rtpmanager/rtptwccstats: track recovery per means of repair (RTX/FEC)

[MishaBaranov]

What

Track TWCC delivery/recovery per means of repair (RTX vs FEC) independently, so
each recovery mechanism can be credited on its own rather than lumped into a single
aggregate recovery figure.

Details

• Each sent packet carries an array of repair-instrument IDs ({ssrc, pt} pairs,
  TWCCRepairId), populated from RTPRepairMeta. A packet's RECOVERED state is
  derived from status == LOST && recovered_by_count > 0 rather than stored.
• Per-window stats now expose two structures:
  • RTPTWCCRepairStats — per-repair recovered count, recovery %, and bitrate-sent
    (bitrate spent on RTX/FEC redundant traffic).
  • RTPTWCCRecoveryOverlap — share of losses recoverable by both RTX and FEC
    (pairwise overlap), useful to detect redundant repair spend.
• Separate accounting of redundant (repair) vs primary (non-recovery) bitrate.
• Fix redundant-packet detection so RTX and FEC blocks are keyed correctly when they
  coexist (RedBlockKey = protected seqnums + repair id).

Tests

• tests/check/elements/rtpsession.c: cooperation tests feeding overlapping RTX + FEC
  and asserting each means of repair is credited independently.

Scope

2 files (gst/rtpmanager/rtptwccstats.c, tests/check/elements/rtpsession.c),
single commit on top of main.

Consumed downstream by the Media stack (per-repair recovery reported on PmxStatsTX).

[Copilot]

Pull request overview

This PR extends rtptwccstats to attribute TWCC loss recovery per means of repair (e.g., RTX vs FEC) instead of a single aggregate, and adds tests to validate independent crediting and overlap reporting.

Changes:

• Track per-packet recovery attribution via {ssrc, pt} repair IDs and derive “recovered” state from status == LOST && recovered_by_count > 0.
• Compute and expose per-window recovery-by-repair stats (recovered count/pct + redundant bitrate) and recovery-overlap stats (pairwise overlap cells).
• Update rtpsession tests to assert per-repair recovery, overlap behavior, and repair bitrate accounting.

Reviewed changes

Copilot reviewed 2 out of 2 changed files in this pull request and generated 3 comments.

File	Description
subprojects/gst-plugins-good/gst/rtpmanager/rtptwccstats.c	Adds per-repair attribution, overlap accounting, and exposes new stats arrays in the TWCC stats structure.
subprojects/gst-plugins-good/tests/check/elements/rtpsession.c	Expands TWCC tests to validate per-repair recovery/overlap and repair bitrate reporting.

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[Copilot]

Pull request overview

Copilot reviewed 2 out of 2 changed files in this pull request and generated no new comments.

Comments suppressed due to low confidence (1)

subprojects/gst-plugins-good/tests/check/elements/rtpsession.c:210

• generate_rtx_buffer() no longer sets the RTX packet's RTP header timestamp (previously copied from the original packet). gst_buffer_copy_into(..., GST_BUFFER_COPY_TIMESTAMPS, ...) only copies the GstBuffer PTS/DTS, not the RTP header timestamp field, so the RTX packet will likely carry a zero/incorrect RTP timestamp.

  gst_rtp_buffer_map (new_buffer, GST_MAP_WRITE, &new_rtp);
  gst_rtp_buffer_set_payload_type (&new_rtp, TEST_RTX_BUF_PT);
  gst_rtp_buffer_set_ssrc (&new_rtp, TEST_RTX_BUF_SSRC);
  gst_rtp_buffer_set_seq (&new_rtp, rtx_seqnum);
  gst_rtp_buffer_unmap (&new_rtp);

  /* Copy over timestamps */
  gst_buffer_copy_into (new_buffer, buffer, GST_BUFFER_COPY_TIMESTAMPS, 0, -1);
  gst_rtp_repair_meta_add (new_buffer, 0, 1, orig_ssrc, &orig_seqnum,
      &orig_timestamp, 1);