[magic-peach]

Stacked on #1831. It targets fix/mt-export-hang, so the diff shows only this work. Merge #1831 first, then change this PR's base to main.

A correction: the earlier diagnosis was wrong

I said earlier that production exports hang because getVideoDuration returns Infinity and the export gets -t Infinity. That was wrong. The worker only adds -t when the speed isn't 1×, and the hangs I saw were at 1×.

The real cause is that the multi-threaded FFmpeg core deadlocks when FFmpeg picks its own thread count. Production is cross-origin isolated, so every visitor loaded that core and every export hung, normal MP4s included. Locally, all four fixtures hang with COOP/COEP and finish without it. #1831 fixes this with a thread cap, plus a watchdog that falls back to the single-threaded core.

This PR fixes a separate bug. Browser-recorded files such as screen recordings have no duration in their header, so <video>.duration is Infinity. The editor turned that into 0:

• the trim fields had no limit and showed "Duration: 0:00";
• a sped-up export had no -t;
• a builder copy without the guard would have passed -t Infinity.

None of this caused the production hang.

What changes

Durations

• src/lib/mediaDuration.ts: when the duration is Infinity, seek past the end so the browser works out the real length. Used by the editor and the export.
• src/lib/ffmpegArgs.ts:
  • withSafeTimes repairs a non-finite or negative trim start, a trim end that isn't after the start, and an unusable speed before any filter is built.
  • -t is added only for a finite, positive length.
  • -shortest is added when looped music would otherwise never end.
  • assertFiniteArgs throws if any argument still contains Infinity or NaN.
  • An infinite speed no longer makes the audio filter builder loop forever.
• I checked every other duration, fps and time value that reaches FFmpeg: trim and atrim, setpts, atempo, GIF -t, and text and overlay positions. They all go through these builders. FPS isn't set anywhere.

Stalls

• The worker computes progress from encoded time ÷ expected length, instead of FFmpeg's own ratio, which needs a header duration. It also reports the encoded seconds.
• The watchdog stops an export when:
  • there's no activity for 60 s (the deadlock);
  • encoded time doesn't change for 120 s while FFmpeg keeps logging (stuck at 100%);
  • encoding runs past 1.25 × the expected length + 15 s (an overrun).
• Each case shows a clear message with a Retry button. On the multi-threaded core, fix(ffmpeg): stop multi-threaded exports from hanging at 100% #1831's automatic single-threaded retry happens first.

Telemetry

• New failure category unknown_duration: a clip whose length couldn't be found that then stalled or failed.
• export_failed.stall_reason: no_activity, no_progress or overrun.
• export_cancelled.stuck: true when the user cancelled after 60 s with no progress. Monitors count these as failures.
• The dashboard JSON gets a "Stalled exports by reason and engine" widget, and the schema is documented in docs/DATADOG.md.

Monitoring (already live in Datadog, not part of the diff)

• New monitor 340168191, "Exports start but none succeed": more than 3 export_started and 0 export_succeeded on production over 6 h. Email only.
• Monitor 340137974 now fires on exports not completing: started − succeeded − ordinary cancellations, over 1 h (more than 10, warning at 5). That covers failures, stuck cancellations and abandoned exports.
• Why the old monitors stayed OK during the outage: hung exports produced no export_failed, only cancellations or nothing at all. Note that builds before feat(telemetry): export reliability by browser and device #1829 used older event names, so these monitors only see traffic from feat(telemetry): export reliability by browser and device #1829 onward.

Tests and CI

• scripts/make-fixtures.sh makes the fixtures in test/fixtures/ (gitignored), all with the ffmpeg CLI:
  • normal 10 s H.264/AAC;
  • WebM with no duration (-live 1, the MediaRecorder layout);
  • portrait;
  • no audio;
  • about 300 MB.
• scripts/serve-isolated.mjs serves out/ with or without COOP/COEP.
• test/e2e/export.spec.ts (Playwright) runs on both the multi- and single-threaded engines:
  • every fixture: 1:1 preset, trim 1–5 s, rotate 90°, 2×, export, download, then ffprobe checks 1080×1080, about 2 s, and the expected audio stream count;
  • the WebM's real length shows in the editor;
  • the WebM at 2× exports its full 3 s.
• New Tests workflow: Vitest, plus Playwright in headless Chromium with ffmpeg from apt. main didn't run unit tests in CI before.
• New Vitest tests: every combination of bad time values, resolveDuration, the watchdog's three rules, no false stall without time reports, unknown_duration / stall_reason / stuck, and the hook passing its known duration.

Checks run locally

• Vitest: 30 files, 362 tests pass. Typecheck and lint are clean.
• Playwright: 12 passed (6 per engine). The ~300 MB clip (E2E_LARGE=1, multi-threaded) also passed: trim, rotate, 2× and export in 22.9 s.
• Build passes with NEXT_PUBLIC_DATADOG_ENV=off.
• CI for the new workflow will run on this PR. I haven't seen it run yet.

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