ASIO: collecting output_devices() into Vec only returns the first device

[mikai233]

Summary

On Windows ASIO, host.output_devices() behaves differently depending on whether the iterator is collected first.

• Iterating directly prints all devices.
• Collecting into Vec<_> first, then iterating, only prints the first device.

collect should not change the visible device set.

Environment

• cpal: 0.17.1
• OS: Windows (x64)
• Backend: ASIO (cpal::HostId::Asio)
• Devices in my setup:
  • Focusrite USB ASIO
  • Realtek ASIO
  • USB DAC ASIO

Minimal Repro

use anyhow::Result;
use cpal::traits::{DeviceTrait, HostTrait};

fn list_output_devices() -> Result<()> {
    let host = cpal::host_from_id(cpal::HostId::Asio)?;
    let devices = host.output_devices()?;
    // let devices = devices.collect::<Vec<_>>();
    for device in devices {
        println!("{}", device.description()?);
    }
    Ok(())
}

fn main() -> Result<()> {
    list_output_devices()
}

Actual Behavior

Without collect:

Focusrite USB ASIO
Realtek ASIO
USB DAC ASIO

With:

let devices = devices.collect::<Vec<_>>();

Only the first device is printed:

Focusrite USB ASIO

Expected Behavior

Both versions should enumerate the same set of devices (all 3 in this environment).

Related

Possibly related to ASIO enumeration/lifetime behavior in #798 (this repro is single-threaded and triggered by collecting Device values).

[roderickvd]

Thanks for the report. This is due to ASIO only allowing one driver being loaded at a time. Please try out #1107.