Stop moving input devices

[achronop]

Stop moving to a new device when the default device changes or when the current device is removed.

[achronop]

This is needed for BMO 1628634. In Ubuntu PA is configured to make a new input device the system default. We don't want to switch to that new device because it is against the spec.

[kinetiknz]

We've been discussing this change via PM. Changing the device switching behaviour of cubeb streams is fine, but we need a clear idea of the desired behaviour at the cubeb API level. With that in hand, we can address any behavioural inconsistencies existing in the various backends.

Currently:

• PulseAudio always follows the default device (before this PR)
• AudioUnit follows the default output device if cubeb_devid is null, but seems to follow the default input device only if the current input device has disappeared (even if a cubeb_devid was specified)
• WASAPI follows the default device if cubeb_devid is null. If a cubeb_devid is specified, it's always used when (re)opening streams.

I think the WASAPI behaviour is closest to the original intent of the API: passing a valid cubeb_devid means the stream only ever uses that device and passing a null cubeb_devid allows the stream to follow that device type's OS default.

My proposal is to change the PulseAudio backend to match the current Windows behaviour, which means setting DONT_MOVE on an input or output stream only if a cubeb_devid was specified.

If we need to support a use case where the caller passes a null cubeb_devid but doesn't want the stream to follow the default device, the caller could indicate that by setting CUBEB_STREAM_PREF_DISABLE_DEVICE_SWITCHING during stream initialization. That pref is currently only implemented for WASAPI, but it'd be simple to add to the other backends.

[padenot]

I think this plan is good, because it makes sense on an API level and allows us to implement the behaviour we need for the web.

I can take care of aligning the AudioUnit bit today if nobody is doing that.

[achronop]

Thanks for summarizing our discussion. I totally agree and I will follow up on the implementation here. One thing I would like to mention explicitly is that the use of CUBEB_STREAM_PREF_DISABLE_DEVICE_SWITCHING pref will disable switching for all available sides which are both input and output in case of a duplex stream. This also windows current behavior.

[kinetiknz]

One thing I would like to mention explicitly is that the use of CUBEB_STREAM_PREF_DISABLE_DEVICE_SWITCHING pref will disable switching for all available sides which are both input and output in case of a duplex stream. This also windows current behavior.

You're right that the WASAPI backend handles it that way. That's due to the very limited requirements of the existing use inside Gecko, and that is no longer needed due to sandboxing and AudioSession remoting. The flag is per-cubeb_stream_param, so backends can treat it per-side.

[ChunMinChang]

For AudioUnit side, the input device will do its best to follow the cubeb_devid user defines. When the stream is reinitializing, we will try using the mic with the specified cubeb_devid. If that fails, then the mic becomes the default input device.

[kinetiknz]

LGTM, thanks!