moq-gst: Fix MoqSink CAPS handling and per-pad EOS aggregation

[SteveMcFarlin]

This fixes two correctness issues in sink/imp.rs: CAPS events are now passed to event_default() after configuring the background task, so pad state still updates normally, and EOS tracking is now per-pad instead of using a single counter. That avoids premature or missed element EOS when pads are dynamic, dropped, or emit EOS more than once.

[SteveMcFarlin]

@sidsethupathi @kixelated. Let me know if you have any questions. I tested with:

gst-launch-1.0 -v -e videotestsrc is-live=true num-buffers=120 ! video/x-raw,width=1280,height=720,framerate=60/1,format=I420 ! x264enc ! moqsink name=mux url="https://cdn.moq.dev/anon" broadcast="videotestsrc"

My main concern was with a possible race conditions between add/remove pad and EOS. Some of the other items are just for gst state.

[coderabbitai]

No actionable comments were generated in the recent review. 🎉

ℹ️ Recent review info

⚙️ Run configuration

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Run ID: 84c6ebef-1e27-4bff-810a-45c5c351b314

📥 Commits

Reviewing files that changed from the base of the PR and between 6adf3c3 and afdf280.

📒 Files selected for processing (1)

• rs/moq-gst/src/sink/imp.rs

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Walkthrough

MoqSink's sink-pad event handling was refactored to aggregate EOS signals per pad instead of using a global counter. The ControlMessage::Eos variant now includes a pad_name field to identify which pad sent EOS. A HashSet named eos_pads replaces the previous counter to track which pads have emitted EOS. The handle_event method converts CAPS and EOS events to control messages with per-pad information. The background session's loop now aggregates EOS by checking that all currently configured pads have reported EOS before posting a single element EOS, and removes pad names from the set when pads are dropped.

🚥 Pre-merge checks | ✅ 4 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 0.00% which is insufficient. The required threshold is 80.00%.	Write docstrings for the functions missing them to satisfy the coverage threshold.

✅ Passed checks (4 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title 'moq-gst: Fix MoqSink CAPS handling and per-pad EOS aggregation' accurately describes the two main fixes in the changeset: CAPS event handling and per-pad EOS aggregation logic.
Description check	✅ Passed	The description directly addresses both correctness issues fixed in the changeset: CAPS event routing to event_default() and per-pad EOS tracking instead of a single counter.
Linked Issues check	✅ Passed	Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check	✅ Passed	Check skipped because no linked issues were found for this pull request.

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[sidsethupathi]

Should gst::Pad::event_default be called for EOS as well?

[SteveMcFarlin]

No. This is a terminal SINK. It will never forward on EOS downstream. The caps need to for internal state.

[sidsethupathi]

Confirmed that multiple pads still works as expected.

gst-launch-1.0 -v -e videotestsrc is-live=true num-buffers=120 ! video/x-raw,width=1280,height=720,framerate=60/1,format=I420 ! x264enc ! moqsink name=mux url="https://cdn.moq.dev/anon" broadcast="videotestsrc" audiotestsrc is-live=true num-buffers=100 samplesperbuffer=882 ! avenc_aac ! mux.

[SteveMcFarlin]

Hand tested a failure case pointed at /dev/null.

2026-05-12T03:45:22.159469Z  WARN moq_native::client: QUIC connection failed err=failed DNS lookup
2026-05-12T03:45:22.228507Z  WARN moq_native::websocket: WebSocket connection failed err=failed to connect WebSocket

ERROR: from element /GstPipeline:pipeline0/GstVideoTestSrc:videotestsrc0: Internal data stream error.

I will look into this more for proper handling. For now I think this will prevent a few hard to find bugs with EOS.

[kixelated]

Yep this is closer to how I expected EOS handling to work.