Ensure AssetLoader callbacks are always called in the right order

Before, it was possible for onDurationUs() and onAllTracksRegistered()
to be called before onTrackAdded() because they are called from
different threads. onDurationUs() and onAllTracksRegistered() are called
from the Transformer internal thread, and onTrackAdded() is called from
the playback thread.

PiperOrigin-RevId: 497102556

library/transformer/src/main/java/com/google/android/exoplayer2/transformer/ExoPlayerAssetLoader.java

@@ -235,7 +235,6 @@ public final class ExoPlayerAssetLoader implements AssetLoader {
   public void start() {
     player.setMediaItem(mediaItem);
     player.prepare();
-    player.play();
     progressState = PROGRESS_STATE_WAITING_FOR_AVAILABILITY;
   }
 
@@ -355,9 +354,13 @@ public final class ExoPlayerAssetLoader implements AssetLoader {
                 "The asset loader has no track to output.",
                 /* cause= */ null,
                 ERROR_CODE_FAILED_RUNTIME_CHECK));
+        return;
       } else {
         assetLoaderListener.onTrackCount(trackCount);
       }
+      // Start the renderers after having registered all the tracks to make sure the AssetLoader
+      // listener callbacks are called in the right order.
+      player.play();
     }
 
     @Override

library/transformer/src/test/java/com/google/android/exoplayer2/transformer/ExoPlayerAssetLoaderTest.java

@@ -0,0 +1,149 @@
+/*
+ * Copyright 2022 The Android Open Source Project
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *      http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+package com.google.android.exoplayer2.transformer;
+
+import static com.google.android.exoplayer2.robolectric.RobolectricUtil.runLooperUntil;
+import static com.google.common.truth.Truth.assertThat;
+
+import android.content.Context;
+import android.os.Handler;
+import android.os.HandlerThread;
+import android.os.Looper;
+import androidx.annotation.Nullable;
+import androidx.test.core.app.ApplicationProvider;
+import androidx.test.ext.junit.runners.AndroidJUnit4;
+import com.google.android.exoplayer2.Format;
+import com.google.android.exoplayer2.MediaItem;
+import com.google.android.exoplayer2.decoder.DecoderInputBuffer;
+import com.google.android.exoplayer2.util.Clock;
+import java.time.Duration;
+import java.util.concurrent.atomic.AtomicBoolean;
+import java.util.concurrent.atomic.AtomicReference;
+import org.junit.Test;
+import org.junit.runner.RunWith;
+import org.robolectric.shadows.ShadowSystemClock;
+
+/** Unit tests for {@link ExoPlayerAssetLoader}. */
+@RunWith(AndroidJUnit4.class)
+public class ExoPlayerAssetLoaderTest {
+
+  @Test
+  public void exoPlayerAssetLoader_callsListenerCallbacksInRightOrder() throws Exception {
+    HandlerThread assetLoaderThread = new HandlerThread("AssetLoaderThread");
+    assetLoaderThread.start();
+    Looper assetLoaderLooper = assetLoaderThread.getLooper();
+    AtomicReference<Exception> exceptionRef = new AtomicReference<>();
+    AtomicBoolean isTrackAdded = new AtomicBoolean();
+    AssetLoader.Listener listener =
+        new AssetLoader.Listener() {
+
+          private volatile boolean isDurationSet;
+          private volatile boolean isTrackCountSet;
+
+          @Override
+          public void onDurationUs(long durationUs) {
+            // Sleep to increase the chances of the test failing.
+            sleep();
+            isDurationSet = true;
+          }
+
+          @Override
+          public void onTrackCount(int trackCount) {
+            // Sleep to increase the chances of the test failing.
+            sleep();
+            isTrackCountSet = true;
+          }
+
+          @Override
+          public SamplePipeline.Input onTrackAdded(
+              Format format,
+              @AssetLoader.SupportedOutputTypes int supportedOutputTypes,
+              long streamStartPositionUs,
+              long streamOffsetUs) {
+            if (!isDurationSet) {
+              exceptionRef.set(
+                  new IllegalStateException("onTrackAdded() called before onDurationUs()"));
+            } else if (!isTrackCountSet) {
+              exceptionRef.set(
+                  new IllegalStateException("onTrackAdded() called before onTrackCount()"));
+            }
+            isTrackAdded.set(true);
+            return new FakeSamplePipelineInput();
+          }
+
+          @Override
+          public void onError(Exception e) {
+            exceptionRef.set(e);
+          }
+
+          private void sleep() {
+            try {
+              Thread.sleep(10);
+            } catch (InterruptedException e) {
+              exceptionRef.set(e);
+            }
+          }
+        };
+    // Use default clock so that messages sent on different threads are not always executed in the
+    // order in which they are received.
+    Clock clock = Clock.DEFAULT;
+    AssetLoader assetLoader = getAssetLoader(assetLoaderLooper, listener, clock);
+
+    new Handler(assetLoaderLooper).post(assetLoader::start);
+    runLooperUntil(
+        Looper.myLooper(),
+        () -> {
+          ShadowSystemClock.advanceBy(Duration.ofMillis(10));
+          return isTrackAdded.get() || exceptionRef.get() != null;
+        });
+
+    assertThat(exceptionRef.get()).isNull();
+  }
+
+  private static AssetLoader getAssetLoader(
+      Looper looper, AssetLoader.Listener listener, Clock clock) {
+    Context context = ApplicationProvider.getApplicationContext();
+    MediaItem mediaItem = MediaItem.fromUri("asset:///media/mp4/sample.mp4");
+    return new ExoPlayerAssetLoader.Factory()
+        .setContext(context)
+        .setMediaItem(mediaItem)
+        .setRemoveAudio(false)
+        .setRemoveVideo(false)
+        .setFlattenVideoForSlowMotion(false)
+        .setDecoderFactory(new DefaultDecoderFactory(context))
+        .setLooper(looper)
+        .setListener(listener)
+        .setClock(clock)
+        .createAssetLoader();
+  }
+
+  private static final class FakeSamplePipelineInput implements SamplePipeline.Input {
+
+    @Override
+    public boolean expectsDecodedData() {
+      return false;
+    }
+
+    @Nullable
+    @Override
+    public DecoderInputBuffer dequeueInputBuffer() {
+      return null;
+    }
+
+    @Override
+    public void queueInputBuffer() {}
+  }
+}