Report 1 channel for builtin input devices

The builtin mic is always mono. Except when VPIO is live somewhere on
the system, as it is then 3, at least on modern MacBooks (with a mic
array). For this reason, and that mixing those 3 channels does not
result in good audio, we already force VPIO with the builtin mic. To
properly pass tests on a local machine with a builtin mic, and to have
clients not be fooled in a way resulting in us having to upmix input
audio, we override the builtin mic's reported channel count and always
report 1.

src/backend/mod.rs

@@ -1621,10 +1621,16 @@ fn get_channel_count(
     debug_assert_running_serially();
 
     let devstreams = get_device_streams(devid, devtype)?;
-    let mut count = 0;
+    let mut count: u32 = 0;
     for ds in devstreams {
-        if let Ok(format) = get_stream_virtual_format(ds.stream) {
-            count += format.mChannelsPerFrame;
+        if devtype == DeviceType::INPUT
+            && CoreStreamData::should_force_vpio_for_input_device(ds.device)
+        {
+            count += 1;
+        } else {
+            count += get_stream_virtual_format(ds.stream)
+                .map(|f| f.mChannelsPerFrame)
+                .unwrap_or(0);
         }
     }
     Ok(count)
@@ -3212,14 +3218,15 @@ impl<'ctx> CoreStreamData<'ctx> {
     }
 
     #[allow(non_upper_case_globals)]
-    fn should_force_vpio_for_input_device(&self, in_device: &device_info) -> bool {
-        assert!(in_device.id != kAudioObjectUnknown);
-        self.debug_assert_is_on_stream_queue();
-        match get_device_transport_type(in_device.id, DeviceType::INPUT) {
+    fn should_force_vpio_for_input_device(id: AudioDeviceID) -> bool {
+        assert!(id != kAudioObjectUnknown);
+        debug_assert_running_serially();
+        match get_device_transport_type(id, DeviceType::INPUT) {
             Ok(kAudioDeviceTransportTypeBuiltIn) => {
                 cubeb_log!(
-                    "Forcing VPIO because input device is built in, and its volume \
-                     is known to be very low without VPIO whenever VPIO is hooked up to it elsewhere."
+                    "Input device {} is on the VPIO force list because it is built in, \
+                     and its volume is known to be very low without VPIO whenever VPIO \
+                     is hooked up to it elsewhere."
                 );
                 true
             }
@@ -3296,7 +3303,7 @@ impl<'ctx> CoreStreamData<'ctx> {
                 .input_stream_params
                 .prefs()
                 .contains(StreamPrefs::VOICE)
-                || self.should_force_vpio_for_input_device(&self.input_device))
+                || CoreStreamData::should_force_vpio_for_input_device(self.input_device.id))
             && !self.should_block_vpio_for_device_pair(&self.input_device, &self.output_device)
             && macos_kernel_major_version() != Ok(MACOS_KERNEL_MAJOR_VERSION_MONTEREY);