Fix the cleanup logic for the Android render thread

On Android the exit logic goes through `Godot#onDestroy()` who attempts to cleanup the engine using the following code:

```
runOnRenderThread {
	GodotLib.ondestroy()
	forceQuit()
}
```

The issue however is that by the time we ran this code, the render thread has already been paused (but not yet destroyed), and thus `GodotLib.ondestroy()` and `forceQuit()` which are scheduled on the render thread are not executed.

To address this, we instead explicitly request the render thread to exit and block until it does. As part of it exit logic, the render thread has been updated to properly destroy and clean the native instance of the Godot engine, resolving the issue.

platform/android/java/lib/src/org/godotengine/godot/Godot.kt

@@ -577,10 +577,7 @@ class Godot(private val context: Context) : SensorEventListener {
 			plugin.onMainDestroy()
 		}
 
-		runOnRenderThread {
-			GodotLib.ondestroy()
-			forceQuit()
-		}
+		renderView?.onActivityDestroyed()
 	}
 
 	/**
@@ -814,11 +811,7 @@ class Godot(private val context: Context) : SensorEventListener {
 		} ?: return false
 	}
 
-	fun onBackPressed(host: GodotHost) {
-		if (host != primaryHost) {
-			return
-		}
-
+	fun onBackPressed() {
 		var shouldQuit = true
 		for (plugin in pluginRegistry.allPlugins) {
 			if (plugin.onMainBackPressed()) {

platform/android/java/lib/src/org/godotengine/godot/GodotActivity.kt

@@ -85,12 +85,8 @@ abstract class GodotActivity : FragmentActivity(), GodotHost {
 	protected open fun getGodotAppLayout() = R.layout.godot_app_layout
 
 	override fun onDestroy() {
-		Log.v(TAG, "Destroying Godot app...")
+		Log.v(TAG, "Destroying GodotActivity $this...")
 		super.onDestroy()
-
-		godotFragment?.let {
-			terminateGodotInstance(it.godot)
-		}
 	}
 
 	override fun onGodotForceQuit(instance: Godot) {

platform/android/java/lib/src/org/godotengine/godot/GodotFragment.java

@@ -187,7 +187,12 @@ public void onCreate(Bundle icicle) {
 		final Activity activity = getActivity();
 		mCurrentIntent = activity.getIntent();
 
-		godot = new Godot(requireContext());
+		if (parentHost != null) {
+			godot = parentHost.getGodot();
+		}
+		if (godot == null) {
+			godot = new Godot(requireContext());
+		}
 		performEngineInitialization();
 		BenchmarkUtils.endBenchmarkMeasure("Startup", "GodotFragment::onCreate");
 	}
@@ -325,7 +330,7 @@ public void onResume() {
 	}
 
 	public void onBackPressed() {
-		godot.onBackPressed(this);
+		godot.onBackPressed();
 	}
 
 	/**

platform/android/java/lib/src/org/godotengine/godot/GodotGLRenderView.java

@@ -42,7 +42,6 @@
 import org.godotengine.godot.xr.regular.RegularFallbackConfigChooser;
 
 import android.annotation.SuppressLint;
-import android.content.Context;
 import android.content.res.AssetManager;
 import android.graphics.Bitmap;
 import android.graphics.BitmapFactory;
@@ -77,7 +76,7 @@
  *   that matches it exactly (with regards to red/green/blue/alpha channels
  *   bit depths). Failure to do so would result in an EGL_BAD_MATCH error.
  */
-public class GodotGLRenderView extends GLSurfaceView implements GodotRenderView {
+class GodotGLRenderView extends GLSurfaceView implements GodotRenderView {
 	private final GodotHost host;
 	private final Godot godot;
 	private final GodotInputHandler inputHandler;
@@ -140,9 +139,14 @@ public void onActivityStarted() {
 		resumeGLThread();
 	}
 
+	@Override
+	public void onActivityDestroyed() {
+		requestRenderThreadExitAndWait();
+	}
+
 	@Override
 	public void onBackPressed() {
-		godot.onBackPressed(host);
+		godot.onBackPressed();
 	}
 
 	@Override

platform/android/java/lib/src/org/godotengine/godot/GodotRenderView.java

@@ -44,6 +44,9 @@ public interface GodotRenderView {
 	 */
 	void startRenderer();
 
+	/**
+	 * Queues a runnable to be run on the rendering thread.
+	 */
 	void queueOnRenderThread(Runnable event);
 
 	void onActivityPaused();
@@ -54,6 +57,8 @@ public interface GodotRenderView {
 
 	void onActivityStarted();
 
+	void onActivityDestroyed();
+
 	void onBackPressed();
 
 	GodotInputHandler getInputHandler();

platform/android/java/lib/src/org/godotengine/godot/GodotVulkanRenderView.java

@@ -50,7 +50,7 @@
 
 import java.io.InputStream;
 
-public class GodotVulkanRenderView extends VkSurfaceView implements GodotRenderView {
+class GodotVulkanRenderView extends VkSurfaceView implements GodotRenderView {
 	private final GodotHost host;
 	private final Godot godot;
 	private final GodotInputHandler mInputHandler;
@@ -118,9 +118,14 @@ public void onActivityResumed() {
 		});
 	}
 
+	@Override
+	public void onActivityDestroyed() {
+		requestRenderThreadExitAndWait();
+	}
+
 	@Override
 	public void onBackPressed() {
-		godot.onBackPressed(host);
+		godot.onBackPressed();
 	}
 
 	@Override

platform/android/java/lib/src/org/godotengine/godot/gl/GLSurfaceView.java

@@ -595,6 +595,15 @@ protected final void pauseGLThread() {
 	protected final void resumeGLThread() {
 		mGLThread.onResume();
 	}
+
+	/**
+	 * Requests the render thread to exit and block until it does.
+	 */
+	protected final void requestRenderThreadExitAndWait() {
+		if (mGLThread != null) {
+			mGLThread.requestExitAndWait();
+		}
+	}
 	// -- GODOT end --
 
 	/**
@@ -783,6 +792,11 @@ public interface Renderer {
 		 * @return true if the buffers should be swapped, false otherwise.
 		 */
 		boolean onDrawFrame(GL10 gl);
+
+		/**
+		 * Invoked when the render thread is in the process of shutting down.
+		 */
+		void onRenderThreadExiting();
 		// -- GODOT end --
 	}
 
@@ -1621,6 +1635,12 @@ private void guardedRun() throws InterruptedException {
 				 * clean-up everything...
 				 */
 				synchronized (sGLThreadManager) {
+					Log.d("GLThread", "Exiting render thread");
+					GLSurfaceView view = mGLSurfaceViewWeakRef.get();
+					if (view != null) {
+						view.mRenderer.onRenderThreadExiting();
+					}
+
 					stopEglSurfaceLocked();
 					stopEglContextLocked();
 				}

platform/android/java/lib/src/org/godotengine/godot/gl/GodotRenderer.java

@@ -34,13 +34,17 @@
 import org.godotengine.godot.plugin.GodotPlugin;
 import org.godotengine.godot.plugin.GodotPluginRegistry;
 
+import android.util.Log;
+
 import javax.microedition.khronos.egl.EGLConfig;
 import javax.microedition.khronos.opengles.GL10;
 
 /**
  * Godot's GL renderer implementation.
  */
 public class GodotRenderer implements GLSurfaceView.Renderer {
+	private final String TAG = GodotRenderer.class.getSimpleName();
+
 	private final GodotPluginRegistry pluginRegistry;
 	private boolean activityJustResumed = false;
 
@@ -62,6 +66,12 @@ public boolean onDrawFrame(GL10 gl) {
 		return swapBuffers;
 	}
 
+	@Override
+	public void onRenderThreadExiting() {
+		Log.d(TAG, "Destroying Godot Engine");
+		GodotLib.ondestroy();
+	}
+
 	public void onSurfaceChanged(GL10 gl, int width, int height) {
 		GodotLib.resize(null, width, height);
 		for (GodotPlugin plugin : pluginRegistry.getAllPlugins()) {

platform/android/java/lib/src/org/godotengine/godot/vulkan/VkRenderer.kt

@@ -31,11 +31,9 @@
 @file:JvmName("VkRenderer")
 package org.godotengine.godot.vulkan
 
+import android.util.Log
 import android.view.Surface
-
-import org.godotengine.godot.Godot
 import org.godotengine.godot.GodotLib
-import org.godotengine.godot.plugin.GodotPlugin
 import org.godotengine.godot.plugin.GodotPluginRegistry
 
 /**
@@ -52,6 +50,11 @@ import org.godotengine.godot.plugin.GodotPluginRegistry
  * @see [VkSurfaceView.startRenderer]
  */
 internal class VkRenderer {
+
+	companion object {
+		private val TAG = VkRenderer::class.java.simpleName
+	}
+
 	private val pluginRegistry: GodotPluginRegistry = GodotPluginRegistry.getPluginRegistry()
 
 	/**
@@ -101,8 +104,10 @@ internal class VkRenderer {
 	}
 
 	/**
-	 * Called when the rendering thread is destroyed and used as signal to tear down the Vulkan logic.
+	 * Invoked when the render thread is in the process of shutting down.
 	 */
-	fun onVkDestroy() {
+	fun onRenderThreadExiting() {
+		Log.d(TAG, "Destroying Godot Engine")
+		GodotLib.ondestroy()
 	}
 }

platform/android/java/lib/src/org/godotengine/godot/vulkan/VkSurfaceView.kt

@@ -113,12 +113,10 @@ open internal class VkSurfaceView(context: Context) : SurfaceView(context), Surf
 	}
 
 	/**
-	 * Tear down the rendering thread.
-	 *
-	 * Must not be called before a [VkRenderer] has been set.
+	 * Requests the render thread to exit and block until it does.
 	 */
-	fun onDestroy() {
-		vkThread.blockingExit()
+	fun requestRenderThreadExitAndWait() {
+		vkThread.requestExitAndWait()
 	}
 
 	override fun surfaceChanged(holder: SurfaceHolder, format: Int, width: Int, height: Int) {

platform/android/java/lib/src/org/godotengine/godot/vulkan/VkThread.kt

@@ -75,6 +75,9 @@ internal class VkThread(private val vkSurfaceView: VkSurfaceView, private val vk
 
 	private fun threadExiting() {
 		lock.withLock {
+			Log.d(TAG, "Exiting render thread")
+			vkRenderer.onRenderThreadExiting()
+
 			exited = true
 			lockCondition.signalAll()
 		}
@@ -93,7 +96,7 @@ internal class VkThread(private val vkSurfaceView: VkSurfaceView, private val vk
 	/**
 	 * Request the thread to exit and block until it's done.
 	 */
-	fun blockingExit() {
+	fun requestExitAndWait() {
 		lock.withLock {
 			shouldExit = true
 			lockCondition.signalAll()
@@ -171,7 +174,6 @@ internal class VkThread(private val vkSurfaceView: VkSurfaceView, private val vk
 					while (true) {
 						// Code path for exiting the thread loop.
 						if (shouldExit) {
-							vkRenderer.onVkDestroy()
 							return
 						}