Studio mode + camera on macOS: every audio-input.ogg is 0 bytes; editor fails with "Failed to open audio: End of file"

[arendon1]

Description

Recording in Studio mode with the camera enabled produces a .cap bundle that Cap marks Complete, but every per-segment audio-input.ogg is written as a 0-byte file (mode 0600, no OggS header, ffprobe → End of file). The per-segment mux-audio task logs Task finished successfully. The bug only surfaces when opening the recording, which shows:

Unable to Open Recording — segment 0 / mic / Failed to open audio: End of file

Visual tracks (display.mp4, camera.mp4) are unaffected and decode cleanly. The mic audio is not recoverable from the broken bundle — raw PCM is never persisted.

Expected: a valid Opus-in-OGG track per segment, matching the duration reported by recording-meta.json.

Reproduction

1. macOS 26.5.x, Cap 0.5.7. Open Cap and select Studio mode.
2. Enable camera + microphone. Reproduces with both the built-in 96 kHz mic and a USB 48 kHz mic.
3. Record several segments (reproduced with 18 s, 56 s, 58 s, 63 s, and 85 s durations).
4. Stop recording. Cap reports status: Complete.
5. Open the recording → editor shows the dialog above.

Additional Context

• Cap version: 0.5.7 (bundle so.cap.desktop)
• Operating system, version: macOS 26.5.2 (25F84), arm64
• Device: Apple silicon MacBook Air (M5). Built-in Micrófono de MacBook Air (F32 96 kHz mono) and external USB PD200X Podcast Microphone (F32 48 kHz mono) both reproduce the bug.
• Out-of-process muxer: enabled (default outOfProcessMuxer: true)

On-disk evidence from a failing bundle:

$ ls -l content/segments/segment-*/audio-input.ogg
… segment-0/audio-input.ogg  size=0  mode=-rw-------
… segment-1/audio-input.ogg  size=0  mode=-rw-------
… segment-5/audio-input.ogg  size=0  mode=-rw-------

$ ffprobe -v error content/segments/segment-0/audio-input.ogg
[ERROR] code=-541478725 string=End of file

Same paths in a working bundle show -rw-r--r-- Opus at 48 kHz with durations matching the segment length.

Log excerpt (no error, only success):

INFO recording:studio_recording:segment{0}:mic-out:
     Microphone source configured device=… source_rate=96000 … target_rate=48000 …
INFO recording:studio_recording:segment{0}:mic-out:
     Built pipeline for output …/content/segments/segment-0/audio-input.ogg
INFO recording:studio_recording:segment{0}:mic-out:
     Padded audio tail with silence padding_ms=57 samples=2775 audio_end_ms=56489 …
INFO recording:studio_recording:segment{0}:mic-out:{task="mux-audio"}:
     Task finished successfully
INFO recording: recording wait actor done: Ok(())

Hypothesis from code review of v0.5.7: studio-with-camera uses OggMuxer (crates/recording/src/studio_recording.rs ~L1654). The chain ends in AudioEncoderBase::send_frame() (crates/enc-ffmpeg/src/audio/base.rs), whose while let Some(out_frame) = self.resampler.get_frame(…) loop silently returns Ok(()) if the BufferedResampler never accumulates enough samples to fill an Opus frame (120 / 240 / 480 / 960 / 1920 / 2880). OggFile::finish() then writes a trailer, but FFmpeg's Ogg muxer has no packets to flush, so the file stays 0 bytes. OggFile lacks the has_frames guard that FragmentedAudioFile (the M4A path used when the camera is off) has.

Workaround: Disabling Settings → Advanced → outOfProcessMuxer and re-recording produces valid audio.

Impact: Total loss of the mic track for any Studio + camera session on Cap 0.5.7 / macOS 26.5.x. Visual tracks remain usable; users can re-record only the audio or composite it externally, but that is not obvious from the UI.

[linear]

CAP-773