fix(linux): write fragmented MP4 so a killed helper keeps its take

[EtienneLescot]

Conclusion. The Linux helper now writes fragmented MP4 with 1 s fragments, like Windows and macOS. A killed helper keeps every fragment closed before the kill instead of losing the whole take.

Muxer options (FRAGMENT_OPTIONS in encoder.rs, passed to avformat_write_header)

• movflags=+frag_keyframe+empty_moov+default_base_moof+delay_moov
• min_frag_duration=1000000: cut at a keyframe once a fragment holds 1 s. The GOP stays 0.5 s, so a fragment is two GOPs. Same floor as kFragmentDurationHns on Windows.
• flush_packets=1: without it the moov and fragments sit in avio's 32 KB buffer, and a low-bitrate take killed after 3 s left a 28-byte file.
• delay_moov: keeps the AAC priming edit list. Without it libavformat shifts every track 21 ms and video frames drift 21 ms off the cursor clock. With it, timestamps match the old plain MP4 packet for packet.
• An option the muxer does not consume is now an error, so a typo cannot silently bring back the unfragmented file.

Verified under WSL (nix, ffmpeg n8.1.2)

• cargo test --release: 93 passed, 1 ignored.
• New test a_killed_muxer_leaves_a_file_that_opens_and_holds_its_frames: 3 s of video + AAC through the real Software encoder and muxer, then std::mem::forget(muxer) (no trailer, no buffer flush). Read back with avformat_open_input + avformat_find_stream_info: 2 streams, 60 video frames, frame 1 at exactly 1/30 s.
• On the old code the same test fails: a killed take must open: Invalid data found (48-byte file, no moov).
• Without delay_moov it fails on timing: frame 1 at 0.0547 s.
• ffmpeg CLI with the same libs: a fragmented file seeks to the keyframe at 2.0 s, and one truncated to 2/3 of its size still opens with 60 frames.

Not covered

• CI does not run this crate's tests.
• A real take killed mid-recording, on a Linux desktop.
• A seek in the editor on a fragmented Linux take, measured before and after.

Fixes #944
Part of #920

🤖 Generated with Claude Code

Summary by CodeRabbit

• New Features
  • Recordings are saved in fragments, so a recording interrupted before finalization can still retain playable video and audio. Partial recordings can include footage captured in the preceding seconds.

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• electron/native/pipewire-capture/src/encoder.rs

📝 Walkthrough

Walkthrough

The Linux MP4 muxer now writes fragmented output with configured fragment and packet-flushing options. A new test checks that a recording remains readable after the muxer is dropped without writing its trailer.

Changes

Linux fragmented MP4 recording

Layer / File(s)	Summary
Configure fragmented MP4 output electron/native/pipewire-capture/src/encoder.rs	The muxer passes fragmentation options to libavformat, frees the options dictionary, and reports unrecognized options. The documentation describes fragmented output and recovery through the last complete fragment.
Check recovery without a trailer electron/native/pipewire-capture/src/encoder.rs, electron/native/pipewire-capture/src/main.rs	The test drops the muxer without writing its trailer, then checks the reopened file for both tracks and retained video frames. The finalization comment describes loss of the final fragment when no trailer is written.

Priority: ⬆️ High

Estimated code review effort: 3 (Moderate) | ~20 minutes

Change: Bug fix · Severity of issue fixed: High

Sequence Diagram(s)

sequenceDiagram
  participant Encoder
  participant libavformat
  participant MP4File
  participant TestReader
  Encoder->>libavformat: Write header with fragment options
  Encoder->>libavformat: Write video and audio packets
  libavformat->>MP4File: Write fragmented output
  TestReader->>MP4File: Reopen after muxer is dropped without a trailer
  MP4File-->>TestReader: Provide readable tracks and retained frames

Loading

Merge Risk: 🔵 Low · up to 136de

The fragmented-output change has no established production-blocking defect. Concurrent test runs can interfere through the shared temporary file; use a unique path to avoid flaky results.

Security Architecture Review

Security architecture risk: 🔵 Low · up to 136de

The change improves recovery without expanding recording permissions or output-file authority in the inspected paths. Remaining uncertainty concerns real termination behavior and compatibility with recording playback and export.

Retained concerns
No architecture-level concerns identified.

Security review details

Security Blast Radius

• inferred — The demonstrated design impact is confined to Linux recording output and its readers. The inspected change does not expand filesystem-writing authority beyond the existing caller-supplied recording destination; interrupted files may now expose readable captured content through that same destination.

Trust Boundaries and Controls

• observed — Fragment options are constants rather than caller-controlled strings. Header initialization rejects unconsumed options before marking the header successful, and Capture propagates initialization failure rather than proceeding with recording packets.

Resilience and Maintainability Implications

• observed — Normal finalization consumes the muxer, while Drop closes its file handle and frees its format context on ordinary error paths. Output is still opened directly, so initialization or write failures can leave a partial file; that exposure predates this change. Abrupt termination bypasses this cleanup and remains outside the inspected test's execution model.

🚥 Pre-merge checks | ✅ 4 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	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 6 functions across 2 files.	Write docstrings for the functions missing them to satisfy the coverage threshold.

✅ Passed checks (4 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title clearly and concisely identifies the main change: fragmented MP4 output on Linux preserves recordings after a helper kill.
Description check	✅ Passed	The description provides a detailed summary, linked issue references, implementation details, testing results, and known limitations. It does not reproduce the template headings or checkboxes for chan…
Linked Issues check	✅ Passed	The changes implement #944's coding objective. encoder.rs configures fragmented MP4 output with frag_keyframe, empty_moov, default_base_moof, delayed moov creation, a one-second minimum fragme…
Out of Scope Changes check	✅ Passed	The changed files support #944. The encoder changes implement fragmented output and the kill simulation. The finish_capture documentation updates describe the related finalization behavior. No unrel…

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Actionable comments posted: 1

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• 🪄 Fix CodeRabbit comments on this PR

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Inline comments:
Review comments at @electron/native/pipewire-capture/src/encoder.rs:
- Around line 1528-1529: Update the test’s output path near `Muxer::create` to
include a per-process unique identifier, such as `std::process::id()`, so
concurrent runs do not share the same file. Remove the file after the assertions
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• electron/native/pipewire-capture/src/encoder.rs
• electron/native/pipewire-capture/src/main.rs

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