame::metadata from an LKTS packet-trailer SEI, the convention used by the LiveKit Go SDK.

LengthPrefixedAccessUnitParser::Options options;
options.single_slice_flush = true;
LengthPrefixedAccessUnitParser parser(VideoCodec::H265, options);
parser.push(buf, n);
while (auto frame = parser.next()) {
  source->captureFrame(*frame);
}

This is a C++ port of the LengthPrefixedAccessUnitParser added to livekit-capture in livekit/rust-sdks#1579, not a wrapper over FFI, so it works with the current Rust SDK and needs no new FFI messages; the cost is keeping the two parsers in sync. If a single implementation is preferred, a Rust-side source that takes raw length-prefixed bytes over FFI would be the follow-up. That PR also publishes pre-encoded video as a single layer, which matters for C++ because TrackPublishOptions::simulcast defaults to on.

Testing

• 10 new unit tests (LengthPrefixedAccessUnitParser*), including chunked-vs-one-shot equivalence and emulation-prevention handling in the SEI.
• Manual: published a 1080p30 hev1 stream from GStreamer over TCP; steady-state publish latency p50 0.83 ms.
• Also verified with a GStreamer camera pipeline that sends length-prefixed H.265 with SEI timestamp metadata over TCP, published through this parser (single_slice_flush, frame_metadata_from_sei) to livekit-server --dev and LiveKit Cloud; every frame carried its SEI metadata and the subscriber received 720p30 with the user timestamp and frame ID intact.
• Unrelated: AudioProcessingModuleTest.AGCAttenuatesLoudSpeech fails locally both with and without this change.

Created with Claude Opus 5.5 in Claude Code.

[devin-ai-integration]

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[haffi96]

The main motivation here is to enable the H265 pre-encoded ingest path with length prefixed NAL units more or less trying to mirror Livekit Go SDK pre-encoded publish path

See rust PR livekit/rust-sdks#1579