Support for WASM with +mutable-globals,+atomics target features

[Oberdiah]

I realise this is quite an obscure use-case, but currently if you try to play audio on WASM with the +atomics and +mutable-globals features enabled in Rust Nightly, Firefox and Chrome error out since AudioBuffer.copyToChannel doesn't support SharedArrayBuffers.

Error on Firefox:

panicked at 'Unable to write sample data into the audio context buffer: JsValue(TypeError: AudioBuffer.copyToChannel: Argument 1 can't be a SharedArrayBuffer or an ArrayBufferView backed by a SharedArrayBuffer

Error on Chrome:

web.js:446 panicked at 'Unable to write sample data into the audio context buffer: JsValue(TypeError: Failed to execute 'copyToChannel' on 'AudioBuffer': The provided Float32Array value must not be shared.
TypeError: Failed to execute 'copyToChannel' on 'AudioBuffer': The provided Float32Array value must not be shared.

init/imports.wbg.__wbg_copyToChannel_74f68c0678489450/<@http://localhost/web.js:1244:25
handleError@http://localhost/web.js:310:18
init/imports.wbg.__wbg_copyToChannel_74f68c0678489450@http://localhost/web.js:1243:76
web_sys::features::gen_AudioBuffer::AudioBuffer::copy_to_channel::ha856702a3af061e5@http://localhost/web_bg.wasm:wasm-function[6669]:0x71ba4f
<cpal::host::webaudio::Device as cpal::traits::DeviceTrait>::build_output_stream_raw::{{closure}}::ha128e8e549ebdafb@http://localhost/web_bg.wasm:wasm-function[669]:0x2bcd81

EDIT:

I have managed to get it working by modifying host\webaudio\mod.rs by adding an external javascript call

webaudio\mod.rs:

#[wasm_bindgen(module = "/audio-shim.js")]
extern "C" {
    fn copy_audio_buffer(dest: &AudioBuffer, src: &[f32], channel: i32);
}

// New, working version
copy_audio_buffer(&ctx_buffer, &temporary_channel_buffer, channel as i32);

// Previous, broken version
// ctx_buffer
//     .copy_to_channel(&mut temporary_channel_buffer, channel as i32)
//     .expect("Unable to write sample data into the audio context buffer");

audio-shim.js:

export function copy_audio_buffer(dest, src, channel) {
    // Turn the array view into owned memory.
    var standalone = [...src];
    // Make it a Float32Array.
    var buffer = new Float32Array(standalone);

    // Copy the data.
    dest.copyToChannel(buffer, channel);
}

( Many thanks to this webpage for that trick )

[Nek]

Huge enormous thanks! You've saved my week.

[Davidster]

would the maintainers of this project be willing to accept a pull request containing this change?

I want to use this but if it's not added into the main branch then I will have to maintain a fork of cpal

[Oberdiah]

No idea, but it's worth trying I guess, feel free. The branch I'm using at the moment is the one attempting to be merged in https://github.com/RustAudio/cpal/pull/774, which has this change plus more fixes, so personally I'm hoping that that gets merged.