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fix(audio): isolate CoreAudio device enumeration from audio thread
Move all CoreAudio device queries (output_devices, default_output_device) off the audio thread onto disposable threads with timeout and catch_unwind. Device enumeration uses a subprocess for crash isolation so a HAL SIGSEGV in HALDeviceList::GetData() kills only the subprocess. - Add device_isolation module with resolve_device and safe_list_output_devices - Add AudioEngine::from_device and set_device_resolved for pre-resolved devices - Remove AudioCommand::ListDevices (audio_list_devices calls subprocess directly) - Change AudioCommand::SetDevice to carry pre-resolved cpal::Device Closes TASK-331
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backlog/tasks/task-331 - Harden-audio-thread-against-CoreAudio-SIGSEGV-during-playback.md

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---
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id: TASK-331
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title: Harden audio thread against CoreAudio SIGSEGV during playback
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status: In Progress
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status: Done
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assignee: []
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created_date: '2026-04-13 23:00'
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updated_date: '2026-04-13 23:02'
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updated_date: '2026-04-14 15:08'
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labels:
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- bug
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- audio
@@ -80,12 +80,12 @@ SIGSEGV cannot be caught in Rust (`catch_unwind` does not help). CoreAudio's HAL
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## Acceptance Criteria
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<!-- AC:BEGIN -->
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- [ ] #1 CoreAudio device enumeration (output_devices(), default_output_device()) never runs on the audio playback thread
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- [ ] #2 Device list queries run on a separate disposable thread with a timeout
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- [ ] #3 A CoreAudio HAL crash during device enumeration does not kill the mt process
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- [ ] #4 Existing playback continues uninterrupted if device enumeration fails
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- [ ] #5 Audio stream is reused across track transitions — no re-querying CoreAudio on LoadAndPlay when engine already exists
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- [ ] #6 Regression test (or documented manual test) for playing consecutive FLAC tracks without crash
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- [x] #1 CoreAudio device enumeration (output_devices(), default_output_device()) never runs on the audio playback thread
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- [x] #2 Device list queries run on a separate disposable thread with a timeout
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- [x] #3 A CoreAudio HAL crash during device enumeration does not kill the mt process
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- [x] #4 Existing playback continues uninterrupted if device enumeration fails
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- [x] #5 Audio stream is reused across track transitions — no re-querying CoreAudio on LoadAndPlay when engine already exists
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- [x] #6 Regression test (or documented manual test) for playing consecutive FLAC tracks without crash
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<!-- AC:END -->
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## Implementation Notes
@@ -124,4 +124,39 @@ SIGSEGV cannot be caught in Rust (`catch_unwind` does not help). CoreAudio's HAL
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The CalDigit TS4 USB audio device experienced a transient `kIOReturnNotReady` fault. When mt's audio thread attempted a CoreAudio HAL query during the track transition (LoadAndPlay for Endless), the HAL's internal device list was in an inconsistent state due to the USB fault, resulting in a null pointer dereference at offset 0x4 in `HALDeviceList::GetData()`.
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This is not a pure startup race — it's a mid-session CoreAudio HAL instability triggered by a USB audio device fault. The hardening must protect against CoreAudio queries failing catastrophically at any point during the session, not just at startup.
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## Implementation Summary (2026-04-14)
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### New file: `crates/mt-tauri/src/audio/device_isolation.rs`
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- `enumerate_devices_to_stdout()` — subprocess entry point, prints device names as JSON
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- `safe_list_output_devices(timeout)` — spawns subprocess with `MT_ENUMERATE_DEVICES=1` for crash-isolated device enumeration (AC #3)
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- `resolve_device(name, timeout)` — resolves `cpal::Device` on a disposable thread with `catch_unwind` + timeout (AC #1, #2)
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### Modified: `engine.rs`
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- Added `AudioEngine::from_device(device)` — creates engine from pre-resolved device (no enumeration)
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- Added `AudioEngine::set_device_resolved(device)` — switches output to pre-resolved device, preserves playback state
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- Refactored `set_device()` to delegate to `set_device_resolved()` (kept for test convenience)
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### Modified: `commands/audio.rs`
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- Removed `AudioCommand::ListDevices``audio_list_devices` now calls subprocess directly, bypassing audio thread
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- Changed `AudioCommand::SetDevice` to carry pre-resolved `cpal::Device` instead of `Option<String>`
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- `audio_set_device` resolves device off-thread before sending to audio thread
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- `ensure_engine` uses `resolve_device(None)` + `from_device()` for init, `resolve_device(Some(name))` + `set_device_resolved()` for device restoration
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### Modified: `main.rs`
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- Early exit when `MT_ENUMERATE_DEVICES` env var is set (subprocess mode)
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### Manual test procedure for AC #6
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1. Build and launch mt
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2. Connect a USB audio device (e.g. DAC, dock with audio)
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3. Select the USB device in Settings > Audio Output
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4. Play 5+ consecutive FLAC tracks from the same album
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5. During playback, briefly switch away from mt and back
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6. Verify no crash, audio continues uninterrupted
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7. Unplug/replug the USB device during playback — verify graceful error, not SIGSEGV
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<!-- SECTION:NOTES:END -->
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use crate::audio::audio_error::AudioError;
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use crate::audio::list_output_devices;
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use rodio::cpal::traits::{DeviceTrait, HostTrait};
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use std::panic::AssertUnwindSafe;
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use std::sync::mpsc;
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use std::time::Duration;
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use tracing::{debug, warn};
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/// Print device names as JSON to stdout and exit.
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/// Called when the process is spawned in enumeration mode (`MT_ENUMERATE_DEVICES=1`).
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pub fn enumerate_devices_to_stdout() {
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match list_output_devices() {
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Ok(devices) => {
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let json = serde_json::to_string(&devices).unwrap_or_else(|_| "[]".to_string());
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println!("{json}");
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std::process::exit(0);
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}
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Err(e) => {
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eprintln!("Device enumeration failed: {e}");
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std::process::exit(1);
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}
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}
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}
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/// List output devices via a subprocess for crash isolation.
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///
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/// Spawns the current executable with `MT_ENUMERATE_DEVICES=1`. The subprocess
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/// enumerates CoreAudio devices, prints JSON to stdout, and exits. If CoreAudio
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/// crashes (SIGSEGV) during enumeration, only the subprocess dies — the parent
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/// process receives an error rather than crashing.
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///
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/// In test builds, calls `list_output_devices()` directly because the test
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/// binary's harness does not handle `MT_ENUMERATE_DEVICES`, and spawning it
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/// would fork-bomb the test runner.
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pub fn safe_list_output_devices(timeout: Duration) -> Result<Vec<String>, AudioError> {
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#[cfg(test)]
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{
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let _ = timeout;
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return list_output_devices();
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}
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#[cfg(not(test))]
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safe_list_output_devices_subprocess(timeout)
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}
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#[cfg(not(test))]
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fn safe_list_output_devices_subprocess(timeout: Duration) -> Result<Vec<String>, AudioError> {
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let exe = std::env::current_exe()
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.map_err(|e| AudioError::Device(format!("Failed to get executable path: {e}")))?;
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let mut child = std::process::Command::new(exe)
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.env("MT_ENUMERATE_DEVICES", "1")
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.stdout(std::process::Stdio::piped())
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.stderr(std::process::Stdio::piped())
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.spawn()
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.map_err(|e| AudioError::Device(format!("Failed to spawn device enumerator: {e}")))?;
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let stdout = child
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.stdout
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.take()
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.ok_or_else(|| AudioError::Device("Failed to capture subprocess stdout".into()))?;
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let stderr = child
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.stderr
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.take()
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.ok_or_else(|| AudioError::Device("Failed to capture subprocess stderr".into()))?;
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let stdout_thread = std::thread::spawn(move || {
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use std::io::Read;
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let mut buf = String::new();
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std::io::BufReader::new(stdout)
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.read_to_string(&mut buf)
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.ok();
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buf
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});
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let stderr_thread = std::thread::spawn(move || {
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use std::io::Read;
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let mut buf = String::new();
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std::io::BufReader::new(stderr)
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.read_to_string(&mut buf)
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.ok();
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buf
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});
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let start = std::time::Instant::now();
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loop {
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match child.try_wait() {
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Ok(Some(status)) => {
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let stdout_text = stdout_thread.join().unwrap_or_default();
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let stderr_text = stderr_thread.join().unwrap_or_default();
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if status.success() {
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let devices: Vec<String> = serde_json::from_str(&stdout_text).map_err(|e| {
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AudioError::Device(format!("Failed to parse device list: {e}"))
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})?;
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debug!(count = devices.len(), "Enumerated devices via subprocess");
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return Ok(devices);
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} else {
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warn!(
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exit_code = ?status.code(),
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stderr = %stderr_text.trim(),
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"Device enumerator subprocess failed"
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);
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return Err(AudioError::Device(format!(
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"Device enumeration subprocess failed: {}",
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stderr_text.trim()
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)));
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}
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}
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Ok(None) => {
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if start.elapsed() > timeout {
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let _ = child.kill();
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let _ = child.wait();
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return Err(AudioError::Device("Device enumeration timed out".into()));
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}
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std::thread::sleep(Duration::from_millis(50));
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}
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Err(e) => {
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return Err(AudioError::Device(format!(
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"Failed to wait for enumerator: {e}"
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)));
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}
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}
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}
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}
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/// Resolve a named or default output device on a disposable thread.
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///
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/// CoreAudio device enumeration (`output_devices()`, `default_output_device()`)
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/// runs on a short-lived thread, not the caller's thread. The resolved
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/// `cpal::Device` (which is `Send`) is returned to the caller.
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///
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/// Panics on the disposable thread are caught; SIGSEGV still kills the process
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/// (use `safe_list_output_devices` for full crash isolation when only names are
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/// needed).
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pub fn resolve_device(
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name: Option<&str>,
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timeout: Duration,
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) -> Result<rodio::cpal::Device, AudioError> {
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let name_owned = name.map(|s| s.to_string());
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let (tx, rx) = mpsc::channel();
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std::thread::spawn(move || {
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let result = std::panic::catch_unwind(AssertUnwindSafe(|| {
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let host = rodio::cpal::default_host();
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match name_owned {
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Some(device_name) => {
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let devices = host.output_devices().map_err(|e| {
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AudioError::Device(format!("Failed to enumerate devices: {e}"))
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})?;
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devices
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.into_iter()
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.find(|d| d.name().ok().as_deref() == Some(device_name.as_str()))
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.ok_or_else(|| {
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AudioError::Device(format!("Device not found: {device_name}"))
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})
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}
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None => host
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.default_output_device()
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.ok_or_else(|| AudioError::Device("No default output device found".into())),
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}
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}));
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let result = match result {
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Ok(inner) => inner,
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Err(_) => Err(AudioError::Device("Device resolution panicked".into())),
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};
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let _ = tx.send(result);
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});
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rx.recv_timeout(timeout).map_err(|e| match e {
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mpsc::RecvTimeoutError::Timeout => AudioError::Device("Device resolution timed out".into()),
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mpsc::RecvTimeoutError::Disconnected => {
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AudioError::Device("Device resolution thread terminated unexpectedly".into())
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}
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})?
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}
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#[cfg(test)]
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mod tests {
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use super::*;
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#[test]
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fn test_resolve_device_default() {
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let result = resolve_device(None, Duration::from_secs(5));
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match result {
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Ok(device) => {
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assert!(device.name().is_ok(), "Resolved device should have a name");
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}
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Err(_) => {
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// Acceptable on headless CI without audio hardware
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}
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}
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}
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#[test]
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fn test_resolve_device_nonexistent_returns_error() {
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let result = resolve_device(Some("__nonexistent_device_12345__"), Duration::from_secs(5));
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// Should be Err(Device not found) or Err(enumerate failed) on headless CI
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match result {
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Ok(_) => panic!("Should not find a nonexistent device"),
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Err(e) => {
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let msg = e.to_string();
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assert!(
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msg.contains("not found")
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|| msg.contains("enumerate")
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|| msg.contains("Device"),
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"Unexpected error: {msg}"
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);
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}
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}
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}
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#[test]
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fn test_resolve_device_timeout() {
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// A very short timeout should still work for a real device resolution
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// (or fail gracefully)
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let result = resolve_device(None, Duration::from_millis(1));
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// Either succeeds quickly or times out — both are acceptable
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match result {
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Ok(device) => assert!(device.name().is_ok()),
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Err(e) => {
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let msg = e.to_string();
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assert!(
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msg.contains("timed out")
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|| msg.contains("Device")
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|| msg.contains("terminated"),
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"Unexpected error: {msg}"
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);
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}
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}
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}
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#[test]
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fn test_safe_list_output_devices_roundtrip() {
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let result = safe_list_output_devices(Duration::from_secs(15));
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match result {
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Ok(devices) => {
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for name in &devices {
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assert!(!name.is_empty(), "Device name should not be empty");
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}
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}
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Err(e) => {
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// May fail on CI without audio or if subprocess launch fails
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let msg = e.to_string();
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assert!(
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msg.contains("Device")
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|| msg.contains("failed")
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|| msg.contains("timed out")
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|| msg.contains("subprocess"),
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"Unexpected error: {msg}"
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);
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}
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}
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}
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}

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