a/android: Monitor whether MonadoView is still being used by native code.

src/xrt/auxiliary/android/android_custom_surface.cpp

@@ -27,12 +27,27 @@ using wrap::android::view::SurfaceHolder;
 
 struct android_custom_surface
 {
-	Activity activity;
-	jni::Class monadoViewClass;
-	jni::Object monadoView;
-	jni::method_t waitGetSurfaceHolderMethod;
+	~android_custom_surface();
+	Activity activity{};
+	jni::Class monadoViewClass{};
+	jni::Object monadoView{};
+	jni::method_t waitGetSurfaceHolderMethod = nullptr;
+	jni::method_t markAsDiscardedMethod = nullptr;
 };
 
+android_custom_surface::~android_custom_surface()
+{
+	if (!monadoView.isNull() && markAsDiscardedMethod != nullptr) {
+		try {
+			monadoView.call<void>(markAsDiscardedMethod);
+		} catch (std::exception const &e) {
+			U_LOG_E(
+			    "Failure while marking MonadoView as discarded: %s",
+			    e.what());
+		}
+	}
+}
+
 constexpr auto FULLY_QUALIFIED_CLASSNAME =
     "org.freedesktop.monado.auxiliary.MonadoView";
 
@@ -89,6 +104,8 @@ android_custom_surface_async_start(struct _JavaVM *vm, void *activity)
 		    ret->monadoViewClass.getMethod(
 		        "waitGetSurfaceHolder",
 		        "(I)Landroid/view/SurfaceHolder;");
+		ret->markAsDiscardedMethod = ret->monadoViewClass.getMethod(
+		    "markAsDiscardedByNative", "()V;");
 
 		attachToActivity = ret->monadoViewClass.getStaticMethod(
 		    "attachToActivity",

src/xrt/auxiliary/android/android_custom_surface.h

@@ -55,7 +55,8 @@ android_custom_surface_async_start(struct _JavaVM *vm, void *activity);
  * Destroy the native handle for the custom surface.
  *
  * Depending on the state, this may not necessarily destroy the underlying
- * surface, if other references exist.
+ * surface, if other references exist. However, a flag will be set to indicate
+ * that native code is done using it.
  *
  * @param ptr_custom_surface Pointer to the opaque pointer: will be set to NULL.
  *

src/xrt/auxiliary/android/src/main/java/org/freedesktop/monado/auxiliary/MonadoView.java

@@ -43,13 +43,18 @@ public class MonadoView extends SurfaceView implements SurfaceHolder.Callback, S
 
     /// Guards currentSurfaceHolder
     private final Object currentSurfaceHolderSync = new Object();
+    /// Guards the usedByNativeCode flag
+    private final Object usedByNativeCodeSync = new Object();
     private final Method viewSetSysUiVis;
+
     public int width = -1;
     public int height = -1;
     public int format = -1;
 
     /// Guarded by currentSurfaceHolderSync
     private SurfaceHolder currentSurfaceHolder = null;
+    /// Guarded by usedByNativeCodeSync
+    private boolean usedByNativeCode = false;
 
     private MonadoView(Activity activity) {
         super(activity);
@@ -124,9 +129,34 @@ public class MonadoView extends SurfaceView implements SurfaceHolder.Callback, S
                 }
             }
         }
+        if (ret != null) {
+            synchronized (usedByNativeCodeSync) {
+                usedByNativeCode = true;
+                usedByNativeCodeSync.notifyAll();
+            }
+        }
         return ret;
     }
 
+    /**
+     * Change the flag and notify those waiting on it, to indicate that native code is done with
+     * this object.
+     * <p>
+     * Called by native code!
+     */
+    @Keep
+    public void markAsDiscardedByNative() {
+
+        synchronized (usedByNativeCodeSync) {
+            if (!usedByNativeCode) {
+                Log.w(TAG, "This should not have happened: Discarding by native code, but not marked as used!");
+            }
+            usedByNativeCode = false;
+            usedByNativeCodeSync.notifyAll();
+        }
+
+    }
+
     private boolean makeFullscreen() {
         if (activity == null) {
             return false;
@@ -190,11 +220,26 @@ public class MonadoView extends SurfaceView implements SurfaceHolder.Callback, S
 
     @Override
     public void surfaceDestroyed(@NonNull SurfaceHolder surfaceHolder) {
-        //! @todo this function should block until the surface is no longer used in the native code.
         Log.i(TAG, "surfaceDestroyed: Lost our surface.");
+        boolean lost = false;
         synchronized (currentSurfaceHolderSync) {
             if (surfaceHolder == currentSurfaceHolder) {
                 currentSurfaceHolder = null;
+                lost = true;
+            }
+        }
+        if (lost) {
+            //! @todo this function should notify native code that the surface is gone.
+            try {
+                synchronized (usedByNativeCodeSync) {
+                    while (usedByNativeCode) {
+                        usedByNativeCodeSync.wait();
+                    }
+                }
+            } catch (InterruptedException e) {
+                e.printStackTrace();
+                Log.i(TAG,
+                        "Interrupted in surfaceDestroyed while waiting for native code to finish up: " + e.toString());
             }
         }
     }