Relax requirement that MergingMediaSource children are not dynamic

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=176664332

library/core/src/androidTest/java/com/google/android/exoplayer2/source/MergingMediaSourceTest.java

@@ -0,0 +1,80 @@
+/*
+ * Copyright (C) 2017 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.source;
+
+import com.google.android.exoplayer2.C;
+import com.google.android.exoplayer2.Timeline;
+import com.google.android.exoplayer2.source.MergingMediaSource.IllegalMergeException;
+import com.google.android.exoplayer2.testutil.FakeMediaSource;
+import com.google.android.exoplayer2.testutil.FakeTimeline;
+import com.google.android.exoplayer2.testutil.FakeTimeline.TimelineWindowDefinition;
+import com.google.android.exoplayer2.testutil.MediaSourceTestRunner;
+import java.io.IOException;
+import junit.framework.TestCase;
+
+/**
+ * Unit tests for {@link MergingMediaSource}.
+ */
+public class MergingMediaSourceTest extends TestCase {
+
+  public void testMergingDynamicTimelines() throws IOException {
+    FakeTimeline firstTimeline = new FakeTimeline(
+        new TimelineWindowDefinition(true, true, C.TIME_UNSET));
+    FakeTimeline secondTimeline = new FakeTimeline(
+        new TimelineWindowDefinition(true, true, C.TIME_UNSET));
+    testMergingMediaSourcePrepare(firstTimeline, secondTimeline);
+  }
+
+  public void testMergingStaticTimelines() throws IOException {
+    FakeTimeline firstTimeline = new FakeTimeline(
+        new TimelineWindowDefinition(true, false, 20));
+    FakeTimeline secondTimeline = new FakeTimeline(
+        new TimelineWindowDefinition(true, false, 10));
+    testMergingMediaSourcePrepare(firstTimeline, secondTimeline);
+  }
+
+  public void testMergingTimelinesWithDifferentPeriodCounts() throws IOException {
+    FakeTimeline firstTimeline = new FakeTimeline(new TimelineWindowDefinition(1, null));
+    FakeTimeline secondTimeline = new FakeTimeline(new TimelineWindowDefinition(2, null));
+    try {
+      testMergingMediaSourcePrepare(firstTimeline, secondTimeline);
+      fail("Expected merging to fail.");
+    } catch (IllegalMergeException e) {
+      assertEquals(IllegalMergeException.REASON_PERIOD_COUNT_MISMATCH, e.reason);
+    }
+  }
+
+  /**
+   * Wraps the specified timelines in a {@link MergingMediaSource}, prepares it and checks that it
+   * forwards the first of the wrapped timelines.
+   */
+  private static void testMergingMediaSourcePrepare(Timeline... timelines) throws IOException {
+    MediaSource[] mediaSources = new MediaSource[timelines.length];
+    for (int i = 0; i < timelines.length; i++) {
+      mediaSources[i] = new FakeMediaSource(timelines[i], null);
+    }
+    MergingMediaSource mediaSource = new MergingMediaSource(mediaSources);
+    MediaSourceTestRunner testRunner = new MediaSourceTestRunner(mediaSource, null);
+    try {
+      Timeline timeline = testRunner.prepareSource();
+      // The merged timeline should always be the one from the first child.
+      assertEquals(timelines[0], timeline);
+    } finally {
+      testRunner.release();
+    }
+  }
+
+}

library/core/src/main/java/com/google/android/exoplayer2/source/MergingMediaSource.java

@@ -28,8 +28,7 @@ import java.util.Arrays;
 /**
  * Merges multiple {@link MediaSource}s.
  * <p>
- * The {@link Timeline}s of the sources being merged must have the same number of periods, and must
+ * The {@link Timeline}s of the sources being merged must have the same number of periods.
- * not have any dynamic windows.
  */
 public final class MergingMediaSource implements MediaSource {
 
@@ -42,26 +41,20 @@ public final class MergingMediaSource implements MediaSource {
      * The reason the merge failed.
      */
     @Retention(RetentionPolicy.SOURCE)
-    @IntDef({REASON_WINDOWS_ARE_DYNAMIC, REASON_PERIOD_COUNT_MISMATCH})
+    @IntDef({REASON_PERIOD_COUNT_MISMATCH})
     public @interface Reason {}
-    /**
-     * One of the sources being merged has a dynamic window.
-     */
-    public static final int REASON_WINDOWS_ARE_DYNAMIC = 0;
     /**
      * The sources have different period counts.
      */
-    public static final int REASON_PERIOD_COUNT_MISMATCH = 1;
+    public static final int REASON_PERIOD_COUNT_MISMATCH = 0;
 
     /**
-     * The reason the merge failed. One of {@link #REASON_WINDOWS_ARE_DYNAMIC} and
+     * The reason the merge failed.
-     * {@link #REASON_PERIOD_COUNT_MISMATCH}.
      */
     @Reason public final int reason;
 
     /**
-     * @param reason The reason the merge failed. One of {@link #REASON_WINDOWS_ARE_DYNAMIC} and
+     * @param reason The reason the merge failed.
-     *     {@link #REASON_PERIOD_COUNT_MISMATCH}.
      */
     public IllegalMergeException(@Reason int reason) {
       this.reason = reason;
@@ -73,7 +66,6 @@ public final class MergingMediaSource implements MediaSource {
 
   private final MediaSource[] mediaSources;
   private final ArrayList<MediaSource> pendingTimelineSources;
-  private final Timeline.Window window;
   private final CompositeSequenceableLoaderFactory compositeSequenceableLoaderFactory;
 
   private Listener listener;
@@ -100,7 +92,6 @@ public final class MergingMediaSource implements MediaSource {
     this.mediaSources = mediaSources;
     this.compositeSequenceableLoaderFactory = compositeSequenceableLoaderFactory;
     pendingTimelineSources = new ArrayList<>(Arrays.asList(mediaSources));
-    window = new Timeline.Window();
     periodCount = PERIOD_COUNT_UNSET;
   }
 
@@ -170,12 +161,6 @@ public final class MergingMediaSource implements MediaSource {
   }
 
   private IllegalMergeException checkTimelineMerges(Timeline timeline) {
-    int windowCount = timeline.getWindowCount();
-    for (int i = 0; i < windowCount; i++) {
-      if (timeline.getWindow(i, window, false).isDynamic) {
-        return new IllegalMergeException(IllegalMergeException.REASON_WINDOWS_ARE_DYNAMIC);
-      }
-    }
     if (periodCount == PERIOD_COUNT_UNSET) {
       periodCount = timeline.getPeriodCount();
     } else if (timeline.getPeriodCount() != periodCount) {