AudioParam automationRate does not instantly update on the control thread

[orottier]

The following test fails:

#[test]
fn test_automation_rate_synchronicity() {
    let context = OfflineAudioContext::new(1, 0, 48000.);

    let opts = AudioParamDescriptor {
        name: String::new(),
        automation_rate: AutomationRate::A, // start with A
        default_value: 0.,
        min_value: 0.,
        max_value: 1.,
    };
    let (param, _render) = audio_param_pair(opts, context.mock_registration());

    // set to K
    param.set_automation_rate(AutomationRate::K);
    assert_eq!(param.automation_rate(), AutomationRate::K);
}

This is quite tricky to fix. We could keep a local copy of the automation rate on the control thread, but because these methods all take &self it needs to be wrapped in an atomic and is susceptible to data races on concurrent calls. Best would be to require &mut on the methods but it is incompatible with how we return AudioParams from the AudioNodes (and the Clone impl)

[orottier]

A Mutex could resolve this issue, when we hold the lock while sending the control message to the render thread. A bit ugly though