fix(producer): audioPadTrim FFmpeg-8.x-compatible apad invocation

[vanceingalls]

What

Change audioPadTrim's pad-concat step to use bare file paths in the concat script instead of file:// URLs. One-line change to concatFileLine in packages/producer/src/services/render/audioPadTrim.ts; drops the pathToFileURL import; updates the arg-shape unit tests + adds a Windows-path regression pin.

Why

Four field-signal reports over ~24h — all on Windows, CLI 0.7.59:

• ts=1784169914 (:warning: 7/10, win32/x64, Baoyu) — 60s render, native audio assembly failed; workaround = video-only render + external ffmpeg mux.
• ts=1784177061 (:red_circle: 5/10, win32/x64, andre 22cores) — 345.87s composition, 9 WAV audio elements, same error shape.
• ts=1784177375 (:red_circle: 3/10, win32/x64, ffmpeg 8.1.1-full_build gyan.dev) — KEY DIAGNOSTIC. 13 mono 44.1kHz mp3 tracks. Reporter isolated the failure: "Same project rendered fine in July with an older ffmpeg. Manual ffmpeg -i track.mp3 -af apad=whole_dur=16 -t 16 -c:a aac out.aac works with the same binary, so the tool's audioPadTrim invocation appears incompatible with ffmpeg 8.x."
• ts=1784177375 (follow-up from same reporter).

Error signature in all four:
```
[audioPadTrim] FFmpeg exited with code 4294967294
Impossible to open file:///…/Temp/hf-render-*/audio.aac
```
4294967294 = -2 unsigned = FFmpeg's AVERROR(ENOENT). The "Impossible to open …" string is the concat demuxer's message for a file it couldn't open.

How

Ground-truth audit findings:

1. packages/producer/src/services/render/audioPadTrim.ts (line 233-236, pre-fix) built each concat directive as:
   ```ts
   function concatFileLine(path: string): string {
   const normalized = pathToFileURL(path).href;
   return `file '${normalized.replace(/'/g, "'\\''")}'`;
   }
   ```
   On Windows, pathToFileURL("C:\\…\\audio.aac").href produces file:///C:/…/audio.aac. FFmpeg's file: protocol handler (libavformat/file.c) strips the file: prefix, leaving ///C:/…/audio.aac. FFmpeg 8.x on Windows rejects this triple-slash form.

2. The sibling concat scripts in this repo do NOT use URLs. packages/producer/src/services/distributed/assemble.ts (line 184), packages/engine/src/services/chunkEncoder.ts (line 680), and the smoke test packages/producer/tests/distributed/_smoke/webm-concat-copy.test.ts all use bare paths with only single-quote escaping (file '${path.replace(/'/g, "'\\\\''")}') — those work on Windows today. audioPadTrim.ts was the outlier.

3. Regression provenance. The original implementation (commit 62317f7f3, 2026-05-13) used apad=pad_dur=Δ — the same filter family as the reporter's proven-working manual command. PR fix(engine): preserve AAC start time during MP4 mux #1615 (commit 0473254bd, 2026-06-20, "fix(engine): preserve AAC start time during MP4 mux") refactored the pad path to the concat-copy approach and introduced the pathToFileURL conversion. That refactor is the direct source of the Windows-8.x incompatibility.

4. Manual command vs. module command diff:

   • Reporter's working command: ffmpeg -i track.mp3 -af apad=whole_dur=16 -t 16 -c:a aac out.aac — filter-based padding, no concat demuxer.
   • Module's failing command (step 2, pad-concat): ffmpeg -f concat -safe 0 -protocol_whitelist file,pipe,crypto,data -i pipe:0 -c:a copy -y outputPath with stdin file 'file:///C:/…/audio.aac'\nfile 'file:///C:/…/silence.aac'\n.
   • The concat approach is intentional (avoids re-encoding the pre-mixed AAC — that was the point of PR fix(engine): preserve AAC start time during MP4 mux #1615's refactor). Fix is to keep the concat approach but switch to bare paths so it survives FFmpeg 8.x on Windows.

The fix (packages/producer/src/services/render/audioPadTrim.ts, concatFileLine):

```diff
function concatFileLine(path: string): string {

• const normalized = pathToFileURL(path).href;
• return `file '${normalized.replace(/'/g, "'\\''")}'`;

• // Bare paths in concat directives — NOT `file://` URLs. FFmpeg 8.x on
• // Windows fails to open URL-form paths from the concat demuxer with
• // "Impossible to open file:///C:/…" (its file protocol strips the
• // `file:` prefix leaving `///C:/…`, which Windows path parsing then
• // rejects). …
• return `file '${path.replace(/'/g, "'\\''")}'`;
  }
  ```

Also drops the now-unused pathToFileURL import.

Test plan

• bun test packages/producer/src/services/render/audioPadTrim.test.ts — 17/17 pass (16 pre-existing + 1 new regression pin asserting the pad-concat stdin never contains file:// for a Windows-shaped input path).
• node scripts/run-test-lane.mjs unit bun (producer) — 365 pass / 0 fail.
• bun x vitest run src/services/render/stages/assembleStage.test.ts — 3/3 pass (audioPadTrim consumer, mocks the module).
• bun run --filter @hyperframes/producer typecheck — clean.
• oxlint + oxfmt --check on the two changed files — clean.
• End-to-end verification on Windows + FFmpeg 8.x — I don't have a Windows box with FFmpeg 8.x to reproduce the pre-fix failure and verify the post-fix success. Reviewer with access, please:
  1. Reproduce with main: render a composition on Windows that triggers the pad branch (audio shorter than video by >1ms) with ffmpeg 8.1.1 on PATH → expect the "Impossible to open file:///…/audio.aac" error.
  2. Repeat with this branch → expect the render to complete with the concat-copy pad step succeeding.

Enterprise release / feature flag holdout

• N/A — no user-facing behavior change beyond the bug fix
• Feature-flagged and held out from enterprise until validated

UX/Screenshot recording

• No UI impact — this is a producer-side FFmpeg arg shape fix, backend-only.

Co-Authored-By: Claude noreply@anthropic.com

🤖 Generated with Claude Code

— Via

[miga-heygen]

Review — #2525: audioPadTrim FFmpeg 8.x-compatible apad invocation

Invariant: concatFileLine must produce bare-path concat directives (no file:// URLs) with the concat demuxer's own single-quote escaping.

Bug: pathToFileURL(path).href produced file:///C:/... URLs. FFmpeg 8.x's concat demuxer on Windows strips the file: prefix, leaving ///C:/..., which Windows path parsing rejects.

Fix: Drop pathToFileURL, use bare path directly. Single-quote escaping unchanged.

What I verified

• Sibling convention (assemble.ts:184): Verified — uses identical bare-path pattern, no pathToFileURL
• pathToFileURL import removed: node:url import dropped cleanly
• Escaping correctness: Concat demuxer's documented escape rule for single quotes
• Existing test updated: Assertions switched to bare paths, plus negative regression pin
• New Windows regression test: Bare Windows paths verified, no file:// or file:\\ URLs
• Special characters: Bare paths are MORE correct than percent-encoded URLs for the concat demuxer

Disconfirming probe: is there any code path in this file that still produces file:// URLs? Searched: pathToFileURL removed, no other file:// construction. The only concat-script builder is concatFileLine, now bare-path. Clean.

Verdict

Code: clean. Mechanical fix, well-documented with field-signal references, matches established sibling convention. CI: all checks in progress.

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Review by Miga