AudioPluginHost: Allow out-of-process scanner to scan AUv3 plugins

extras/AudioPluginHost/Source/HostStartup.cpp

@@ -35,22 +35,23 @@ class PluginScannerSubprocess : private ChildProcessWorker,
                                 private AsyncUpdater
 {
 public:
-    PluginScannerSubprocess()
-    {
-        formatManager.addDefaultFormats();
-    }
-
     using ChildProcessWorker::initialiseFromCommandLine;
 
 private:
     void handleMessageFromCoordinator (const MemoryBlock& mb) override
     {
+        if (mb.isEmpty())
+            return;
+
+        if (! doScan (mb))
         {
-            const std::lock_guard<std::mutex> lock (mutex);
-            pendingBlocks.emplace (mb);
-        }
+            {
+                const std::lock_guard<std::mutex> lock (mutex);
+                pendingBlocks.emplace (mb);
+            }
 
-        triggerAsyncUpdate();
+            triggerAsyncUpdate();
+        }
     }
 
     void handleConnectionLost() override
@@ -58,7 +59,6 @@ private:
         JUCEApplicationBase::quit();
     }
 
-    // It's important to run the plugin scan on the main thread!
     void handleAsyncUpdate() override
     {
         for (;;)
@@ -78,27 +78,55 @@ private:
             if (block.isEmpty())
                 return;
 
-            MemoryInputStream stream { block, false };
-            const auto formatName = stream.readString();
-            const auto identifier = stream.readString();
+            doScan (block);
+        }
+    }
+
+    bool doScan (const MemoryBlock& block)
+    {
+        AudioPluginFormatManager formatManager;
+        formatManager.addDefaultFormats();
+
+        MemoryInputStream stream { block, false };
+        const auto formatName = stream.readString();
+        const auto identifier = stream.readString();
 
-            OwnedArray<PluginDescription> results;
+        PluginDescription pd;
+        pd.fileOrIdentifier = identifier;
+        pd.uniqueId = pd.deprecatedUid = 0;
 
+        const auto matchingFormat = [&]() -> AudioPluginFormat*
+        {
             for (auto* format : formatManager.getFormats())
                 if (format->getName() == formatName)
-                    format->findAllTypesForFile (results, identifier);
-
-            XmlElement xml ("LIST");
+                    return format;
 
-            for (const auto& desc : results)
-                xml.addChildElement (desc->createXml().release());
+            return nullptr;
+        }();
 
-            const auto str = xml.toString();
-            sendMessageToCoordinator ({ str.toRawUTF8(), str.getNumBytesAsUTF8() });
+        if (matchingFormat == nullptr
+            || (! MessageManager::getInstance()->isThisTheMessageThread()
+                && ! matchingFormat->requiresUnblockedMessageThreadDuringCreation (pd)))
+        {
+            return false;
         }
+
+        OwnedArray<PluginDescription> results;
+        matchingFormat->findAllTypesForFile (results, identifier);
+        sendPluginDescriptions (results);
+        return true;
     }
 
-    AudioPluginFormatManager formatManager;
+    void sendPluginDescriptions (const OwnedArray<PluginDescription>& results)
+    {
+        XmlElement xml ("LIST");
+
+        for (const auto& desc : results)
+            xml.addChildElement (desc->createXml().release());
+
+        const auto str = xml.toString();
+        sendMessageToCoordinator ({ str.toRawUTF8(), str.getNumBytesAsUTF8() });
+    }
 
     std::mutex mutex;
     std::queue<MemoryBlock> pendingBlocks;

modules/juce_audio_processors/format/juce_AudioPluginFormat.h

@@ -137,6 +137,9 @@ public:
     */
     virtual FileSearchPath getDefaultLocationsToSearch() = 0;
 
+    /** Returns true if instantiation of this plugin type must be done from a non-message thread. */
+    virtual bool requiresUnblockedMessageThreadDuringCreation (const PluginDescription&) const = 0;
+
 protected:
     //==============================================================================
     friend class AudioPluginFormatManager;
@@ -149,9 +152,6 @@ protected:
     virtual void createPluginInstance (const PluginDescription&, double initialSampleRate,
                                        int initialBufferSize, PluginCreationCallback) = 0;
 
-    /** Returns true if instantiation of this plugin type must be done from a non-message thread. */
-    virtual bool requiresUnblockedMessageThreadDuringCreation (const PluginDescription&) const = 0;
-
 private:
     struct AsyncCreateMessage;
     void handleMessage (const Message&) override;

modules/juce_events/interprocess/juce_ConnectedChildProcess.cpp

@@ -204,6 +204,7 @@ struct ChildProcessWorker::Connection  : public InterprocessConnection,
     ~Connection() override
     {
         stopThread (10000);
+        disconnect();
     }
 
 private: