Avoid sending messages of destroyed Engine

And app can call notifyColorsChanged() on a destroyed engine, which
would cause problems, given it doesn't have a surface anymore.

The same is true to visibility notification, where the engine can be
destroyed before it had the opportuinity of drawing its first frame (if
it had to load large assets for example)

Test: manually change wallpapers
Test: WallpaperServiceTest (both CTS and internal)
Fixes: 291053623
Change-Id: Iff83603695cff491f2045586d5b3f4658f7ae55a

core/java/android/service/wallpaper/WallpaperService.java

@@ -868,6 +868,11 @@ public abstract class WallpaperService extends Service {
          * This will trigger a {@link #onComputeColors()} call.
          */
         public void notifyColorsChanged() {
+            if (mDestroyed) {
+                Log.i(TAG, "Ignoring notifyColorsChanged(), Engine has already been destroyed.");
+                return;
+            }
+
             final long now = mClockFunction.get();
             if (now - mLastColorInvalidation < NOTIFY_COLORS_RATE_LIMIT_MS) {
                 Log.w(TAG, "This call has been deferred. You should only call "
@@ -2226,7 +2231,11 @@ public abstract class WallpaperService extends Service {
             }
         }
 
-        void detach() {
+        /**
+         * @hide
+         */
+        @VisibleForTesting
+        public void detach() {
             if (mDestroyed) {
                 return;
             }
@@ -2442,6 +2451,14 @@ public abstract class WallpaperService extends Service {
         }
 
         public void reportShown() {
+            if (mEngine == null) {
+                Log.i(TAG, "Can't report null engine as shown.");
+                return;
+            }
+            if (mEngine.mDestroyed) {
+                Log.i(TAG, "Engine was destroyed before we could draw.");
+                return;
+            }
             if (!mShownReported) {
                 mShownReported = true;
                 Trace.beginSection("WPMS.mConnection.engineShown");

tests/Internal/src/android/service/wallpaper/OWNERS

@@ -0,0 +1,4 @@
+dupin@google.com
+santie@google.com
+pomini@google.com
+poultney@google.com

tests/Internal/src/android/service/wallpaper/WallpaperServiceTest.java

@@ -85,4 +85,17 @@ public class WallpaperServiceTest {
         assertEquals("onAmbientModeChanged should have been called", 2, zoomChangedCount[0]);
     }
 
+    @Test
+    public void testNotifyColorsOfDestroyedEngine_doesntCrash() {
+        WallpaperService service = new WallpaperService() {
+            @Override
+            public Engine onCreateEngine() {
+                return new Engine();
+            }
+        };
+        WallpaperService.Engine engine = service.onCreateEngine();
+        engine.detach();
+
+        engine.notifyColorsChanged();
+    }
 }