feat(host/windows): WASAPI loopback audio capture

Windows AudioCapturer via the wasapi crate (0.23): loopback the default render endpoint (Render device + Direction::Capture + shared mode => STREAMFLAGS_LOOPBACK) at 48 kHz stereo f32 with autoconvert, feeding the existing Opus path with no resampling. Dedicated COM-MTA thread owns the !Send WASAPI objects; interleaved f32 chunks leave over a bounded lossy channel; RAII Drop stops + joins. Bring-up handshake reports a missing endpoint as Err so a session continues without audio. open_audio_capture Windows factory arm + module. Init chain validated live on the VM (open succeeds; next_chunk waits on a silent system). Virtual mic deferred (no Windows virtual-audio endpoint). m3 audio_thread wiring + opus hoist land with the integration task.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Cargo.lock

@@ -2638,6 +2638,7 @@ dependencies = [
  "utoipa",
  "utoipa-axum",
  "utoipa-scalar",
+ "wasapi",
  "wayland-backend",
  "wayland-client",
  "wayland-protocols-misc",
@@ -3965,6 +3966,19 @@ dependencies = [
  "winapi-util",
 ]
 
+[[package]]
+name = "wasapi"
+version = "0.23.0"
+source = "registry+https://github.com/rust-lang/crates.io-index"
+checksum = "80c3aa5d6b0e7acc3ea10cb19c334df0c8d825060f14a30d9e3b03385e6e5175"
+dependencies = [
+ "log",
+ "num-integer",
+ "thiserror 2.0.18",
+ "windows",
+ "windows-core",
+]
+
 [[package]]
 name = "wasi"
 version = "0.11.1+wasi-snapshot-preview1"

crates/punktfunk-host/Cargo.toml

@@ -120,3 +120,5 @@ windows = { version = "0.62", features = [
 # Software H.264 encoder (GPU-less path + NVENC fallback). The default `source` feature statically
 # compiles OpenH264 (BSD-2) — no system lib, builds on MSVC; nasm on PATH adds the SIMD fast path.
 openh264 = "0.9"
+# WASAPI loopback audio capture (default render endpoint -> 48 kHz stereo f32 for the Opus path).
+wasapi = "0.23"

crates/punktfunk-host/src/audio.rs

@@ -37,9 +37,14 @@ pub fn open_audio_capture(channels: u32) -> Result<Box<dyn AudioCapturer>> {
     linux::PwAudioCapturer::open(channels).map(|c| Box::new(c) as Box<dyn AudioCapturer>)
 }
 
-#[cfg(not(target_os = "linux"))]
+#[cfg(target_os = "windows")]
+pub fn open_audio_capture(channels: u32) -> Result<Box<dyn AudioCapturer>> {
+    wasapi_cap::WasapiLoopbackCapturer::open(channels).map(|c| Box::new(c) as Box<dyn AudioCapturer>)
+}
+
+#[cfg(not(any(target_os = "linux", target_os = "windows")))]
 pub fn open_audio_capture(_channels: u32) -> Result<Box<dyn AudioCapturer>> {
-    anyhow::bail!("audio capture requires Linux + PipeWire")
+    anyhow::bail!("audio capture requires Linux + PipeWire or Windows + WASAPI")
 }
 
 /// The inverse of [`AudioCapturer`]: a virtual microphone the host *produces*. It registers a
@@ -71,3 +76,5 @@ pub fn open_virtual_mic(_channels: u32) -> Result<Box<dyn VirtualMic>> {
 
 #[cfg(target_os = "linux")]
 mod linux;
+#[cfg(target_os = "windows")]
+mod wasapi_cap;

crates/punktfunk-host/src/audio/wasapi_cap.rs

@@ -0,0 +1,189 @@
+//! WASAPI loopback capture of the default render endpoint (system output) — the Windows analogue
+//! of the PipeWire sink-monitor backend. Delivers interleaved f32 PCM at 48 kHz stereo, ready for
+//! the existing Opus path with NO resampling (WASAPI shared-mode autoconvert does any SRC). WASAPI
+//! objects are COM-apartment-bound and not `Send`, so they live on a dedicated thread (mirrors
+//! `linux::PwAudioCapturer`); only the channel + stop flag + join handle are in the struct.
+
+use super::{AudioCapturer, SAMPLE_RATE};
+use anyhow::{anyhow, Context, Result};
+use std::collections::VecDeque;
+use std::sync::atomic::{AtomicBool, Ordering};
+use std::sync::mpsc::{sync_channel, Receiver, RecvTimeoutError, SyncSender};
+use std::sync::Arc;
+use std::thread::{self, JoinHandle};
+use std::time::Duration;
+use wasapi::{DeviceEnumerator, Direction, SampleType, StreamMode, WaveFormat};
+
+// 48 kHz stereo 32-bit float: 2 channels * 4 bytes = 8 bytes per frame.
+const BLOCK_ALIGN: usize = 2 * 4;
+
+pub struct WasapiLoopbackCapturer {
+    chunks: Receiver<Vec<f32>>,
+    channels: u32,
+    stop: Arc<AtomicBool>,
+    join: Option<JoinHandle<()>>,
+}
+
+impl WasapiLoopbackCapturer {
+    pub fn open(channels: u32) -> Result<WasapiLoopbackCapturer> {
+        anyhow::ensure!(
+            channels == 2,
+            "WASAPI loopback backend is stereo-only (got {channels})"
+        );
+        let (tx, rx) = sync_channel::<Vec<f32>>(64);
+        let stop = Arc::new(AtomicBool::new(false));
+        // Bring-up handshake: report open success/failure before returning, so a missing render
+        // endpoint surfaces as Err (caller continues without audio) rather than a silent dead thread.
+        let (ready_tx, ready_rx) = sync_channel::<Result<()>>(1);
+        let stop_t = stop.clone();
+        let join = thread::Builder::new()
+            .name("punktfunk-wasapi-audio".into())
+            .spawn(move || {
+                if let Err(e) = capture_thread(tx, stop_t, ready_tx) {
+                    tracing::error!(error = format!("{e:#}"), "wasapi loopback thread failed");
+                }
+            })
+            .context("spawn wasapi audio thread")?;
+        match ready_rx.recv_timeout(Duration::from_secs(3)) {
+            Ok(Ok(())) => {
+                tracing::info!("WASAPI loopback capture: 48 kHz stereo f32 (default render endpoint)");
+                Ok(WasapiLoopbackCapturer {
+                    chunks: rx,
+                    channels,
+                    stop,
+                    join: Some(join),
+                })
+            }
+            Ok(Err(e)) => Err(e),
+            Err(_) => Err(anyhow!(
+                "wasapi loopback init timed out (no default render endpoint?)"
+            )),
+        }
+    }
+}
+
+impl Drop for WasapiLoopbackCapturer {
+    fn drop(&mut self) {
+        self.stop.store(true, Ordering::SeqCst);
+        if let Some(j) = self.join.take() {
+            let _ = j.join();
+        }
+    }
+}
+
+impl AudioCapturer for WasapiLoopbackCapturer {
+    fn next_chunk(&mut self) -> Result<Vec<f32>> {
+        match self.chunks.recv_timeout(Duration::from_secs(5)) {
+            Ok(c) => Ok(c),
+            Err(RecvTimeoutError::Timeout) => Err(anyhow!("no WASAPI audio within 5s")),
+            Err(RecvTimeoutError::Disconnected) => Err(anyhow!("wasapi audio thread ended")),
+        }
+    }
+    fn channels(&self) -> u32 {
+        self.channels
+    }
+    fn drain(&mut self) {
+        while self.chunks.try_recv().is_ok() {}
+    }
+}
+
+fn capture_thread(
+    tx: SyncSender<Vec<f32>>,
+    stop: Arc<AtomicBool>,
+    ready: SyncSender<Result<()>>,
+) -> Result<()> {
+    // COM must be initialized on THIS thread (MTA), before any device call.
+    if let Err(e) = wasapi::initialize_mta().ok().context("CoInitializeEx (MTA)") {
+        let _ = ready.send(Err(e));
+        return Ok(());
+    }
+    let res = (|| -> Result<()> {
+        // Loopback = capture the RENDER endpoint: get the default render device, but open a CAPTURE
+        // client with loopback=true over it.
+        let device = DeviceEnumerator::new()
+            .context("DeviceEnumerator")?
+            .get_default_device(&Direction::Render)
+            .context("default render endpoint (loopback needs a render device)")?;
+        let mut audio_client = device.get_iaudioclient().context("IAudioClient")?;
+        // 48 kHz stereo f32 interleaved; autoconvert lets WASAPI's shared-mode SRC match the engine
+        // mix format to ours, so we never resample in Rust. Loopback is implied by capturing a
+        // RENDER device with Direction::Capture in shared mode (wasapi sets STREAMFLAGS_LOOPBACK).
+        let desired = WaveFormat::new(32, 32, &SampleType::Float, SAMPLE_RATE as usize, 2, None);
+        let (default_period, _min_period) =
+            audio_client.get_device_period().context("device period")?;
+        let mode = StreamMode::EventsShared {
+            autoconvert: true,
+            buffer_duration_hns: default_period,
+        };
+        audio_client
+            .initialize_client(&desired, &Direction::Capture, &mode)
+            .context("initialize loopback client")?;
+        let h_event = audio_client.set_get_eventhandle().context("event handle")?;
+        let capture_client = audio_client
+            .get_audiocaptureclient()
+            .context("IAudioCaptureClient")?;
+        audio_client.start_stream().context("start loopback stream")?;
+        let _ = ready.send(Ok(()));
+
+        let mut bytes: VecDeque<u8> = VecDeque::new();
+        while !stop.load(Ordering::Relaxed) {
+            // Loopback fires events only while audio renders; the finite timeout keeps `stop` responsive.
+            if h_event.wait_for_event(100).is_err() {
+                continue;
+            }
+            loop {
+                match capture_client.get_next_packet_size() {
+                    Ok(Some(0)) | Ok(None) => break,
+                    Ok(Some(_n)) => {
+                        capture_client
+                            .read_from_device_to_deque(&mut bytes)
+                            .context("read loopback")?;
+                    }
+                    Err(e) => return Err(anyhow!("get_next_packet_size: {e}")),
+                }
+            }
+            let whole = (bytes.len() / BLOCK_ALIGN) * BLOCK_ALIGN;
+            if whole == 0 {
+                continue;
+            }
+            let raw: Vec<u8> = bytes.drain(..whole).collect();
+            let mut samples = Vec::with_capacity(whole / 4);
+            for c in raw.chunks_exact(4) {
+                samples.push(f32::from_le_bytes([c[0], c[1], c[2], c[3]]));
+            }
+            let _ = tx.try_send(samples); // non-blocking, lossy — same discipline as PipeWire
+        }
+        audio_client.stop_stream().ok();
+        Ok(())
+    })();
+    if let Err(ref e) = res {
+        let _ = ready.send(Err(anyhow!("{e:#}")));
+    }
+    res
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    /// Live loopback round trip — skipped unless `PUNKTFUNK_WASAPI_LIVE=1` and a render endpoint
+    /// exists. Opens the capturer and pulls one chunk of interleaved f32.
+    #[test]
+    fn live_open_and_read() {
+        if std::env::var("PUNKTFUNK_WASAPI_LIVE").is_err() {
+            return;
+        }
+        let mut cap = match WasapiLoopbackCapturer::open(2) {
+            Ok(c) => c,
+            Err(e) => {
+                eprintln!("no render endpoint on this box ({e:#}) — skipping");
+                return;
+            }
+        };
+        assert_eq!(cap.channels(), 2);
+        match cap.next_chunk() {
+            Ok(samples) => assert!(samples.len() % 2 == 0, "interleaved stereo => even sample count"),
+            Err(e) => eprintln!("no audio within timeout (silent system?): {e:#}"),
+        }
+    }
+}