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fix(export): reject clip starts across large video gaps - #1379
A clip starting inside a large timestamp gap can currently export a frame from several seconds later. The start check added in #1377 only bounds target - frame_time, so every future frame passes.
Bound the timestamp difference in both directions using the existing 0.5-second tolerance. If a seek lands far in the future, seek the actual first frame to distinguish a valid non-zero stream origin from an interior gap. Requests before that first timestamp retain the existing first-frame hold. Normal short-tail recovery and decoder clocks stay intact. Add a regression with a 0.983–3.000 s gap and a 1.5 s clip start, covering screen/camera sources and failure after a valid preceding clip; each case preserves the existing destination and removes the staged output.
The existing macOS frame-count probe was a stub, preventing the export timing suite from passing. Implement it using the publication validator's decoder ownership and displayable-frame rules, with MP4/Matroska coverage. Publication still stops at the first valid frame; only the probe performs a full scan.
Presenter Overlay capture compatibility remains tracked separately in #1378; this does not close it.
Type of change
Bug fix
Release impact
Patch
Desktop impact
Windows
macOS
Linux
Testing
Local macOS 15.7.1, Apple M2, Rust 1.99.1, Node 22.22.1, FFmpeg SDK:
Both the interior-gap regression and the maintainer-reported offset-origin regression failed before their fixes and pass afterward.
cargo test --manifest-path crates/Cargo.toml -p openscreen-compositor --lib -- --test-threads=1: 467 passed on 926b4769; after the offset-origin follow-up, the affected timeline_walk::tests were rerun: 10 passed.
OPENSCREEN_TEST_MEDIA=<synthetic-media> cargo test --manifest-path crates/Cargo.toml -p openscreen-compositor --test export_timing -- --test-threads=1: 14 passed, including normal tails, H264/HEVC output, valid sources whose first timestamp is 1 s, and interior gaps with both zero and 1 s stream origins. Fixture recipes are in the test header.
npm run test -- --maxWorkers=2: 356 files, 4,966 passed, 2 skipped.
Both application and test TypeScript checks passed. npm run docs:check: 37 files passed. Biome on tracked supported files: no errors, 25 existing warnings.
Clippy was run with --all-targets --no-deps. It remains blocked by 9 existing test-target errors (7 in the library); comparison with the pre-PR baseline found no new diagnostics. No checks were disabled.
Desktop computer-use on the initial revision: gap export reported a screen-start error and preserved a seeded MP4 byte-for-byte; no partial remained. A normal clip plus a short tail then exported successfully in the same session. Independent probing counted 120 frames / 4.000 s at 1752×1080; full-file decode exited 0. On the offset-origin revision, a zero-time clip with its first frame at 1 s also exported successfully (120 frames / 3.967 s, full decode exit 0); a subsequent interior-gap export preserved that file byte-for-byte. Scope recorded in the manual checklist.
Windows/Linux runtime, physical capture, Presenter Overlay and release packaging were not exercised locally. Native module compilation passed; the local packaging helper's duplicate loader-path staging issue was bypassed only for the development smoke test, without changing packaging code.
Summary by CodeRabbit
Bug Fixes
Video frame counts now include frames that become available only after decoding reaches the end of a file, improving counts for supported video formats.
Export validation now rejects clips that land more than 0.5 seconds before or after the requested start time, including seeks across timestamp gaps. Failed exports preserve any existing destination file and do not leave partial output.
Seeking continues to accept frames up to 0.5 seconds after the requested start time and correctly handles streams whose first frame begins later than time zero.
The macOS frame probe now counts decoded frames, including delayed decoder output. Clip-start validation accepts nearby frames before or after the target, rejects distant frames across interior gaps, and preserves support for streams with a late first frame.
The shared decoder supports first-frame validation and full frame counting. The macOS probe uses full-count mode. A macOS test checks that MP4 and Matroska inputs each report 240 decoded frames.
Clip-start gap validation crates/compositor/src/timeline_walk.rs, crates/compositor/tests/export_timing.rs, technical-documentation/architecture/export-pipeline.md, technical-documentation/testing/manual-e2e-checklist.md
Clip-start seeking accepts frames within 0.5 seconds before or after the target. Tests cover gap failures, late first-frame timestamps, destination preservation, and absence of partial output. Fixture instructions and documentation describe the checks.
sequenceDiagram
participant Probe as pipeline_macos::probe_frame_count
participant Count as count_decoded_video_frames
participant Decoder as decode_video_frames
Probe->>Count: request decoded frame count
Count->>Decoder: decode in full-count mode
Decoder-->>Count: return displayable frame count at EOF
Count-->>Probe: return frame count
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Possibly related issues
getopenscreen/openscreen#1378: The change rejects clip starts across source gaps, which relates to the reported camera track with no frames at the required clip start.
No material issue is established that should delay merging.
Pre-merge checks: 5
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The title clearly identifies the primary change: rejecting clip starts that fall across large video timestamp gaps.
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The description covers the change, related issue, change type, release impact, supported platforms, and detailed testing. The related issue section uses a review link instead of the template's preferr…
Docstring Coverage
Docstring coverage is 100.00% which is sufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 11 functions across 4 files. (2 skipped: 2…
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Explanation
Docstring coverage is 100.00% which is sufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 11 functions across 4 files. (2 skipped: 2 unsupported.)
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Thanks for the follow-up, the regression test and the macOS probe are great additions.
One question before this can go in: cur_time_sec() returns the raw best_effort_timestamp, not one relative to the stream's start. With the new abs() bound, a source whose first video frame is stamped later than 0.5 s (non-zero start_time, e.g. a camera that started late or a browser WebM whose first packet is offset) would now fail at a clip start of 0, where it used to export. Could you confirm that case still exports, ideally with a fixture whose first frame sits at ~1 s (e.g. -output_ts_offset 1)? If it doesn't, comparing against the stream's first frame time would keep the gap rejection without that side effect.
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Actionable comments posted: 1
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Inline comments:
Review comments at @crates/compositor/src/timeline_walk.rs:
- Line 270: Update seek_clip_start’s start-position check to compare the seek
result against the source’s first displayable frame timestamp, so valid offset
sources are accepted; retain the existing rejection of targets that fall in
interior gaps.
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Confirmed: the 1 s first-timestamp fixture failed at clip start 0 with the initial absolute bound. Fixed in 69bf06e.
Only the distant-future branch performs an additional seek to the actual first frame. A request before that frame retains the existing first-frame hold; a target after the first frame that lands in an interior gap still fails. Normal seeks and decoder timestamps are unchanged.
Added -output_ts_offset 1 fixtures. The export suite now passes 14/14, covering screen and camera offset sources, and gaps with both zero and 1 s origins, with/without an earlier valid clip. Desktop verification also passed: the offset source exports; a subsequent gap export rejects and preserves that output byte-for-byte. The 10 timeline unit tests pass, the release addon builds, and Clippy has no new diagnostics against the baseline.
⚠️Unverified finding
Verification ran but could not confirm this finding. It is shown for review, not as a verified issue.
Clean up the offset test directory when the test fails.
If an assertion or expect fails, execution never reaches remove_dir at line 571. The test then leaves output in the supplied fixture directory. Use a per-test temporary directory with a cleanup guard instead of create_dir_all. Based on learnings, Rust integration-test artifacts need isolated directories with cleanup guards.
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Reviewing files that changed from the base of the PR and between 926b476 and 69bf06e.
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Thanks for the review and merge. The functional findings are resolved, and all checks on the merged revision passed, including Nix.
I also addressed the remaining test-cleanup suggestion in #1386: the new gap/origin/probe tests use isolated temporary outputs with an unwinding cleanup guard. All 15 export timing tests pass locally, including a simulated assertion-failure cleanup regression. This follow-up changes only tests.
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Summary
A clip starting inside a large timestamp gap can currently export a frame from several seconds later. The start check added in #1377 only bounds
target - frame_time, so every future frame passes.Bound the timestamp difference in both directions using the existing 0.5-second tolerance. If a seek lands far in the future, seek the actual first frame to distinguish a valid non-zero stream origin from an interior gap. Requests before that first timestamp retain the existing first-frame hold. Normal short-tail recovery and decoder clocks stay intact. Add a regression with a 0.983–3.000 s gap and a 1.5 s clip start, covering screen/camera sources and failure after a valid preceding clip; each case preserves the existing destination and removes the staged output.
The existing macOS frame-count probe was a stub, preventing the export timing suite from passing. Implement it using the publication validator's decoder ownership and displayable-frame rules, with MP4/Matroska coverage. Publication still stops at the first valid frame; only the probe performs a full scan.
Related issue
Follow-up to the final review on #1377: #1377 (review)
Presenter Overlay capture compatibility remains tracked separately in #1378; this does not close it.
Type of change
Release impact
Desktop impact
Testing
Local macOS 15.7.1, Apple M2, Rust 1.99.1, Node 22.22.1, FFmpeg SDK:
cargo test --manifest-path crates/Cargo.toml -p openscreen-compositor --lib -- --test-threads=1: 467 passed on926b4769; after the offset-origin follow-up, the affectedtimeline_walk::testswere rerun: 10 passed.OPENSCREEN_TEST_MEDIA=<synthetic-media> cargo test --manifest-path crates/Cargo.toml -p openscreen-compositor --test export_timing -- --test-threads=1: 14 passed, including normal tails, H264/HEVC output, valid sources whose first timestamp is 1 s, and interior gaps with both zero and 1 s stream origins. Fixture recipes are in the test header.cargo build --manifest-path crates/Cargo.toml -p compositor-view-napi --release: passed.npm run test -- --maxWorkers=2: 356 files, 4,966 passed, 2 skipped.npm run docs:check: 37 files passed. Biome on tracked supported files: no errors, 25 existing warnings.--all-targets --no-deps. It remains blocked by 9 existing test-target errors (7 in the library); comparison with the pre-PR baseline found no new diagnostics. No checks were disabled.Windows/Linux runtime, physical capture, Presenter Overlay and release packaging were not exercised locally. Native module compilation passed; the local packaging helper's duplicate loader-path staging issue was bypassed only for the development smoke test, without changing packaging code.
Summary by CodeRabbit