Merge pull request #849 from vangel76/gap-audio-stream-recovery

#840: restart an audio stream the device drops instead of playing silent

apps/filmcraft/src/audio.rs

@@ -1,18 +1,33 @@
 //! cpal audio output: the playback master clock. Settings ▸ Audio Hardware picks the host
 //! ("Device Class"), the output device, the I/O buffer size and the sample rate.
+//!
+//! A stream the device drops while playing (an ALSA xrun the backend does not recover from, which
+//! cpal reports as `poll() returned POLLERR`; an unplugged device) is restarted by
+//! [`AudioOut::recover`], keeping the played-frame clock and the play-ahead mixer, so sound
+//! resumes where it stopped instead of playback going silent (#840).
 
+use std::panic::{AssertUnwindSafe, catch_unwind};
 use std::sync::atomic::{AtomicBool, AtomicU64, Ordering};
+use std::time::{Duration, Instant};
 
-use std::sync::{Arc, Mutex, PoisonError};
+use std::sync::{Arc, Mutex, PoisonError, TryLockError};
 
 use cpal::traits::{DeviceTrait, HostTrait, StreamTrait};
 use filmcraft_engine::settings::AudioHardwarePrefs;
 use filmcraft_ui_egui::{AudioDevices, AudioOut};
 
+/// The playback mixer callback handed to [`AudioOut::start`].
+type Fill = Box<dyn FnMut(&mut [f32], usize) + Send>;
+
 pub struct CpalOut {
     stream: Option<cpal::Stream>,
+    /// The running playback's mixer, shared with the stream so a restarted stream continues it.
+    fill: Option<Arc<Mutex<Fill>>>,
     played: Arc<AtomicU64>,
-    failed: Arc<AtomicBool>,
+    health: Arc<Health>,
+    watchdog: Watchdog,
+    /// Time base for the watchdog.
+    epoch: Instant,
     rate: u32,
     channels: u16,
     hw: AudioHardwarePrefs,
@@ -128,8 +143,11 @@ impl CpalOut {
         };
         Self {
             stream: None,
+            fill: None,
             played: Arc::new(AtomicU64::new(0)),
-            failed: Arc::new(AtomicBool::new(false)),
+            health: Arc::new(Health::default()),
+            watchdog: Watchdog::default(),
+            epoch: Instant::now(),
             rate: cfg.as_ref().map_or(48_000, |c| c.sample_rate().0),
             channels: cfg.as_ref().map_or(2, |c| c.channels()),
             hw: AudioHardwarePrefs::default(),
@@ -153,19 +171,14 @@ impl CpalOut {
         Ok(exact.unwrap_or(default))
     }
 
-    fn converted_stream<T>(
+    fn converted_stream<T>(&self, dev: &cpal::Device, config: &cpal::StreamConfig, fill: Arc<Mutex<Fill>>) -> Result<cpal::Stream, cpal::BuildStreamError>
-        &self,
-        dev: &cpal::Device,
-        config: &cpal::StreamConfig,
-        mut fill: Box<dyn FnMut(&mut [f32], usize) + Send>,
-    ) -> Result<cpal::Stream, cpal::BuildStreamError>
     where
         T: cpal::SizedSample + cpal::FromSample<f32>,
     {
         let channels = usize::from(config.channels);
         let played = self.played.clone();
-        let failed = self.failed.clone();
+        let health = self.health.clone();
-        let error_flag = failed.clone();
+        let error_health = health.clone();
         let mut scratch = Vec::new();
         dev.build_output_stream(
             config,
@@ -173,39 +186,23 @@ impl CpalOut {
                 // Reuse the conversion buffer. A pathological driver buffer must not panic the audio thread.
                 if buf.len() > 1_048_576 || scratch.try_reserve(buf.len().saturating_sub(scratch.len())).is_err() {
                     buf.fill(T::EQUILIBRIUM);
-                    if !failed.swap(true, Ordering::SeqCst) {
+                    if !health.failed.swap(true, Ordering::SeqCst) {
                         eprintln!("filmcraft: unable to allocate the audio conversion buffer");
                     }
                     return;
                 }
                 scratch.resize(buf.len(), 0.0);
-                fill(&mut scratch, channels);
+                render(&fill, &mut scratch, channels, &played, &health);
                 convert_samples(&scratch, buf);
-                played.fetch_add((buf.len() / channels) as u64, Ordering::SeqCst);
             },
-            move |e| stream_error(&error_flag, e),
+            move |e| stream_error(&error_health, e),
             None,
         )
     }
-}
 
-fn stream_error(failed: &AtomicBool, e: cpal::StreamError) {
+    /// Open the configured output device and play `fill` on it. `expect` is the (sample rate,
-    if !failed.swap(true, Ordering::SeqCst) {
+    /// channels) a restarted stream must keep: the mixer was set up for them.
-        eprintln!("filmcraft: audio stream error: {e}");
+    fn open(&mut self, fill: Arc<Mutex<Fill>>, expect: Option<(u32, u16)>) -> Result<(), String> {
-    }
-}
-
-fn convert_samples<T: cpal::Sample + cpal::FromSample<f32>>(input: &[f32], output: &mut [T]) {
-    // Integer conversion represents [-1, 1). In particular I24 cannot represent +1 exactly.
-    let upper = f32::from_bits(1.0_f32.to_bits() - 1);
-    for (sample, converted) in input.iter().zip(output) {
-        *converted = T::from_sample(if sample.is_finite() { sample.clamp(-1.0, upper) } else { 0.0 });
-    }
-}
-
-impl AudioOut for CpalOut {
-    fn start(&mut self, mut fill: Box<dyn FnMut(&mut [f32], usize) + Send>) -> Result<u32, String> {
-        self.stop();
         let host = host(&self.hw.device_class);
         let DeviceChoice { device: dev, fallback } = output_device(&host, &self.hw.default_output, &self.fallback).ok_or("no output device")?;
         self.note = fallback;
@@ -214,6 +211,11 @@ impl AudioOut for CpalOut {
         if channels == 0 || cfg.sample_rate().0 == 0 {
             return Err("the output device returned an invalid channel count or sample rate".into());
         }
+        if let Some((rate, ch)) = expect
+            && (rate, ch) != (cfg.sample_rate().0, cfg.channels())
+        {
+            return Err(format!("the output device now runs at {} Hz with {} channels", cfg.sample_rate().0, cfg.channels()));
+        }
         let mut config: cpal::StreamConfig = cfg.clone().into();
         if let cpal::SupportedBufferSize::Range { min, max } = cfg.buffer_size()
             && (*min..=*max).contains(&self.hw.buffer_size)
@@ -222,20 +224,18 @@ impl AudioOut for CpalOut {
         }
         self.rate = cfg.sample_rate().0;
         self.channels = cfg.channels();
-        self.played.store(0, Ordering::SeqCst);
-        self.failed.store(false, Ordering::SeqCst);
-        let played = self.played.clone();
-        let failed = self.failed.clone();
         let stream = match cfg.sample_format() {
-            cpal::SampleFormat::F32 => dev.build_output_stream(
+            cpal::SampleFormat::F32 => {
-                &config,
+                let played = self.played.clone();
-                move |buf: &mut [f32], _| {
+                let health = self.health.clone();
-                    fill(buf, channels);
+                let error_health = health.clone();
-                    played.fetch_add((buf.len() / channels) as u64, Ordering::SeqCst);
+                dev.build_output_stream(
-                },
+                    &config,
-                move |e| stream_error(&failed, e),
+                    move |buf: &mut [f32], _| render(&fill, buf, channels, &played, &health),
-                None,
+                    move |e| stream_error(&error_health, e),
-            ),
+                    None,
+                )
+            }
             cpal::SampleFormat::I8 => self.converted_stream::<i8>(&dev, &config, fill),
             cpal::SampleFormat::I16 => self.converted_stream::<i16>(&dev, &config, fill),
             cpal::SampleFormat::I24 => self.converted_stream::<cpal::I24>(&dev, &config, fill),
@@ -251,10 +251,172 @@ impl AudioOut for CpalOut {
         .map_err(|e| e.to_string())?;
         stream.play().map_err(|e| e.to_string())?;
         self.stream = Some(stream);
+        Ok(())
+    }
+
+    /// Replace a dropped stream with a new one on the same mixer, clock and format.
+    fn restart(&mut self) -> Result<(), String> {
+        let fill = self.fill.clone().ok_or("nothing is playing")?;
+        let expect = (self.rate, self.channels);
+        // The old stream (and its audio thread) goes first: only one stream ever runs the mixer.
+        self.stream = None;
+        self.health.clear_errors();
+        self.open(fill, Some(expect))
+    }
+}
+
+/// What the audio thread reports to the UI thread.
+#[derive(Default)]
+struct Health {
+    /// The device went away (unplugged): restart the stream at once.
+    lost: AtomicBool,
+    /// A backend error since the last check (cpal keeps the stream alive after these): restart
+    /// only if the stream also stopped consuming samples.
+    errored: AtomicBool,
+    /// Unrecoverable: the mixer panicked, a buffer could not be allocated, or restarts ran out.
+    /// [`AudioOut::played_frames`] reports `None` and playback goes on without sound.
+    failed: AtomicBool,
+    /// The first error of a stream has been printed.
+    reported: AtomicBool,
+}
+
+impl Health {
+    fn clear_errors(&self) {
+        self.lost.store(false, Ordering::SeqCst);
+        self.errored.store(false, Ordering::SeqCst);
+        self.reported.store(false, Ordering::SeqCst);
+    }
+}
+
+/// How long a stream that reported an error may go without consuming samples before it is
+/// restarted. Well under the UI's `AUDIO_STALL_S`, which gives up on the device clock.
+const STALL_RESTART: Duration = Duration::from_millis(300);
+/// At most this many restarts within [`RESTART_WINDOW`]; a device that keeps failing gives up.
+const MAX_RESTARTS: usize = 3;
+const RESTART_WINDOW: Duration = Duration::from_secs(10);
+
+/// What [`Watchdog::check`] decides about the running stream.
+#[derive(Clone, Copy, Debug, PartialEq, Eq)]
+enum Verdict {
+    Healthy,
+    Restart,
+    GiveUp,
+}
+
+/// Decides when a stream that reported errors is restarted (UI thread, every frame).
+#[derive(Default)]
+struct Watchdog {
+    /// Played-frame count last seen, and when it last changed.
+    seen: (u64, Duration),
+    /// An error was reported and the stream has not consumed samples since.
+    pending_error: bool,
+    /// When recent restarts happened (at most [`MAX_RESTARTS`] kept).
+    restarts: Vec<Duration>,
+}
+
+impl Watchdog {
+    fn reset(&mut self, now: Duration) {
+        *self = Self { seen: (0, now), ..Self::default() };
+    }
+
+    fn check(&mut self, now: Duration, played: u64, errored: bool, lost: bool) -> Verdict {
+        if played != self.seen.0 {
+            self.seen = (played, now);
+            self.pending_error = false;
+        }
+        // An error that arrives with the stream still consuming samples is only a hiccup.
+        self.pending_error |= errored;
+        let stalled = now.saturating_sub(self.seen.1) >= STALL_RESTART;
+        if !lost && !(self.pending_error && stalled) {
+            return Verdict::Healthy;
+        }
+        self.restarts.retain(|t| now.saturating_sub(*t) < RESTART_WINDOW);
+        if self.restarts.len() >= MAX_RESTARTS {
+            return Verdict::GiveUp;
+        }
+        self.restarts.push(now);
+        self.seen = (played, now);
+        self.pending_error = false;
+        Verdict::Restart
+    }
+}
+
+/// One device callback: run the shared mixer into `buf` and advance the clock. Never panics: a
+/// panicking mixer plays silence and fails the stream, and a mixer held by another stream (never
+/// expected: a restart drops the old stream first) plays silence without advancing the clock.
+fn render(fill: &Mutex<Fill>, buf: &mut [f32], channels: usize, played: &AtomicU64, health: &Health) {
+    let mut guard = match fill.try_lock() {
+        Ok(guard) => guard,
+        Err(TryLockError::Poisoned(poisoned)) => poisoned.into_inner(),
+        Err(TryLockError::WouldBlock) => {
+            buf.fill(0.0);
+            return;
+        }
+    };
+    if health.failed.load(Ordering::SeqCst) || catch_unwind(AssertUnwindSafe(|| (*guard)(buf, channels))).is_err() {
+        buf.fill(0.0);
+        if !health.failed.swap(true, Ordering::SeqCst) {
+            eprintln!("filmcraft: the audio mixer failed; playing without sound");
+        }
+        return;
+    }
+    played.fetch_add((buf.len() / channels.max(1)) as u64, Ordering::SeqCst);
+}
+
+fn stream_error(health: &Health, e: cpal::StreamError) {
+    match e {
+        cpal::StreamError::DeviceNotAvailable => health.lost.store(true, Ordering::SeqCst),
+        cpal::StreamError::BackendSpecific { .. } => health.errored.store(true, Ordering::SeqCst),
+    }
+    if !health.reported.swap(true, Ordering::SeqCst) {
+        eprintln!("filmcraft: audio stream error: {e}");
+    }
+}
+
+fn convert_samples<T: cpal::Sample + cpal::FromSample<f32>>(input: &[f32], output: &mut [T]) {
+    // Integer conversion represents [-1, 1). In particular I24 cannot represent +1 exactly.
+    let upper = f32::from_bits(1.0_f32.to_bits() - 1);
+    for (sample, converted) in input.iter().zip(output) {
+        *converted = T::from_sample(if sample.is_finite() { sample.clamp(-1.0, upper) } else { 0.0 });
+    }
+}
+
+impl AudioOut for CpalOut {
+    fn start(&mut self, fill: Box<dyn FnMut(&mut [f32], usize) + Send>) -> Result<u32, String> {
+        self.stop();
+        self.played.store(0, Ordering::SeqCst);
+        self.health = Arc::new(Health::default());
+        self.watchdog.reset(self.epoch.elapsed());
+        let fill = Arc::new(Mutex::new(fill));
+        self.open(fill.clone(), None)?;
+        self.fill = Some(fill);
         Ok(self.rate)
     }
     fn stop(&mut self) {
         self.stream = None;
+        self.fill = None;
+    }
+    fn recover(&mut self) {
+        if self.stream.is_none() || self.health.failed.load(Ordering::SeqCst) {
+            return;
+        }
+        let lost = self.health.lost.swap(false, Ordering::SeqCst);
+        let errored = self.health.errored.swap(false, Ordering::SeqCst);
+        match self.watchdog.check(self.epoch.elapsed(), self.played.load(Ordering::SeqCst), errored, lost) {
+            Verdict::Healthy => {}
+            Verdict::Restart => {
+                log::warn!("the audio output stream {}; restarting it", if lost { "lost its device" } else { "stopped after an error" });
+                if let Err(e) = self.restart() {
+                    log::warn!("the audio output stream could not be restarted: {e}");
+                    self.stream = None;
+                    self.health.failed.store(true, Ordering::SeqCst);
+                }
+            }
+            Verdict::GiveUp => {
+                log::warn!("the audio output stream keeps failing ({MAX_RESTARTS} restarts within {} s)", RESTART_WINDOW.as_secs());
+                self.health.failed.store(true, Ordering::SeqCst);
+            }
+        }
     }
     fn sample_rate(&self) -> u32 {
         self.rate
@@ -263,7 +425,7 @@ impl AudioOut for CpalOut {
         self.channels as usize
     }
     fn played_frames(&self) -> Option<u64> {
-        self.stream.as_ref().filter(|_| !self.failed.load(Ordering::SeqCst)).map(|_| self.played.load(Ordering::SeqCst))
+        self.stream.as_ref().filter(|_| !self.health.failed.load(Ordering::SeqCst)).map(|_| self.played.load(Ordering::SeqCst))
     }
     fn devices(&self) -> AudioDevices {
         let h = host(&self.hw.device_class);
@@ -327,11 +489,92 @@ mod tests {
     }
 
     #[test]
-    fn failed_stream_relinquishes_the_playback_clock() {
+    fn stream_errors_ask_for_a_restart_instead_of_failing_playback() {
-        let failed = AtomicBool::new(false);
+        let health = Health::default();
-        stream_error(&failed, cpal::StreamError::DeviceNotAvailable);
+        stream_error(&health, cpal::StreamError::BackendSpecific { err: cpal::BackendSpecificError { description: "`alsa::poll()` returned POLLERR".into() } });
-        assert!(failed.load(Ordering::SeqCst));
+        assert!(health.errored.load(Ordering::SeqCst) && !health.lost.load(Ordering::SeqCst));
+        stream_error(&health, cpal::StreamError::DeviceNotAvailable);
+        assert!(health.lost.load(Ordering::SeqCst));
+        assert!(!health.failed.load(Ordering::SeqCst), "a stream error alone must not silence playback");
         let output = CpalOut::new();
-        assert_eq!(output.played_frames(), None);
+        assert_eq!(output.played_frames(), None, "no stream, no clock");
+    }
+
+    fn ms(n: u64) -> Duration {
+        Duration::from_millis(n)
+    }
+
+    #[test]
+    fn an_error_while_samples_keep_flowing_is_only_a_hiccup() {
+        let mut w = Watchdog::default();
+        w.reset(ms(0));
+        assert_eq!(w.check(ms(10), 480, true, false), Verdict::Healthy);
+        // the stream keeps consuming samples after the error: never restarted
+        for (i, t) in (20..2_000).step_by(20).enumerate() {
+            assert_eq!(w.check(ms(t), 480 * (i as u64 + 2), false, false), Verdict::Healthy);
+        }
+    }
+
+    #[test]
+    fn a_stream_that_errors_and_stops_consuming_is_restarted() {
+        let mut w = Watchdog::default();
+        w.reset(ms(0));
+        assert_eq!(w.check(ms(10), 480, false, false), Verdict::Healthy);
+        // POLLERR: the backend reports errors and no longer calls back
+        assert_eq!(w.check(ms(20), 480, true, false), Verdict::Healthy);
+        assert_eq!(w.check(ms(200), 480, true, false), Verdict::Healthy);
+        assert_eq!(w.check(ms(310), 480, false, false), Verdict::Restart);
+        // the restarted stream gets a fresh grace period
+        assert_eq!(w.check(ms(400), 480, false, false), Verdict::Healthy);
+        // a stall without any reported error is left to the UI's own watchdog
+        assert_eq!(w.check(ms(2_000), 480, false, false), Verdict::Healthy);
+    }
+
+    #[test]
+    fn a_lost_device_restarts_at_once_and_repeated_failures_give_up() {
+        let mut w = Watchdog::default();
+        w.reset(ms(0));
+        assert_eq!(w.check(ms(5), 0, false, true), Verdict::Restart);
+        assert_eq!(w.check(ms(10), 0, false, true), Verdict::Restart);
+        assert_eq!(w.check(ms(15), 0, false, true), Verdict::Restart);
+        assert_eq!(w.check(ms(20), 0, false, true), Verdict::GiveUp);
+        // sporadic failures far apart keep being recovered
+        assert_eq!(w.check(ms(10_006), 0, false, true), Verdict::Restart);
+    }
+
+    #[test]
+    fn the_device_callback_never_panics_and_counts_played_frames() {
+        let (played, health) = (AtomicU64::new(0), Health::default());
+        let fill: Mutex<Fill> = Mutex::new(Box::new(|buf: &mut [f32], _| buf.fill(0.25)));
+        let mut buf = [1.0_f32; 8];
+        render(&fill, &mut buf, 2, &played, &health);
+        assert_eq!((buf, played.load(Ordering::SeqCst)), ([0.25; 8], 4));
+        // a mixer still held elsewhere plays silence without moving the clock
+        {
+            let _held = fill.lock().unwrap();
+            render(&fill, &mut buf, 2, &played, &health);
+        }
+        assert_eq!((buf, played.load(Ordering::SeqCst)), ([0.0; 8], 4));
+        // zero channels from a broken driver must not divide by zero
+        render(&fill, &mut buf, 0, &played, &health);
+        assert_eq!(played.load(Ordering::SeqCst), 12);
+    }
+
+    #[test]
+    fn a_panicking_mixer_plays_silence_and_fails_the_stream() {
+        let (played, health) = (AtomicU64::new(0), Health::default());
+        let fill: Mutex<Fill> = Mutex::new(Box::new(|buf: &mut [f32], _| {
+            buf.fill(0.5);
+            panic!("mixer bug");
+        }));
+        let mut buf = [1.0_f32; 4];
+        render(&fill, &mut buf, 2, &played, &health);
+        assert_eq!(buf, [0.0; 4]);
+        assert_eq!(played.load(Ordering::SeqCst), 0);
+        assert!(health.failed.load(Ordering::SeqCst));
+        // later callbacks stay silent instead of re-entering the broken mixer
+        buf = [1.0; 4];
+        render(&fill, &mut buf, 2, &played, &health);
+        assert_eq!(buf, [0.0; 4]);
     }
 }

crates/ui-egui/src/lib.rs

@@ -83,6 +83,10 @@ pub trait AudioOut {
     }
     /// Frames played since `start` (the playback master clock), if the device reports it.
     fn played_frames(&self) -> Option<u64>;
+    /// Called every frame while playing, before `played_frames`: restart a stream the device
+    /// dropped (a driver error, an unplugged device), keeping the clock and `fill`. A stream that
+    /// cannot be restarted reports `played_frames() == None`.
+    fn recover(&mut self) {}
     /// Hosts and devices that can be chosen in Settings ▸ Audio Hardware.
     fn devices(&self) -> AudioDevices {
         AudioDevices::default()
@@ -1041,6 +1045,11 @@ impl FilmcraftApp {
             self.playback.anchor_time = now;
         }
         let rate = self.session.sequence_rate();
+        if self.playback.audio_clock
+            && let Some(audio) = self.audio.as_mut()
+        {
+            audio.recover();
+        }
         let reading = if self.playback.audio_clock { self.audio.as_ref().and_then(|a| a.played_frames().map(|f| (f, a.sample_rate()))) } else { None };
         if self.playback.audio_clock && reading.is_none() {
             // A device error relinquishes its clock immediately. Resume from the displayed
@@ -2252,6 +2261,52 @@ mod audio_recovery_tests {
         assert!(app.ui.status.contains("Audio output failed"));
         app.stop();
     }
+
+    /// A stream the device dropped, which `recover` restarts on the same clock.
+    struct Restartable {
+        reading: Option<u64>,
+    }
+
+    impl AudioOut for Restartable {
+        fn start(&mut self, _: Box<dyn FnMut(&mut [f32], usize) + Send>) -> Result<u32, String> {
+            Ok(48000)
+        }
+        fn stop(&mut self) {}
+        fn sample_rate(&self) -> u32 {
+            48000
+        }
+        fn played_frames(&self) -> Option<u64> {
+            self.reading
+        }
+        fn recover(&mut self) {
+            self.reading = self.reading.or(Some(48_000));
+        }
+    }
+
+    #[test]
+    fn a_restarted_stream_keeps_the_device_clock_and_the_sound() {
+        let mut session = Session::default();
+        session.execute("file.openDemoProject", json!({})).unwrap();
+        let ctx = egui::Context::default();
+        let mut app = FilmcraftApp::new(session);
+        app.audio = Some(Box::new(Restartable { reading: None }));
+        app.playback.playing = true;
+        app.playback.audio_clock = true;
+        app.playback.anchor_tick = Tick::ZERO;
+        app.playback.anchor_time = 0.0;
+        app.playback.audio_seen = (0, 10.0);
+        let mut output = ctx.run_ui(egui::RawInput { time: Some(10.0), ..Default::default() }, |ui| app.advance_playback(ui.ctx()));
+        output.textures_delta.clear();
+        assert!(app.playback.audio_clock, "the restarted stream is still the master clock");
+        assert!(!app.ui.status.contains("Audio output failed"));
+        // 48 000 frames played at 48 kHz: one second in, on a frame boundary
+        let (shown, second) = (app.session.playhead(), Tick::from_seconds_f64(1.0));
+        assert!(
+            shown <= second && shown + app.session.sequence_rate().frame_duration() > second,
+            "the playhead follows the frames the device played: {shown:?}"
+        );
+        app.stop();
+    }
 }
 
 /// Whether a shortcut chord must be left alone because a text field is focused.

crates/ui-egui/src/source_playback.rs

@@ -213,6 +213,11 @@ impl FilmcraftApp {
             self.source_playback.clock.audio_seen = (0, now);
             self.start_source_audio();
         }
+        if self.source_playback.clock.audio_clock
+            && let Some(audio) = self.audio.as_mut()
+        {
+            audio.recover();
+        }
         let clock = &mut self.source_playback.clock;
         if clock.anchor_time < 0.0 {
             clock.anchor_time = now;

docs/gaps.md

@@ -247,6 +247,10 @@ platforms** G4, G9, G17; **performance** G16; **ecosystem** G11; **measurement**
 
 - **Evidence:** Asus Xonar DX output broken (#605), no audio (#602), audio cuts out with
   overlapping clips (#336), buggy audio (#410), Linux stutter (#617). No ASIO; cpal only.
+- **Have:** a stream the device drops while playing (an ALSA xrun cpal reports as `POLLERR`, an
+  unplugged device) is restarted with the same rate, channels, clock and mixer position, so sound
+  resumes instead of playback going silent (at most 3 restarts in 10 s); a panicking mixer plays
+  silence (#840).
 - **Estimate:** 20–40 h.
 
 ## G18. Effects depth: approximations and missing behaviour