Suppress discontinuities that don't change the position

This is mostly useful for suppressing the initial position
discontinuity reported by ClippingMediaPeriod.

-------------
Created by MOE: https://github.com/google/moe
MOE_MIGRATED_REVID=176758972

library/core/src/androidTest/java/com/google/android/exoplayer2/ExoPlayerTest.java

@@ -343,12 +343,9 @@ public final class ExoPlayerTest extends TestCase {
       @Override
       protected FakeMediaPeriod createFakeMediaPeriod(MediaPeriodId id,
           TrackGroupArray trackGroupArray, Allocator allocator) {
-        return new FakeMediaPeriod(trackGroupArray) {
-          @Override
-          public long seekToUs(long positionUs) {
-            return positionUs + 10; // Adjusts the requested seek position.
-          }
-        };
+        FakeMediaPeriod mediaPeriod = new FakeMediaPeriod(trackGroupArray);
+        mediaPeriod.setSeekToUsOffset(10);
+        return mediaPeriod;
       }
     };
     ActionSchedule actionSchedule = new ActionSchedule.Builder("testSeekDiscontinuityAdjust")
@@ -359,32 +356,39 @@ public final class ExoPlayerTest extends TestCase {
         Player.DISCONTINUITY_REASON_SEEK_ADJUSTMENT);
   }
 
-  public void testInternalDiscontinuity() throws Exception {
+  public void testInternalDiscontinuityAtNewPosition() throws Exception {
     FakeTimeline timeline = new FakeTimeline(1);
     FakeMediaSource mediaSource = new FakeMediaSource(timeline, null, Builder.VIDEO_FORMAT) {
       @Override
       protected FakeMediaPeriod createFakeMediaPeriod(MediaPeriodId id,
           TrackGroupArray trackGroupArray, Allocator allocator) {
-        return new FakeMediaPeriod(trackGroupArray) {
-          boolean discontinuityRead;
-          @Override
-          public long readDiscontinuity() {
-            if (!discontinuityRead) {
-              discontinuityRead = true;
-              return 10; // Return a discontinuity.
-            }
-            return C.TIME_UNSET;
-          }
-        };
+        FakeMediaPeriod mediaPeriod = new FakeMediaPeriod(trackGroupArray);
+        mediaPeriod.setDiscontinuityPositionUs(10);
+        return mediaPeriod;
       }
     };
-    ActionSchedule actionSchedule = new ActionSchedule.Builder("testInternalDiscontinuity")
-        .waitForPlaybackState(Player.STATE_READY).build();
     ExoPlayerTestRunner testRunner = new ExoPlayerTestRunner.Builder().setMediaSource(mediaSource)
-        .setActionSchedule(actionSchedule).build().start().blockUntilEnded(TIMEOUT_MS);
+        .build().start().blockUntilEnded(TIMEOUT_MS);
     testRunner.assertPositionDiscontinuityReasonsEqual(Player.DISCONTINUITY_REASON_INTERNAL);
   }
 
+  public void testInternalDiscontinuityAtInitialPosition() throws Exception {
+    FakeTimeline timeline = new FakeTimeline(1);
+    FakeMediaSource mediaSource = new FakeMediaSource(timeline, null, Builder.VIDEO_FORMAT) {
+      @Override
+      protected FakeMediaPeriod createFakeMediaPeriod(MediaPeriodId id,
+          TrackGroupArray trackGroupArray, Allocator allocator) {
+        FakeMediaPeriod mediaPeriod = new FakeMediaPeriod(trackGroupArray);
+        mediaPeriod.setDiscontinuityPositionUs(0);
+        return mediaPeriod;
+      }
+    };
+    ExoPlayerTestRunner testRunner = new ExoPlayerTestRunner.Builder().setMediaSource(mediaSource)
+        .build().start().blockUntilEnded(TIMEOUT_MS);
+    // If the position is unchanged we do not expect the discontinuity to be reported externally.
+    testRunner.assertNoPositionDiscontinuities();
+  }
+
   public void testAllActivatedTrackSelectionAreReleasedForSinglePeriod() throws Exception {
     Timeline timeline = new FakeTimeline(/* windowCount= */ 1);
     MediaSource mediaSource =

library/core/src/main/java/com/google/android/exoplayer2/ExoPlayerImplInternal.java

@@ -509,10 +509,14 @@ import java.io.IOException;
     long periodPositionUs = playingPeriodHolder.mediaPeriod.readDiscontinuity();
     if (periodPositionUs != C.TIME_UNSET) {
       resetRendererPosition(periodPositionUs);
-      playbackInfo = playbackInfo.fromNewPosition(playbackInfo.periodId, periodPositionUs,
-          playbackInfo.contentPositionUs);
-      eventHandler.obtainMessage(MSG_POSITION_DISCONTINUITY, Player.DISCONTINUITY_REASON_INTERNAL,
-          0, playbackInfo).sendToTarget();
+      // A MediaPeriod may report a discontinuity at the current playback position to ensure the
+      // renderers are flushed. Only report the discontinuity externally if the position changed.
+      if (periodPositionUs != playbackInfo.positionUs) {
+        playbackInfo = playbackInfo.fromNewPosition(playbackInfo.periodId, periodPositionUs,
+            playbackInfo.contentPositionUs);
+        eventHandler.obtainMessage(MSG_POSITION_DISCONTINUITY, Player.DISCONTINUITY_REASON_INTERNAL,
+            0, playbackInfo).sendToTarget();
+      }
     } else {
       rendererPositionUs = mediaClock.syncAndGetPositionUs();
       periodPositionUs = playingPeriodHolder.toPeriodTime(rendererPositionUs);

testutils/src/main/java/com/google/android/exoplayer2/testutil/FakeMediaPeriod.java

@@ -33,9 +33,31 @@ public class FakeMediaPeriod implements MediaPeriod {
   private final TrackGroupArray trackGroupArray;
 
   private boolean preparedPeriod;
+  private long seekOffsetUs;
+  private long discontinuityPositionUs;
 
   public FakeMediaPeriod(TrackGroupArray trackGroupArray) {
     this.trackGroupArray = trackGroupArray;
+    discontinuityPositionUs = C.TIME_UNSET;
+  }
+
+  /**
+   * Sets a discontinuity position to be returned from the next call to
+   * {@link #readDiscontinuity()}.
+   *
+   * @param discontinuityPositionUs The position to be returned, in microseconds.
+   */
+  public void setDiscontinuityPositionUs(long discontinuityPositionUs) {
+    this.discontinuityPositionUs = discontinuityPositionUs;
+  }
+
+  /**
+   * Sets an offset to be applied to positions returned by {@link #seekToUs(long)}.
+   *
+   * @param seekOffsetUs The offset to be applied, in microseconds.
+   */
+  public void setSeekToUsOffset(long seekOffsetUs) {
+    this.seekOffsetUs = seekOffsetUs;
   }
 
   public void release() {
@@ -92,7 +114,9 @@ public class FakeMediaPeriod implements MediaPeriod {
   @Override
   public long readDiscontinuity() {
     Assert.assertTrue(preparedPeriod);
-    return C.TIME_UNSET;
+    long positionDiscontinuityUs = this.discontinuityPositionUs;
+    this.discontinuityPositionUs = C.TIME_UNSET;
+    return positionDiscontinuityUs;
   }
 
   @Override
@@ -104,7 +128,7 @@ public class FakeMediaPeriod implements MediaPeriod {
   @Override
   public long seekToUs(long positionUs) {
     Assert.assertTrue(preparedPeriod);
-    return positionUs;
+    return positionUs + seekOffsetUs;
   }
 
   @Override