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Windows audio (cpal): startup handshake + RT-safe capture ring (W1, W3)

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Two correctness fixes for the cpal/WASAPI backend from the Codex Windows-compat
review, plus device logging.

W1 — start_capture/start_playback no longer return Ok before the stream exists.
The owning thread did device resolution, config selection, build_stream, and
play() and only *logged* failures, so a missing 48 kHz config / unsupported
format / WASAPI error left the UI in a joined-but-silent room. The worker now
reports readiness over a channel and start_* blocks on it via finish_start(),
returning the real AudioError on failure (and joining the dead worker).

W3 — the RT capture callback no longer allocates or sends on a channel. It now
only downmixes and wait-free-pushes mono samples into a preallocated lock-free
HeapRb; the owning thread drains that ring, frames it (the Vec allocation lives
off the RT path), and sends completed frames. A full ring increments an overrun
counter instead of blocking. Restores the no-alloc/no-block-in-callback contract
the PipeWire backend already honors.

Also logs the selected device name / sample format / channels / rate on stream
start (a review nice-to-have) and logs capture overruns when they occur.

Windows-only file (cfg(windows)); Linux build unaffected. Compile-verified via
the windows-gnu cross-build; not yet run on a real WASAPI host.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>
This commit is contained in:
mollusk
2026-06-19 03:57:41 -04:00
co-authored by Claude Opus 4.8
parent 63b45e03ab
commit bbbe2d8f17
1 changed files with 175 additions and 58 deletions
Download Patch File Download Diff File Expand all files Collapse all files
src/audio/cpal_impl.rs
+175 -58
View File
@@ -7,6 +7,9 @@
//!
//! - **Capture**: mono, 48 kHz, S16 PCM, emitted as `Vec<i16>` frames of
//! [`CAPTURE_FRAME`] (960 = 20 ms) samples — matching the encoder/jitter frame.
//! The RT capture callback only downmixes and pushes samples into a lock-free
//! ring; the owning thread drains that ring, frames it, and sends — so the
//! callback never allocates, locks, or touches an mpsc channel.
//! - **Playback**: stereo interleaved ([`PLAYBACK_CHANNELS`]) S16 PCM at 48 kHz,
//! drained from a ring buffer that is paced to the device's hardware clock via
//! `ring_fill` exactly as the PipeWire backend does.
@@ -20,7 +23,12 @@
//! stream, plays it, and keeps it alive until the per-worker `running` flag flips
//! (set by `stop`). The struct holds only `Send` handles (the flag + the join
//! handle). The stream's RT callback does the actual audio work; the owning
//! thread additionally feeds the playback ring from the network mixer.
//! thread additionally feeds the playback ring (or drains the capture ring).
//!
//! `start_*` does not return until the owning thread reports back over a readiness
//! channel that the device resolved and the stream is built and playing — so a
//! device/format/WASAPI failure surfaces as a real `Err` to the caller instead of
//! leaving the UI in a joined-but-silent room.
//!
//! ## Sample rate
//!
@@ -30,7 +38,7 @@
//! sample-rate support (resampling) is a Phase 1.1 follow-up.
use std::sync::atomic::{AtomicBool, AtomicU64, AtomicUsize, Ordering};
use std::sync::mpsc::{Receiver, RecvTimeoutError, Sender};
use std::sync::mpsc::{channel, Receiver, RecvTimeoutError, Sender};
use std::sync::{Arc, Mutex};
use std::thread::{self, JoinHandle};
use std::time::Duration;
@@ -49,6 +57,13 @@ const SAMPLE_RATE: u32 = 48_000;
/// Mono capture frame: 960 samples = 20 ms @ 48 kHz. Matches the PipeWire backend
/// and `core::jitter::FRAME_SAMPLES`.
const CAPTURE_FRAME: usize = 960;
/// Lock-free capture ring capacity (mono samples) between the RT callback and the
/// owning drain thread: 8 frames = 160 ms of headroom, so a scheduling hiccup on
/// the drain thread doesn't immediately overrun the RT producer.
const CAPTURE_RING_CAPACITY: usize = CAPTURE_FRAME * 8;
/// How long the capture drain thread sleeps when the ring is momentarily empty,
/// before polling again. Small enough to stay well under the 20 ms frame cadence.
const CAPTURE_POLL: Duration = Duration::from_millis(5);
/// Playback ring capacity in interleaved samples: 200 ms of stereo @ 48 kHz.
/// Comfortably above [`PLAYBACK_TARGET_SAMPLES`] so the clock-paced producer has
/// headroom and never has to drop frames in steady state.
@@ -90,22 +105,20 @@ impl AudioBackend for CpalBackend {
tx: Sender<Vec<i16>>,
target_node: Option<String>,
) -> Result<(), AudioError> {
let mut guard = self.capture.lock().unwrap();
let guard = self.capture.lock().unwrap();
if guard.is_some() {
return Err(AudioError::Stream("Capture already started".to_string()));
}
let running = Arc::new(AtomicBool::new(true));
let running_thread = running.clone();
let (ready_tx, ready_rx) = channel::<Result<(), AudioError>>();
let thread = thread::Builder::new()
.name("peerspeak-cpal-capture".to_string())
.spawn(move || {
if let Err(e) = run_capture(tx, target_node, running_thread) {
crate::log_msg(&format!("cpal capture error: {e}"));
}
run_capture(tx, target_node, running_thread, ready_tx);
})
.map_err(|e| AudioError::Init(e.to_string()))?;
*guard = Some(StreamWorker { running, thread });
Ok(())
finish_start(guard, StreamWorker { running, thread }, ready_rx, "capture")
}
fn start_playback(
@@ -114,22 +127,20 @@ impl AudioBackend for CpalBackend {
target_node: Option<String>,
ring_fill: Arc<AtomicUsize>,
) -> Result<(), AudioError> {
let mut guard = self.playback.lock().unwrap();
let guard = self.playback.lock().unwrap();
if guard.is_some() {
return Err(AudioError::Stream("Playback already started".to_string()));
}
let running = Arc::new(AtomicBool::new(true));
let running_thread = running.clone();
let (ready_tx, ready_rx) = channel::<Result<(), AudioError>>();
let thread = thread::Builder::new()
.name("peerspeak-cpal-playback".to_string())
.spawn(move || {
if let Err(e) = run_playback(rx, target_node, ring_fill, running_thread) {
crate::log_msg(&format!("cpal playback error: {e}"));
}
run_playback(rx, target_node, ring_fill, running_thread, ready_tx);
})
.map_err(|e| AudioError::Init(e.to_string()))?;
*guard = Some(StreamWorker { running, thread });
Ok(())
finish_start(guard, StreamWorker { running, thread }, ready_rx, "playback")
}
fn stop(&self) -> Result<(), AudioError> {
@@ -143,6 +154,38 @@ impl AudioBackend for CpalBackend {
}
}
/// Block until the just-spawned worker reports (over `ready_rx`) that its stream
/// is built and playing, then either install it (`Ok`) or join it and surface the
/// real error. This is what makes `start_capture`/`start_playback` fail loudly
/// instead of returning `Ok` into a joined-but-silent room (Codex review W1).
fn finish_start(
mut guard: std::sync::MutexGuard<'_, Option<StreamWorker>>,
worker: StreamWorker,
ready_rx: Receiver<Result<(), AudioError>>,
what: &str,
) -> Result<(), AudioError> {
match ready_rx.recv() {
Ok(Ok(())) => {
*guard = Some(worker);
Ok(())
}
// Setup failed (Err) or the worker exited before reporting (recv Err):
// either way it has stopped, so reap it and surface the error.
Ok(Err(e)) => {
worker.running.store(false, Ordering::Relaxed);
let _ = worker.thread.join();
Err(e)
}
Err(_) => {
worker.running.store(false, Ordering::Relaxed);
let _ = worker.thread.join();
Err(AudioError::Init(format!(
"cpal {what} worker exited before reporting readiness"
)))
}
}
}
// ---------------------------------------------------------------------------
// Device enumeration (for the settings device pickers)
// ---------------------------------------------------------------------------
@@ -285,53 +328,108 @@ fn run_capture(
tx: Sender<Vec<i16>>,
target: Option<String>,
running: Arc<AtomicBool>,
) -> Result<(), AudioError> {
let (device, config, sample_format) = resolve(false, target)?;
let channels = config.channels as usize;
ready: Sender<Result<(), AudioError>>,
) {
// The RT callback pushes mono samples into this lock-free ring; we drain it on
// this (non-RT) thread, so the callback never allocates or sends on a channel.
let rb = HeapRb::<i16>::new(CAPTURE_RING_CAPACITY);
let (producer, mut consumer) = rb.split();
let overrun = Arc::new(AtomicU64::new(0));
let stream = match sample_format {
SampleFormat::F32 => build_input::<f32>(&device, &config, tx, channels),
SampleFormat::I16 => build_input::<i16>(&device, &config, tx, channels),
SampleFormat::U16 => build_input::<u16>(&device, &config, tx, channels),
other => Err(AudioError::Stream(format!(
"unsupported capture sample format: {other:?}"
))),
}?;
// Fallible device/stream setup. We report the real error to `start_capture`
// before doing any work, so a join never lands in a silent room.
let setup = || -> Result<(Stream, String, SampleFormat, usize), AudioError> {
let (device, config, sample_format) = resolve(false, target)?;
let channels = config.channels as usize;
let stream = match sample_format {
SampleFormat::F32 => {
build_input::<f32, _>(&device, &config, producer, channels, overrun.clone())
}
SampleFormat::I16 => {
build_input::<i16, _>(&device, &config, producer, channels, overrun.clone())
}
SampleFormat::U16 => {
build_input::<u16, _>(&device, &config, producer, channels, overrun.clone())
}
other => Err(AudioError::Stream(format!(
"unsupported capture sample format: {other:?}"
))),
}?;
stream.play().map_err(|e| AudioError::Stream(e.to_string()))?;
let name = device.name().unwrap_or_else(|_| "<unknown>".to_string());
Ok((stream, name, sample_format, channels))
};
stream.play().map_err(|e| AudioError::Stream(e.to_string()))?;
let (stream, dev_name, sample_format, channels) = match setup() {
Ok(v) => {
let _ = ready.send(Ok(()));
v
}
Err(e) => {
let _ = ready.send(Err(e));
return;
}
};
crate::log_msg(&format!(
"cpal capture started: device='{dev_name}' format={sample_format:?} channels={channels} rate={SAMPLE_RATE} Hz"
));
// The RT callback does the work; this thread just keeps `stream` alive until
// `stop()` flips the flag, at which point the stream is dropped (= stopped).
// Drain the RT ring on this thread: pop mono samples, frame them (the `Vec`
// allocation lives here, off the RT path), and send completed frames. Keep
// `stream` alive until `stop()` flips the flag.
let mut acc = FrameAccumulator::new(CAPTURE_FRAME);
let mut last_overrun = 0u64;
while running.load(Ordering::Relaxed) {
thread::sleep(WORKER_POLL);
let mut drained = false;
while let Some(sample) = consumer.try_pop() {
drained = true;
if let Some(frame) = acc.push(sample) {
// Consumer gone (call ended) → stop feeding; the stream is
// dropped below on the way out.
if tx.send(frame).is_err() {
drop(stream);
return;
}
}
}
let o = overrun.load(Ordering::Relaxed);
if o != last_overrun {
crate::log_msg(&format!(
"cpal capture overrun: dropped {} samples (drain thread fell behind)",
o - last_overrun
));
last_overrun = o;
}
if !drained {
thread::sleep(CAPTURE_POLL);
}
}
Ok(())
drop(stream);
}
fn build_input<T>(
fn build_input<T, P>(
device: &Device,
config: &StreamConfig,
tx: Sender<Vec<i16>>,
mut producer: P,
channels: usize,
overrun: Arc<AtomicU64>,
) -> Result<Stream, AudioError>
where
T: SizedSample + Send + 'static,
i16: FromSample<T>,
P: Producer<Item = i16> + Send + 'static,
{
let mut acc = FrameAccumulator::new(CAPTURE_FRAME);
let err_fn = |e| crate::log_msg(&format!("cpal capture stream error: {e}"));
device
.build_input_stream::<T, _, _>(
config,
move |data: &[T], _| {
// RT-safe: downmix + wait-free push only. A full ring means the
// drain thread stalled; count the drop and keep going.
for frame in data.chunks_exact(channels) {
let mono = downmix_to_mono(frame);
if let Some(full) = acc.push(mono) {
// Consumer gone (call ended) → stop feeding; the owning
// thread will drop the stream on `stop()`.
if tx.send(full).is_err() {
return;
}
if producer.try_push(mono).is_err() {
overrun.fetch_add(1, Ordering::Relaxed);
}
}
},
@@ -393,9 +491,8 @@ fn run_playback(
target: Option<String>,
ring_fill: Arc<AtomicUsize>,
running: Arc<AtomicBool>,
) -> Result<(), AudioError> {
let (device, config, sample_format) = resolve(true, target)?;
ready: Sender<Result<(), AudioError>>,
) {
let rb = HeapRb::<i16>::new(RING_CAPACITY);
let (mut producer, consumer) = rb.split();
@@ -413,22 +510,43 @@ fn run_playback(
let underrun = Arc::new(AtomicU64::new(0));
let dropped = Arc::new(AtomicU64::new(0));
let stream = match sample_format {
SampleFormat::F32 => {
build_output::<f32, _>(&device, &config, consumer, ring_fill.clone(), underrun.clone())
}
SampleFormat::I16 => {
build_output::<i16, _>(&device, &config, consumer, ring_fill.clone(), underrun.clone())
}
SampleFormat::U16 => {
build_output::<u16, _>(&device, &config, consumer, ring_fill.clone(), underrun.clone())
}
other => Err(AudioError::Stream(format!(
"unsupported playback sample format: {other:?}"
))),
}?;
// Fallible device/stream setup; report the real error to `start_playback`
// before any work so a failure surfaces instead of a silent room. `consumer`
// is moved into the output callback here.
let setup = || -> Result<(Stream, String, SampleFormat), AudioError> {
let (device, config, sample_format) = resolve(true, target)?;
let stream = match sample_format {
SampleFormat::F32 => {
build_output::<f32, _>(&device, &config, consumer, ring_fill.clone(), underrun.clone())
}
SampleFormat::I16 => {
build_output::<i16, _>(&device, &config, consumer, ring_fill.clone(), underrun.clone())
}
SampleFormat::U16 => {
build_output::<u16, _>(&device, &config, consumer, ring_fill.clone(), underrun.clone())
}
other => Err(AudioError::Stream(format!(
"unsupported playback sample format: {other:?}"
))),
}?;
stream.play().map_err(|e| AudioError::Stream(e.to_string()))?;
let name = device.name().unwrap_or_else(|_| "<unknown>".to_string());
Ok((stream, name, sample_format))
};
stream.play().map_err(|e| AudioError::Stream(e.to_string()))?;
let (stream, dev_name, sample_format) = match setup() {
Ok(v) => {
let _ = ready.send(Ok(()));
v
}
Err(e) => {
let _ = ready.send(Err(e));
return;
}
};
crate::log_msg(&format!(
"cpal playback started: device='{dev_name}' format={sample_format:?} channels={PLAYBACK_CHANNELS} rate={SAMPLE_RATE} Hz"
));
let logger = spawn_health_logger(
running.clone(),
@@ -456,7 +574,6 @@ fn run_playback(
running.store(false, Ordering::Relaxed);
let _ = logger.join();
drop(stream);
Ok(())
}
fn build_output<T, C>(