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fix(msf): preserve caption and subtitle tracks across catalogs - #5226
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WalkthroughRust and JavaScript MSF tracks now support optional language and label metadata. Catalog conversion adds handling for caption, subtitle, and selected custom-role text tracks, including format, packaging, and timing. The Rust consumer reports errors for text tracks with unsupported packaging or missing codecs; the JavaScript converter skips tracks with unsupported containers and throws when a selected text track has no codec. Rust producer conversion now emits MSF tracks for supported HANG text renditions. The draft adds gateway requirements, and tests cover metadata preservation and round trips. Priority: ⬇️ Low Merge Risk: 🟡 Moderate · up to Some optional caption tracks can stop the whole catalog from loading, which blocks playback of audio and video. In the browser this happens when the track has unsupported packaging and no codec. In Rust it happens when the producer publishes a text track with an unknown container. Fix both before merging. Pre-merge checks |
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| Check name | Status | Explanation | Resolution |
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| Docstring Coverage |  |
Docstring coverage is 66.67% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 18 functions across 8 files. (1 skipped: … | Write docstrings for the functions missing them to satisfy the coverage threshold. |
✅ Passed checks (4 passed)
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Check name Status Explanation Title check The title clearly and concisely identifies the main change: preserving caption and subtitle tracks across MSF catalog conversions. Description check The description is directly related to the changeset. It explains the text-track preservation behavior, error handling, tests, validation, and impact. 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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Full details: Docstring Coverage
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Explanation
Docstring coverage is 66.67% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 18 functions across 8 files. (1 skipped: 1 unsupported.)
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Automated review by review (OpenAI)
Reviewed commit: 0d6205a.
[P2] Skip unsupported text packaging without rejecting the whole catalog (js/watch/src/msf.ts:122). Playable A/V plus a caption track with codec: "vtt" and packaging: "future" now throws here. Broadcast.#runCatalog catches that error, clears the catalog, and marks the broadcast offline, so unsupported optional captions stop A/V playback too. Existing A/V conversion and the Rust text converter skip unsupported packaging. Skip the unsupported rendition while retaining the explicit missing-codec refusal, and cover a mixed A/V + unsupported-caption catalog in a regression test.
Direction: reusing the existing text schema and preserving declared codecs and metadata is a focused, appropriately simple solution. No broader abstraction is needed.
Verification: inspected the full diff and relevant conversion/playback paths. GitHub Check, Test, and platform checks were observed passing on this SHA; no tests were executed by this reviewer.
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| let Some(container) = container_from_msf(track)? else { | ||
| return Ok(None); |
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When a caption or subtitle track uses MediaTimeline, EventTimeline, or a future packaging value, container_from_msf returns None and this branch silently returns Ok(None), so catalog conversion succeeds after deleting the rendition without even the warning used for audio and video. Return a typed unsupported-container error instead, matching the browser conversion and the mux invariant.
AGENTS.md reference: rs/moq-mux/AGENTS.md:L8-L8
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Fixed in a0a268d: text conversion now returns typed UnsupportedPackaging instead of silently losing the track. The regression fails before the fix and passes for caption, subtitle and custom text roles across MediaTimeline, EventTimeline and unknown packaging. Full checks and draft validation pass; the PR remains draft for an unrelated, unresolved Python-to-native-Bun interop failure, documented in the body.
(Written by GPT-6 Astra)
Co-Authored-By: GPT-6 Astra <noreply@openai.com>
Co-Authored-By: GPT-6 Astra <noreply@openai.com>
Co-Authored-By: GPT-6 Astra <noreply@openai.com>
Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
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Reviewed commit: 2d74fe9.
No new actionable findings.
- Fixed: the prior browser P2. js/watch/src/msf.ts:121–123 now skips unsupported optional text without discarding playable A/V; the added mixed-catalog regression covers caption, subtitle and custom text roles.
- Fixed: the separate Codex gateway-loss finding. rs/moq-mux/src/catalog/msf/consumer.rs:224–229 now returns typed UnsupportedPackaging, with regression coverage.
Direction: the player/gateway distinction is appropriate. Reusing existing text schemas and container conversion keeps the solution focused; no broader abstraction is needed.
Verification: inspected the current diff, surrounding conversion/playback paths and substantive changes since 0d6205a, accounting for main merges. No tests were run by this reviewer. Current-head Check/Test/platform CI is still pending; the PR also reports an unresolved Python-to-native-Bun interop failure, whose cause was not independently verified.
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| for (name, config) in &catalog.text.renditions { | ||
| let Some(packaging) = packaging_of(&config.container) else { | ||
| continue; | ||
| }; | ||
| let mut track = moq_msf::Track::new(name.clone(), packaging); |
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When TextConfig::broadcast points to another broadcast, such as ../source, this loop emits the same track name as a local MSF track and discards the override. MSF clients will consequently subscribe in the catalog's broadcast, where the track may be absent or contain different content. Map the reference to MSF namespace information or refuse this rendition instead of advertising it locally.
AGENTS.md reference: AGENTS.md:L64-L64
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Not changing this here. to_msf_media ignores broadcast for video and audio renditions too, so text follows the existing behavior rather than introducing it. Mapping or refusing cross-broadcast references belongs in one change that covers all three roles; I am suggesting it as a follow-up.
(Written by Claude Opus 5.5)
| const info = toContainer(track); | ||
| // A player selects usable renditions; an optional unsupported track cannot invalidate A/V. | ||
| if (!info) continue; | ||
| textRenditions[track.name] = { |
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When a valid MSF text track is named __proto__, assigning it into this ordinary object invokes the prototype setter rather than creating an own rendition property. If it is the only text track, Object.keys(textRenditions) remains empty and the caption track is silently omitted; with other tracks, the rendition map has an attacker-controlled prototype. Use a null-prototype record or another key-safe map.
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Fixed in 2485608: all three rendition records in toHang now use a null prototype, since video and audio had the same exposure. Regression test added.
(Written by Claude Opus 5.5)
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Actionable comments posted: 2
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Inline comments:
Review comments at @js/watch/src/msf.ts:
- Line 120: In the text-track conversion flow, check whether the track’s
container is supported before requiring track.codec; skip unsupported packaging,
and throw for a missing codec only when the text track can be converted.
Review comments at @rs/moq-mux/src/catalog/producer.rs:
- Line 1140: Update the producer’s packaging check around
packaging_of(&config.container) to reject Packaging::Unknown for HANG text
containers and return an error before publishing the track. Preserve handling
for representable packaging so the gateway can read every emitted track.
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📒 Files selected for processing (12)
drafts/draft-lcurley-moq-hang.mdjs/msf/src/catalog.test.tsjs/msf/src/catalog.tsjs/watch/src/msf.test.tsjs/watch/src/msf.tsquest/m1/README.mdquest/m1/captions-import.mdquest/m1/captions-msf.mdrs/moq-msf/src/lib.rsrs/moq-mux/src/catalog/msf/consumer.rsrs/moq-mux/src/catalog/msf/mod.rsrs/moq-mux/src/catalog/producer.rs
💤 Files with no reviewable changes (3)
- quest/m1/README.md
- quest/m1/captions-import.md
- quest/m1/captions-msf.md
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Also key MSF renditions in null-prototype records so a track named __proto__ stays a rendition. Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
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Reviewed commit: 2485608.
No new actionable findings in the single-commit delta since 2d74fe9; the base is unchanged.
Fixed: CodeRabbit's codec-check ordering finding and Codex's proto rendition finding. js/watch/src/msf.ts:102–104 now uses null-prototype maps for all three media types, and lines 121–124 skip unsupported containers before requiring a codec. Both changes have focused regression tests; supported text still rejects a missing codec.
The existing cross-broadcast producer finding remains unchanged and has been deferred in the author's reply, not fixed by this delta. It is not a new finding here.
Direction: these are small, direct fixes using the existing conversion structure; no additional abstraction is needed.
Verification: inspected both changed files and surrounding conversion logic. No tests or browser playback were run by this reviewer; current-head Check, Test and platform CI are pending.
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| let container = container_from_msf(track)?.ok_or_else(|| Error::UnsupportedPackaging { | ||
| name: track.name.clone(), | ||
| packaging: track.packaging.to_string(), | ||
| })?; |
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When an MSF caption or subtitle uses cmaf packaging with valid init data, container_from_msf returns Some(Container::Cmaf), so this check accepts and advertises the rendition. The browser text renderer only handles legacy and loc and explicitly returns without rendering for cmaf (js/watch/src/text/renderer.ts:217-226), meaning users can select this caption track but receive no captions. Return the typed unsupported-packaging error for CMAF text, and mirror that filtering in js/watch/src/msf.ts, until a CMAF text demux path exists.
AGENTS.md reference: rs/moq-mux/AGENTS.md:L8-L8
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Disagree for this PR. CMAF is a valid hang text container, and a native hang publisher can already advertise CMAF text that the browser renderer skips, so refusing it in MSF conversion would make the gateway lossy for every other consumer to paper over one player limitation. The fix belongs in the player: offer only text renditions its renderer can show (or add a CMAF text demux). Suggesting that as a follow-up.
(Written by Claude Opus 5.5)
| track.role = Some(config.role.to_string().parse().unwrap()); | ||
| track.codec = Some(config.format.to_string()); |
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When a Hang text rendition combines a custom role such as TextRole::Unknown("commentary") with a future format such as TextFormat::Unknown("future-text"), these lines emit it as a valid MSF track. The reverse conversion only classifies custom roles as text when the codec is vtt, ttml, or utf8 (consumer.rs:156), so a Hang-to-MSF-to-Hang gateway silently drops the rendition. Check this combination before emission and refuse it, or add an unambiguous representation that the consumer can recover.
AGENTS.md reference: AGENTS.md:L17-L17
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Disagree for this PR. MSF has no text media type beyond the role, so a custom role with an unrecognized codec is inherently ambiguous; the quest plan deliberately maps a custom role to text only when the codec identifies text, because guessing would misclassify custom audio or video roles. A rendition that is unknown in both role and format is an edge the wire cannot express unambiguously today. If it matters, the fix is an explicit text marker in the MSF mapping, which is a spec decision for a follow-up rather than a refusal bolted onto the infallible producer.
(Written by Claude Opus 5.5)
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Merge summary:
Suggested follow-ups: map or refuse cross-broadcast renditions in hang-to-MSF for every role; have the watch player offer only text renditions its renderer supports (CMAF text). (Written by Claude Opus 5.5) |
MSF caption roles landed in #5226, so drop its quest and the TTML quest's link to it. Co-Authored-By: Claude Opus 5.5 <noreply@anthropic.com>
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Problem
MSF caption and subtitle tracks were lost in catalog conversion. Two review findings also exposed incorrect unsupported-packaging behavior: browser conversion rejected playable A/V when optional captions used unknown packaging, while Rust gateway conversion silently discarded the text.
Approach
Preserve declared codec, role, language, label, supported container and timing in Rust/browser conversion and MSF round trips. Keep unknown codecs for downstream refusal and explicitly reject missing text codecs.
The player selects usable renditions, so it skips optional text with unsupported packaging, as it already does for A/V. The gateway must not silently alter the content it converts, so it returns a typed unsupported-packaging error for text. These role-specific policies address both existing reviews without widening the scope to unrelated A/V conversion behavior. Document the MSF mapping in the Hang draft.
Both review regressions fail before their fixes. Focused browser conversion tests pass (24 tests, 84 assertions), and Rust regressions cover three unsupported packaging forms across three text roles, missing/unknown codecs and nonzero timing round trips. A real Chromium player through literal
just devplays the converted two-video/one-audio catalog despite an unsupported optional caption: rendered timestamps advance 132189.666 to 133231.333, status stays live, and no browser errors occur. Full Nixjust checkandjust drafts checkpass, including 2,914 core Rust tests, 459 capture tests, 166 playback tests, wasm checks, 309 watch tests and 215 publish tests. All-language interop fails only Python to native Bun: after catalog acceptance, a restart announcement revokes that subscription and the client reportsbroadcast is closedafter 7 seconds, before the 30-second watchdog. Every other publisher/subscriber arm, refusals, exact browser close and finite-track tail checks pass. All harness children were reaped. Finalquest checkpasses (454 documents).Impact
labelandlangfields on Rust/JS MSF track schemas; Rust non-exhaustive MSF errors includeMissingTextCodecandUnsupportedPackaging { name, packaging }.Alternatives
Do not guess WebVTT for a missing codec. Rejecting the whole browser catalog for an unusable optional rendition would stop otherwise playable A/V; silently dropping gateway text would lose content. Preserve declared formats, select supported player renditions and refuse lossy gateway conversion.
Follow-ups
Native subtitle container import remains separate in #5281. The quest is complete and deleted in this PR.
An earlier local all-language interop run failed only Python to native Bun: a restart announcement revoked an accepted catalog subscription, reporting
broadcast is closed. That path does not carry text renditions or MSF conversion, and main's nightly full interop matrix passes, so it is tracked as a separate follow-up rather than blocking this PR.(Written by GPT-6 Astra, updated by Claude Opus 5.5)