[magic-peach]

Production exports are hanging

https://reframe-os.netlify.app is cross-origin isolated, so every browser loads the multi-threaded FFmpeg core. On main, FFmpeg chooses its own thread count, core-mt's 32-worker pool deadlocks, and every export sits at "Exporting 100%" forever. Datadog has no successful production export since 2026-10-22.

Reproduced locally with Playwright (headless Chromium, production build served with COOP/COEP):

Fixture	main, isolated	main, not isolated	this PR, isolated
normal.mp4 (720p H.264/AAC, 10 s)	hang (>120 s)	done 16.9 s	done 7.5 s
no-duration.webm (screen-recording style)	hang	done 12.7 s	done 6.4 s
no-audio.mp4	hang	—	done 6.3 s
portrait.mp4	hang	—	done 5.9 s
normal.mp4 → WebM / GIF / MKV	—	—	done 12.2 s / 54.6 s / 7.7 s

What changes

Ports the export hardening from the earlier branch (commits 33b2c5a and bf26cf1). These files hadn't changed on main since then except for #1829's telemetry, which this keeps.

• src/lib/ffmpegArgs.ts: one command builder, used by the worker. The copy in ffmpeg.ts was dead code, and the worker's own copy ignored sharpness and volume.
  • Decoder, filter and encoder threads are capped at 4, on the multi-threaded core only.
• Stall watchdog. The worker sends heartbeats. 60 s without activity fails the export with ExportStalledError (telemetry category timeout), and later attempts in the page use the single-threaded core.
• WebM uses VP8 + Vorbis. libvpx-vp9 and libopus trap in @ffmpeg/core 0.12.10.
• Engine loading. The multi-threaded core files are integrity-checked (SRI). If core-mt fails to load, the worker falls back to the single-threaded core.
• No more stuck "Exporting" state after an error with no export id, a cancel during engine load, or a string rejection from a wasm trap.

Changes after review (3fe0335, 29db120)

• Threads: per stage = min(4, navigator.hardwareConcurrency || 2). Before, an unreported core count meant 4.
• Automatic fallback: when the multi-threaded engine stalls, the same export (same file and edits) restarts once on the single-threaded engine instead of failing.
  • The overlay says it switched engines and started again.
  • RUM gets engine_fallback with stall_reason. The attempt's one outcome then has engine: single and fell_back: true.
  • A single-threaded stall is not retried; its message tells the user to press Retry.
• Duration: the editor passes the duration it already read to the export.
• Cleanup: 3fe0335 accidentally committed 31 untracked local duplicate files (names ending in " 2"), and 29db120 removes them. The PR diff no longer contains them. Please squash-merge so they never appear in main's history.

Checks

• bun run test: 29 files, 338 tests pass. They include the fallback (retried with the same edits, not retried when single-threaded or cancelled) and thread-count tests.
• Typecheck and lint are clean, and the build passes.
• Exports verified in headless Chromium as in the table above, re-run after the fallback change: normal 8.6 s, no-duration 6.1 s, portrait 6.7 s. A real engine stall couldn't be reproduced once the thread cap was in, so the fallback is covered by unit tests only. Not yet verified on the Netlify deploy preview: please do one export there before merging.

Follow-up (stacked PR, Phase 1 of the plan)

• Treat Infinity durations as unknown everywhere. outputDurationArgs still lets Infinity through when speed ≠ 1.
• Detect progress stuck at 100%.
• Report an unknown_duration failure category.
• Fixtures script and a Playwright end-to-end test in CI.

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[github-actions]

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