Handle !resetPosition as initial seek position.

The flag in ExoPlayer.prepare is documented as keeping the current window
index and window position. We are currently using the current period UID and
period position instead. This causes problems when the media source is changed
but the position is not reset.

Using the initial seek position instead ensures we actually use the window
index and position.

Issue:#5520
PiperOrigin-RevId: 236101024

RELEASENOTES.md

@@ -64,6 +64,9 @@
   ([#5179](https://github.com/google/ExoPlayer/issues/5179)).
 * Fix issue with `TimelineQueueNavigator` not publishing the queue in shuffled
   order when in shuffle mode.
+* Fix issue where not resetting the position for a new `MediaSource` in calls to
+  `ExoPlayer.prepare` causes an `IndexOutOfBoundsException`
+  ([#5520](https://github.com/google/ExoPlayer/issues/5520)).
 
 ### 2.9.6 ###
 

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

@@ -436,6 +436,12 @@ import java.util.concurrent.atomic.AtomicBoolean;
 
   private void prepareInternal(MediaSource mediaSource, boolean resetPosition, boolean resetState) {
     pendingPrepareCount++;
+    if (!resetPosition && pendingInitialSeekPosition == null && !playbackInfo.timeline.isEmpty()) {
+      playbackInfo.timeline.getPeriodByUid(playbackInfo.periodId.periodUid, period);
+      long windowPositionUs = playbackInfo.positionUs + period.getPositionInWindowUs();
+      pendingInitialSeekPosition =
+          new SeekPosition(Timeline.EMPTY, period.windowIndex, windowPositionUs);
+    }
     resetInternal(
         /* resetRenderers= */ false, /* releaseMediaSource= */ true, resetPosition, resetState);
     loadControl.onPrepared();

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

@@ -1124,6 +1124,45 @@ public final class ExoPlayerTest {
     testRunner.assertPlayedPeriodIndices(0, 1);
   }
 
+  @Test
+  public void testReprepareAndKeepPositionWithNewMediaSource() throws Exception {
+    Timeline timeline =
+        new FakeTimeline(
+            new TimelineWindowDefinition(/* periodCount= */ 1, /* id= */ new Object()));
+    Timeline secondTimeline =
+        new FakeTimeline(
+            new TimelineWindowDefinition(/* periodCount= */ 1, /* id= */ new Object()));
+    MediaSource secondSource = new FakeMediaSource(secondTimeline, /* manifest= */ null);
+    AtomicLong positionAfterReprepare = new AtomicLong();
+    ActionSchedule actionSchedule =
+        new ActionSchedule.Builder("testReprepareAndKeepPositionWithNewMediaSource")
+            .pause()
+            .waitForPlaybackState(Player.STATE_READY)
+            .playUntilPosition(/* windowIndex= */ 0, /* positionMs= */ 2000)
+            .prepareSource(secondSource, /* resetPosition= */ false, /* resetState= */ true)
+            .waitForTimelineChanged(secondTimeline)
+            .executeRunnable(
+                new PlayerRunnable() {
+                  @Override
+                  public void run(SimpleExoPlayer player) {
+                    positionAfterReprepare.set(player.getCurrentPosition());
+                  }
+                })
+            .play()
+            .build();
+    ExoPlayerTestRunner testRunner =
+        new ExoPlayerTestRunner.Builder()
+            .setTimeline(timeline)
+            .setActionSchedule(actionSchedule)
+            .build(context)
+            .start()
+            .blockUntilActionScheduleFinished(TIMEOUT_MS)
+            .blockUntilEnded(TIMEOUT_MS);
+
+    testRunner.assertTimelinesEqual(timeline, Timeline.EMPTY, secondTimeline);
+    assertThat(positionAfterReprepare.get()).isAtLeast(2000L);
+  }
+
   @Test
   public void testStopDuringPreparationOverwritesPreparation() throws Exception {
     Timeline timeline = new FakeTimeline(/* windowCount= */ 1);