CoreAudio: Prevent audio glitches before microphone permission granted

If input channels have been requested but no data is being sent from CoreAudio (usually due to permissions not being granted yet) then avoid waiting unnecessarily in AudioIODeviceCombiner::readInput() which could previously cause audio glitches. Also changed the wait time to use an exponential backoff.

modules/juce_audio_devices/native/juce_mac_CoreAudio.cpp

@@ -1663,11 +1663,16 @@ private:
     void readInput (AudioBuffer<float>& buffer, const int numSamples, const int blockSizeMs)
     {
         for (auto* d : devices)
-            d->done = (d->numInputChans == 0);
+            d->done = (d->numInputChans == 0 || d->isWaitingForInput);
 
-        for (int tries = 5;;)
+        float totalWaitTimeMs = blockSizeMs * 5.0f;
+        constexpr int numReadAttempts = 6;
+        auto sumPower2s = [] (int maxPower) { return (1 << (maxPower + 1)) - 1; };
+        float waitTime = totalWaitTimeMs / (float) sumPower2s (numReadAttempts - 2);
+
+        for (int numReadAttemptsRemaining = numReadAttempts;;)
         {
-            bool anyRemaining = false;
+            bool anySamplesRemaining = false;
 
             for (auto* d : devices)
             {
@@ -1679,17 +1684,20 @@ private:
                         d->done = true;
                     }
                     else
-                        anyRemaining = true;
+                    {
+                        anySamplesRemaining = true;
+                    }
                 }
             }
 
-            if (! anyRemaining)
+            if (! anySamplesRemaining)
                 return;
 
-            if (--tries == 0)
+            if (--numReadAttemptsRemaining == 0)
                 break;
 
-            wait (blockSizeMs);
+            wait (jmax (1, roundToInt (waitTime)));
+            waitTime *= 2.0f;
         }
 
         for (auto* d : devices)
@@ -1717,7 +1725,9 @@ private:
                         d->done = true;
                     }
                     else
+                    {
                         anyRemaining = true;
+                    }
                 }
             }
 
@@ -1808,6 +1818,8 @@ private:
             numInputChans  = useInputs  ? device->getActiveInputChannels().countNumberOfSetBits()  : 0;
             numOutputChans = useOutputs ? device->getActiveOutputChannels().countNumberOfSetBits() : 0;
 
+            isWaitingForInput = numInputChans > 0;
+
             inputIndex = channelIndex;
             outputIndex = channelIndex + numInputChans;
 
@@ -1892,6 +1904,8 @@ private:
         {
             if (numInputChannels > 0)
             {
+                isWaitingForInput = false;
+
                 int start1, size1, start2, size2;
                 inputFifo.prepareToWrite (numSamples, start1, size1, start2, size2);
 
@@ -1972,7 +1986,7 @@ private:
         AudioIODeviceCombiner& owner;
         std::unique_ptr<CoreAudioIODevice> device;
         int inputIndex = 0, numInputChans = 0, outputIndex = 0, numOutputChans = 0;
-        bool useInputs = false, useOutputs = false;
+        bool useInputs = false, useOutputs = false, isWaitingForInput = false;
         AbstractFifo inputFifo { 32 }, outputFifo { 32 };
         bool done = false;