CoreAudio: Forward errors to callback during device initialisation

modules/juce_audio_devices/audio_io/juce_AudioDeviceManager.cpp

@@ -720,6 +720,12 @@ String AudioDeviceManager::setAudioDeviceSetup (const AudioDeviceSetup& newSetup
         currentDeviceType = currentAudioDevice->getTypeName();
 
         currentAudioDevice->start (callbackHandler.get());
+
+        error = currentAudioDevice->getLastError();
+    }
+
+    if (error.isEmpty())
+    {
         updateCurrentSetup();
 
         for (int i = 0; i < availableDeviceTypes.size(); ++i)

modules/juce_audio_devices/native/juce_mac_CoreAudio.cpp

@@ -1605,11 +1605,17 @@ public:
             stop();
             fifos.clear();
 
+            {
+                ScopedErrorForwarder forwarder (*this, newCallback);
+
                 for (auto* d : devices)
                     d->start();
 
-            if (newCallback != nullptr)
+                if (! forwarder.encounteredError() && newCallback != nullptr)
                     newCallback->audioDeviceAboutToStart (this);
+                else if (lastError.isEmpty())
+                    lastError = TRANS("Failed to initialise all requested devices.");
+            }
 
             const ScopedLock sl (callbackLock);
             previousCallback = callback = newCallback;
@@ -2079,6 +2085,60 @@ private:
         JUCE_DECLARE_NON_COPYABLE_WITH_LEAK_DETECTOR (DeviceWrapper)
     };
 
+    /* If the current AudioIODeviceCombiner::callback is nullptr, it sets itself as the callback
+       and forwards error related callbacks to the provided callback
+    */
+    class ScopedErrorForwarder  : public AudioIODeviceCallback
+    {
+    public:
+        ScopedErrorForwarder (AudioIODeviceCombiner& ownerIn, AudioIODeviceCallback* cb)
+            : owner (ownerIn),
+              target (cb)
+        {
+            const ScopedLock sl (owner.callbackLock);
+
+            if (owner.callback == nullptr)
+                owner.callback = this;
+        }
+
+        ~ScopedErrorForwarder() override
+        {
+            const ScopedLock sl (owner.callbackLock);
+
+            if (owner.callback == this)
+                owner.callback = nullptr;
+        }
+
+        // We only want to be notified about error conditions when the owner's callback is nullptr.
+        // This class shouldn't be relied on for forwarding this call.
+        void audioDeviceAboutToStart (AudioIODevice*) override {}
+
+        void audioDeviceStopped() override
+        {
+            if (target != nullptr)
+                target->audioDeviceStopped();
+
+            error = true;
+        }
+
+        void audioDeviceError (const String& errorMessage) override
+        {
+            owner.lastError = errorMessage;
+
+            if (target != nullptr)
+                target->audioDeviceError (errorMessage);
+
+            error = true;
+        }
+
+        bool encounteredError() const { return error; }
+
+    private:
+        AudioIODeviceCombiner& owner;
+        AudioIODeviceCallback* target;
+        bool error = false;
+    };
+
     OwnedArray<DeviceWrapper> devices;
 
     JUCE_DECLARE_NON_COPYABLE_WITH_LEAK_DETECTOR (AudioIODeviceCombiner)