Wasapi capture doesn't work: Stream is spawned but callback not called

[Boscop]

I'm trying to do Wasapi loopback capture but it's not working:
Even though my input stream is spawned, the stream callback never gets called. Why could that be?

It prints this:

[DEBUG] Try capturing system audio
[INFO ] Capturing audio from: Line (Steinberg UR22mkII )
[INFO ] Default audio cfg: SupportedStreamConfig { channels: 2, sample_rate: SampleRate(44100), buffer_size: Unknown, sample_format: F32 }
build_stream F32
stream is playing
still playing
still playing
still playing

But it should ALSO print wave_reader called (from inside the wave_reader stream callback), but it never prints this and so I'm not receiving any audio frames through the channel in my application.

I based my code on the implementation of loopback capture in https://github.com/dheijl/swyh-rs:
https://github.com/dheijl/swyh-rs/blob/e6709b2f546af6cf272ded9529f6dc5d95145453/src/main.rs#L849

In swyh it apparently works (but I can't test that application because I don't have a streaming server/receiver).
I just refactored the code a little bit, but apart from that it's the same.
Any idea why the stream callback (wave_reader) doesn't get called?

(Note: On linux, wave_reader does get called and I receive audio frames over the channel. They are all 0.0, but that's a different issue, probably a linux issue rather than cpal issue?)

use super::*;
use anyhow::{Context, Result};
use cpal::{Device, Sample, SampleFormat, Stream, StreamError, SupportedStreamConfig};
use device::*;
use std::{
    sync::{
        atomic::{AtomicBool, Ordering},
        mpsc::channel,
        Arc,
    },
    thread,
};

pub fn capture_live_audio_input(
    m_audio_device: Option<String>,
    terminate_capture: Arc<AtomicBool>,
) -> Result<(BidirAudioChannel, AudioFormat)> {
    let (audio_to_vis, vis_to_audio) = BidirAudioChannel::duplex_unbounded();

    // spawn new thread as workaround: https://github.com/RustAudio/rodio/issues/214
    let (tx, rx) = channel();
    let _thread_capture = thread::Builder::new()
        .name("capture".to_string())
        .spawn(move || -> Result<_> {
            let output_device = match m_audio_device {
                Some(name) => unwropt!(
                    get_audio_output_devices()
                        .into_iter()
                        .find_map(|device| (device.name().ok().as_deref() == Some(&name)).then_some(device)),
                    "audio device not found"
                ),
                None => unwropt!(get_default_audio_output_device(), "no default audio device"),
            };

            // capture system audio
            debug!("Try capturing system audio");
            let (stream, audio_format) =
                capture_output_audio(&output_device, audio_to_vis).context("Could not capture audio")?;
            tx.send(audio_format).unwrap();
            stream.play().context("stream.play()")?;
            println!("stream is playing");
            // stream shouldn't be dropped, but it's not Send, so we need to keep this thread alive
            while !terminate_capture.load(Ordering::SeqCst) {
                std::thread::sleep(std::time::Duration::from_millis(100));
                println!("still playing");
            }
            Ok(())
        })
        .expect("spawn");
    let audio_format = rx.recv()?;

    Ok((vis_to_audio, audio_format))
}

/// capture the audio stream from the default audio output device
/// sets up an input stream for the wave_reader in the appropriate format (f32/i16/u16)
fn capture_output_audio(
    output_device: &Device,
    audio_to_vis: BidirAudioChannel,
) -> Result<(Stream, AudioFormat)> {
    info!(
        "Capturing audio from: {}",
        output_device.name().context("Could not get default audio device name")?
    );

    let audio_cfg = output_device.default_output_config().context("No default output config found")?;
    let audio_format =
        AudioFormat { sample_rate: audio_cfg.sample_rate().0, channel_count: audio_cfg.channels() as usize };
    info!("Default audio cfg: {:?}", audio_cfg);

    fn build_stream<T: Sample>(
        output_device: &Device,
        audio_cfg: SupportedStreamConfig,
        audio_format: AudioFormat,
        audio_to_vis: BidirAudioChannel,
    ) -> Result<(Stream, AudioFormat)> {
        println!("build_stream {:?}", T::FORMAT);
        fn wave_reader<T: Sample>(samples: &[T], audio_to_vis: &BidirAudioChannel) {
			println!("wave_reader called");
            let mut f32_samples =
                audio_to_vis.rx.try_recv().unwrap_or_else(|_| Vec::with_capacity(samples.len()));
            f32_samples.clear();
            f32_samples.extend(samples.iter().map(|x| x.to_f32()));
            println!("wave_reader {}", f32_samples.len());
            let _ = audio_to_vis.tx.send(f32_samples);
        }

        match output_device.build_input_stream(
            &audio_cfg.config(),
            move |data, _: &_| wave_reader::<T>(data, &audio_to_vis),
            |e| error!("Error on audio input stream: {}", e),
        ) {
            Ok(stream) => Ok((stream, audio_format)),
            Err(e) => {
                bail!("Error capturing {:?} audio stream: {}", T::FORMAT, e);
            }
        }
    }

    match audio_cfg.sample_format() {
        SampleFormat::F32 => build_stream::<f32>(output_device, audio_cfg, audio_format, audio_to_vis),
        SampleFormat::I16 => build_stream::<i16>(output_device, audio_cfg, audio_format, audio_to_vis),
        SampleFormat::U16 => build_stream::<u16>(output_device, audio_cfg, audio_format, audio_to_vis),
    }
}

---

Btw, if you're wondering why I'm spawning a new thread (compared to swyh):
If I don't create the stream in the separate thread, I get this:

Os { code: -2147417850, kind: Other, message: "Cannot change thread mode after it is set." }', C:\Users\me.cargo\registry\src\github.com-1ecc6299db9ec823\cpal-0.13.1\src\host\wasapi\com.rs:13:77

It's this issue: RustAudio/rodio#214
My application uses winit, and apparently this combination causes this issue:
RustAudio/rodio#214 (comment)
rust-windowing/winit#1185
That's why I'm spawning a new thread for the stream (which I'm then keeping alive to prevent the stream from getting dropped).
But that's the only difference to swyh that I see. So I'm wondering why the stream callback is not being called.

[dheijl]

I don't know if you are aware of the following (ignore it if you are): it seems that WasApi will not start calling the callback until something is actually playing on an audio output (real device or virtual device like HiFi Cable). For instance on some systems if the speakers are muted, depending on the hardware, the callback will not be called at all.

[toastmod]

Was this bug ever worked around? It's also happening to me where the data_callback is not being called.
Also I am playing audio on the device aside from my own application and it's still not using the callback.
I am also using winit.