Host side of AUv3 IPC forwards incoming transactions to dedicated dis…

…patch queues

because AUMessageChannel does not allow to call back from the receiving thread...

IPC/ARAIPCAudioUnit_v3.mm

@@ -166,6 +166,11 @@ void _sendMessage (MessageID messageID, MessageEncoder * encoder) override
     : AudioUnitMessageChannel { this },
     _audioUnitChannel { audioUnitChannel }
     {
+        // \todo there's also QOS_CLASS_USER_INTERACTIVE which seems more appropriate but is undocumented...
+        if (_instanceCount == 0)
+            _readAudioQueue = dispatch_queue_create ("ARA read audio samples", dispatch_queue_attr_make_with_qos_class(DISPATCH_QUEUE_SERIAL, QOS_CLASS_USER_INITIATED, -1));
+        ++_instanceCount;
+
 #if !__has_feature(objc_arc)
         [_audioUnitChannel retain];
 #endif
@@ -180,6 +185,27 @@ void _sendMessage (MessageID messageID, MessageEncoder * encoder) override
     ~ProxyPlugInMessageChannel () override
     {
         _audioUnitChannel.callHostBlock = nil;
+
+        --_instanceCount;
+        if (_instanceCount == 0)
+            dispatch_release (_readAudioQueue);
+    }
+
+    DispatchTarget getDispatchTargetForIncomingTransaction (MessageID messageID) override
+    {
+        // AUMessageChannel cannot be called back from the same thread it receives the message,
+        // so we dispatch to the main queue for playback requests and to a dedicated read samples queue for audio requests
+        // \todo maybe we should make this configurable, so hosts can set these queues if they already have appropriate ones?
+        if (messageID == ARA_IPC_HOST_METHOD_ID (ARAAudioAccessControllerInterface, readAudioSamples).getMessageID ())
+        {
+            return _readAudioQueue;
+        }
+        else
+        {
+            ARA_INTERNAL_ASSERT ((ARA_IPC_HOST_METHOD_ID (ARAPlaybackControllerInterface, requestStartPlayback).getMessageID () <= messageID) &&
+                                 (messageID <= ARA_IPC_HOST_METHOD_ID (ARAPlaybackControllerInterface, requestEnableCycle).getMessageID ()));
+            return dispatch_get_main_queue ();
+        }
     }
 
 protected:
@@ -191,8 +217,13 @@ void _sendMessage (MessageID messageID, MessageEncoder * encoder) override
 
 private:
     NSObject<AUMessageChannel> * __strong _Nonnull _audioUnitChannel;
+    static dispatch_queue_t _readAudioQueue;
+    static int _instanceCount;
 };
 
+dispatch_queue_t ProxyPlugInMessageChannel::_readAudioQueue { nullptr };
+int ProxyPlugInMessageChannel::_instanceCount { 0 };
+
 
 // host side: proxy plug-in message channel further specialization for plug-in extension messages
 class ProxyPlugInExtensionMessageChannel : public ProxyPlugInMessageChannel