fix(mux): start fMP4 passthrough video at a sync sample by kixelated · Pull Request #6743 · moq-dev/moq

kixelated · 2026-10-25T06:37:40Z

Summary

An fMP4 passthrough video track that joined mid-GOP published its leading non-sync samples as the start of a group, frames a decoder cannot use. The unwrapped path already skipped them.

Both modes now share one rule, the Fmp4Track::orphaned flag from moq-dev/moq#6686: a video track starts orphaned, so run drops every sample until the first sync sample. A fragment that starts mid-GOP and reaches a sync sample is trimmed through the existing kept rewrite, so passthrough opens its group on the sync sample.

Choices made without asking:

Import::seek orphans video tracks too, since a seek is the other mid-stream join. Before, passthrough opened the seeked group on whatever fragment came next; unwrapped skipped via MissingKeyframe.
With both joins covered, the unwrapped path's MissingKeyframe swallow is unreachable, so it is gone and an unexpected one now fails loud. unwrap no longer reports whether a group opened; a skipped fragment never reaches it.
Audio stays un-orphaned at the start: its samples decode alone whatever their sync flags say.
A wholly dropped leading fragment no longer anchors the catalog clock, the same as a fragment the edit list drops.

Impact

Public API: none. fmp4::Import::seek docs now say video resumes at its next sync sample.
Wire: none.

Tests

New video_joined_mid_gop_starts_at_a_sync_sample covers both modes: a fully non-sync first fragment, a fragment whose sync sample is in the middle, and a mid-GOP fragment after seek. Before the fix, passthrough failed it.
just rs test -p moq-mux: 1743 passed.
just check: passed (the one shear warning, an unlinked moq-audio file, predates this branch).

Closes the quest quest/m1/fmp4-passthrough-sync.md.

(Written by Claude Opus 5.5)

🤖 Generated with Claude Code

Quest-Claim: 59c0063404e1a60d

A passthrough video track joined mid-GOP, at the start or after a seek, published its leading non-sync samples as the start of a group. Video tracks now start orphaned, and seek orphans them again, so both modes drop samples until the next sync sample through the edit-list rule. The unwrapped path's MissingKeyframe skip is unreachable and goes. Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>

chatgpt-codex-connector · 2026-10-25T07:08:35Z

Codex Review Summary

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Review	Status	Commit	Review trigger
📝 Code Review	✅ Completed 2026-10-25T18:09:43.071455Z	`279225a`	New commits

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kixelated

Automated review by review (OpenAI)

Reviewed commit: 78dac27

No actionable bugs found in the changed code. Reusing the orphaned-sample filter for initial joins and seeks is a focused fix: both output modes wait for a sync sample, while passthrough trimming preserves retained samples' decode/composition timing. Removing the downstream MissingKeyframe swallow follows from that shared invariant without a second mechanism.

Verification: static review of the patch and surrounding filtering, fragment rewriting, grouping and producer code. The regression covers whole-fragment drops, a sync sample inside a fragment and post-seek drops in both modes. I did not run tests or independently verify reported test results.

coderabbitai · 2026-10-25T07:15:41Z

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Reviewing files that changed from the base of the PR and between 78dac27 and 279225a.

Files selected for processing: 4

quest/m1/README.md
quest/m1/fmp4-passthrough-sync.md
rs/moq-mux/src/container/fmp4/import.rs
rs/moq-mux/src/container/fmp4/import_test.rs

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Reviewing files that changed from the base of the PR and between e8f3bed and 78dac27.

Files selected for processing: 4

quest/m1/README.md
quest/m1/fmp4-passthrough-sync.md
rs/moq-mux/src/container/fmp4/import.rs
rs/moq-mux/src/container/fmp4/import_test.rs

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quest/m1/README.md
quest/m1/fmp4-passthrough-sync.md

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Walkthrough

The fMP4 importer now starts video tracks in an orphaned state and drops video samples until a sync sample. Audio tracks start non-orphaned. Seeking marks video tracks orphaned again. The importer updates the current segment after processing a returned run, and unwrap propagates errors from producer.write. A new test checks startup and post-seek behavior in passthrough and non-passthrough modes. The related m1 planning entry and document were removed.

Priority: Normal

Merge Risk: Minimal · up to 78dac

No merge-blocking issue was established for the sync-sample gating change. The identified failed-seek path does not publish samples from the partially changed state.

Pre-merge checks: 4 · 1

Failed checks: 1 warning

Check name	Status	Explanation	Resolution
Docstring Coverage		Docstring coverage is 57.14% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 7 functions across 2 files.	Write docstrings for the functions missing them to satisfy the coverage threshold.

Passed checks: 4 passed

Check name	Status	Explanation
Title check		The title clearly summarizes the main change: fMP4 passthrough video now starts at a sync sample.
Description check		The description directly explains the sync-sample behavior, orphaned video tracks, seek handling, tests, and impact of the changes.
Linked Issues check		Check skipped because no linked issues were found for this pull request.
Out of Scope Changes check		Check skipped because no linked issues were found for this pull request.

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kixelated · 2026-10-25T16:25:07Z

Landing as is: CI is green, Codex and the OpenAI review of 78dac27 found nothing actionable, and quest check --base origin/main passes.

(Written by Claude Opus 5.5)

# Conflicts: # quest/m1/README.md

kixelated · 2026-10-25T18:13:49Z

Left the merge queue on a conflict with main. Merged main: only quest/m1/README.md conflicted, and the fMP4 edit-list change in import.rs merged cleanly with moq-mux fmp4 tests and clippy passing. Re-queued after a Codex thumbs-up on 279225a.

(Written by Claude Opus 5.5)

kixelated and others added 2 commits October 24, 2026 23:37

chore(quest): claim quest/m1/fmp4-passthrough-sync.md

08da3d4

Quest-Claim: 59c0063404e1a60d

kixelated changed the title ~~fMP4 passthrough starts at a sync sample~~ fix(mux): start fMP4 passthrough video at a sync sample Oct 25, 2026

kixelated marked this pull request as ready for review October 25, 2026 07:08

kixelated commented Oct 25, 2026

View reviewed changes

kixelated mentioned this pull request Oct 25, 2026

quest: record pre-cut follow-ups from the quest-spawn run #6790

Merged

kixelated added the quest:merge label Oct 25, 2026

kixelated added this pull request to the merge queue Oct 25, 2026

github-merge-queue Bot removed this pull request from the merge queue due to a conflict with the base branch Oct 25, 2026

kixelated added quest:merge and removed quest:merge labels Oct 25, 2026

Merge remote-tracking branch 'origin/main' into HEAD

279225a

# Conflicts: # quest/m1/README.md

kixelated added this pull request to the merge queue Oct 25, 2026

Merged via the queue into main with commit b797848 Oct 25, 2026
11 checks passed

kixelated deleted the quest/m1/fmp4-passthrough-sync branch October 25, 2026 20:31

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fix(mux): start fMP4 passthrough video at a sync sample - #6743

fix(mux): start fMP4 passthrough video at a sync sample#6743
kixelated merged 3 commits into
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