Demos: Fix data race in AudioPluginDemo

examples/Plugins/AudioPluginDemo.h

@@ -185,8 +185,6 @@ public:
                  { std::make_unique<AudioParameterFloat> ("gain",  "Gain",           NormalisableRange<float> (0.0f, 1.0f), 0.9f),
                    std::make_unique<AudioParameterFloat> ("delay", "Delay Feedback", NormalisableRange<float> (0.0f, 1.0f), 0.5f) })
     {
-        lastPosInfo.resetToDefault();
-
         // Add a sub-tree to store the state of our UI
         state.state.addChild ({ "uiState", { { "width",  400 }, { "height", 200 } }, {} }, -1, nullptr);
 
@@ -324,6 +322,33 @@ public:
         return trackProperties;
     }
 
+    class SpinLockedPosInfo
+    {
+    public:
+        SpinLockedPosInfo() { info.resetToDefault(); }
+
+        // Wait-free, but setting new info may fail if the main thread is currently
+        // calling `get`. This is unlikely to matter in practice because
+        // we'll be calling `set` much more frequently than `get`.
+        void set (const AudioPlayHead::CurrentPositionInfo& newInfo)
+        {
+            const juce::SpinLock::ScopedTryLockType lock (mutex);
+
+            if (lock.isLocked())
+                info = newInfo;
+        }
+
+        AudioPlayHead::CurrentPositionInfo get() const noexcept
+        {
+            const juce::SpinLock::ScopedLockType lock (mutex);
+            return info;
+        }
+
+    private:
+        juce::SpinLock mutex;
+        AudioPlayHead::CurrentPositionInfo info;
+    };
+
     //==============================================================================
     // These properties are public so that our editor component can access them
     // A bit of a hacky way to do it, but it's only a demo! Obviously in your own
@@ -335,7 +360,7 @@ public:
 
     // this keeps a copy of the last set of time info that was acquired during an audio
     // callback - the UI component will read this and display it.
-    AudioPlayHead::CurrentPositionInfo lastPosInfo;
+    SpinLockedPosInfo lastPosInfo;
 
     // Our plug-in's current state
     AudioProcessorValueTreeState state;
@@ -422,7 +447,7 @@ private:
 
         void timerCallback() override
         {
-            updateTimecodeDisplay (getProcessor().lastPosInfo);
+            updateTimecodeDisplay (getProcessor().lastPosInfo.get());
         }
 
         void hostMIDIControllerIsAvailable (bool controllerIsAvailable) override
@@ -622,19 +647,23 @@ private:
 
     void updateCurrentTimeInfoFromHost()
     {
-        if (auto* ph = getPlayHead())
+        const auto newInfo = [&]
         {
-            AudioPlayHead::CurrentPositionInfo newTime;
-
-            if (ph->getCurrentPosition (newTime))
+            if (auto* ph = getPlayHead())
             {
-                lastPosInfo = newTime;  // Successfully got the current time from the host..
-                return;
-            }
-        }
+                AudioPlayHead::CurrentPositionInfo result;
 
-        // If the host fails to provide the current time, we'll just reset our copy to a default..
-        lastPosInfo.resetToDefault();
+                if (ph->getCurrentPosition (result))
+                    return result;
+            }
+
+            // If the host fails to provide the current time, we'll just use default values
+            AudioPlayHead::CurrentPositionInfo result;
+            result.resetToDefault();
+            return result;
+        }();
+
+        lastPosInfo.set (newInfo);
     }
 
     static BusesProperties getBusesProperties()