CallbackCamera operations hang once camera stream is open

[RReverser]

Simple repro:

use nokhwa::pixel_format::RgbFormat;
use nokhwa::utils::{RequestedFormat, RequestedFormatType};
use nokhwa::CallbackCamera;

fn main() -> anyhow::Result<()> {
    println!("Querying...");
    let cameras = nokhwa::query(nokhwa::utils::ApiBackend::Auto)?;
    println!("Opening camera...");
    let mut camera = CallbackCamera::new(
        cameras[0].index().clone(),
        RequestedFormat::new::<RgbFormat>(RequestedFormatType::AbsoluteHighestResolution),
        |_frame| {
            println!("Got frame!");
        },
    )?;
    camera.open_stream()?;
    println!("Waiting 1 sec...");
    std::thread::sleep(std::time::Duration::from_secs(1));
    println!("Trying to change format...");
    camera.set_camera_requset(RequestedFormat::new::<RgbFormat>(
        RequestedFormatType::AbsoluteHighestFrameRate,
    ))?;
    println!("Changed format successfully!");
    Ok(())
}

will currently output:

Querying...
Opening camera...
Waiting 1 sec...
Got frame!
Got frame!
Got frame!
Got frame!
Got frame!
Got frame!
Got frame!
Got frame!
Got frame!
Trying to change format...
Got frame!
Got frame!
Got frame!
Got frame!
Got frame!
Got frame!
Got frame!
Got frame!
Got frame!
Got frame!
Got frame!
Got frame!
Got frame!
Got frame!
Got frame!
Got frame!
Got frame!
Got frame!
[...]

That is, "Changed format successfully!" is never reached.

[RReverser]

What's interesting, I thought stopping stream and then changing camera format might help, but CallbackCamera::stop_stream itself also hangs:

use nokhwa::pixel_format::RgbFormat;
use nokhwa::utils::{RequestedFormat, RequestedFormatType};
use nokhwa::CallbackCamera;

fn main() -> anyhow::Result<()> {
    println!("Querying...");
    let cameras = nokhwa::query(nokhwa::utils::ApiBackend::Auto)?;
    println!("Opening camera...");
    let mut camera = CallbackCamera::new(
        cameras[0].index().clone(),
        RequestedFormat::new::<RgbFormat>(RequestedFormatType::AbsoluteHighestResolution),
        |_frame| {
            println!("Got frame!");
        },
    )?;
    camera.open_stream()?;
    println!("Waiting 1 sec...");
    std::thread::sleep(std::time::Duration::from_secs(1));
    println!("Trying to stop stream...");
    camera.stop_stream()?;
    println!("Stopped stream successfully!");
    Ok(())
}

Querying...
Opening camera...
Waiting 1 sec...
Got frame!
Got frame!
Got frame!
Got frame!
Got frame!
Got frame!
Got frame!
Got frame!
Trying to stop stream...
Got frame!
Got frame!
Got frame!
Got frame!
Got frame!
Got frame!
Got frame!
Got frame!
Got frame!
Got frame!
Got frame!
Got frame!
[...]

Looks like there's some lock issue in CallbackCamera that doesn't let it process any incoming messages once the stream is open.

[RReverser]

Looks like there's some lock issue in CallbackCamera that doesn't let it process any incoming messages once the stream is open.

Yeah I think the problem is here: https://github.com/l1npengtul/nokhwa/blob/12cfe670e89be6486f11efda7fa5b38c87233c4b/src/threaded.rs#L555-L556

This locks-unlocks camera in a tight loop, which is effectively the same as holding the lock forever outside the loop, so no other operation that tries to also take camera.lock() has a chance to get through.

[RReverser]

Semi-related question: I see you're using unsafe impl Send for Camera {}. Is it definitely safe for all camera backends?

I know some device drivers (not sure about webcams) really don't like being accessed from threads other than the one that created them, so I thought if they're not Send automatically, this might be problematic?