Fix potential NPE in DefaultPlaybackSessionManager

When handling timeline updates or position discontinuities, we
may remove the current session and later attempt to access it
again if we haven't explicitly cleared the currentSessionId.
This could happen if this currentSessionId has not been marked
as isCreated. Part of this change guards against that gap by
more clearly clearing the currentSessionId in any case.

This whole situation shouldn't happen though because every
situation where we set the currentSessionId should also ensure
that this session is marked as isCreated. One way to reproduce
an issue is to generate data for a not-created placeholder session
and then making this session the current one by matching it
with an event for a different window index. This would hit one
of the early returns in updateSession before the session is
marked as isCreated. To prevent this root cause, we can make
sure to not match such sessions in belongsToSession.
Additionally, the check in updateSessions for the sequence number
should only be used if there actually is a sequence number
to check (a bug that got accidentally introduced by e0191dd)

Issue: #2885
PiperOrigin-RevId: 832263214
(cherry picked from commit 7784e73)

Author: tonihei

RELEASENOTES.md

@@ -11,6 +11,8 @@
         a non-empty media source.
     *   Fix bug where seeking into other media items while in scrubbing mode
         could cause `IllegalStateException`.
+    *   Fix potential `NullPointerException` in `DefaultPlaybackSessionManager`
+        ([#2885](https://github.com/androidx/media/issues/2885)).
 *   CompositionPlayer:
 *   Transformer:
 *   Track Selection:

libraries/exoplayer/src/main/java/androidx/media3/exoplayer/analytics/DefaultPlaybackSessionManager.java

@@ -113,7 +113,8 @@ public synchronized void updateSessions(EventTime eventTime) {
       return;
     }
     if (eventTime.mediaPeriodId != null) {
-      if (eventTime.mediaPeriodId.windowSequenceNumber < getMinWindowSequenceNumber()) {
+      if (eventTime.mediaPeriodId.windowSequenceNumber != C.INDEX_UNSET
+          && eventTime.mediaPeriodId.windowSequenceNumber < getMinWindowSequenceNumber()) {
         // Ignore event because it is part of a past window that has already been finished.
         return;
       }
@@ -184,10 +185,10 @@ public synchronized void updateSessionsWithTimelineChange(EventTime eventTime) {
       if (!session.tryResolvingToNewTimeline(previousTimeline, currentTimeline)
           || session.isFinishedAtEventTime(eventTime)) {
         iterator.remove();
+        if (session.sessionId.equals(currentSessionId)) {
+          clearCurrentSession(session);
+        }
         if (session.isCreated) {
-          if (session.sessionId.equals(currentSessionId)) {
-            clearCurrentSession(session);
-          }
           listener.onSessionFinished(
               eventTime, session.sessionId, /* automaticTransitionToNextPlayback= */ false);
         }
@@ -206,13 +207,13 @@ public synchronized void updateSessionsWithDiscontinuity(
       SessionDescriptor session = iterator.next();
       if (session.isFinishedAtEventTime(eventTime)) {
         iterator.remove();
+        boolean isRemovingCurrentSession = session.sessionId.equals(currentSessionId);
+        if (isRemovingCurrentSession) {
+          clearCurrentSession(session);
+        }
         if (session.isCreated) {
-          boolean isRemovingCurrentSession = session.sessionId.equals(currentSessionId);
           boolean isAutomaticTransition =
               hasAutomaticTransition && isRemovingCurrentSession && session.isActive;
-          if (isRemovingCurrentSession) {
-            clearCurrentSession(session);
-          }
           listener.onSessionFinished(eventTime, session.sessionId, isAutomaticTransition);
         }
       }
@@ -278,7 +279,7 @@ private void updateCurrentSession(EventTime eventTime) {
   }
 
   private void clearCurrentSession(SessionDescriptor currentSession) {
-    if (currentSession.windowSequenceNumber != C.INDEX_UNSET) {
+    if (currentSession.windowSequenceNumber != C.INDEX_UNSET && currentSession.isCreated) {
       lastRemovedCurrentWindowSequenceNumber = currentSession.windowSequenceNumber;
     }
     currentSessionId = null;
@@ -375,7 +376,7 @@ public boolean tryResolvingToNewTimeline(Timeline oldTimeline, Timeline newTimel
 
     public boolean belongsToSession(
         int eventWindowIndex, @Nullable MediaPeriodId eventMediaPeriodId) {
-      if (eventMediaPeriodId == null) {
+      if (eventMediaPeriodId == null || eventMediaPeriodId.windowSequenceNumber == C.INDEX_UNSET) {
         // Events without concrete media period id are for all sessions of the same window.
         return eventWindowIndex == windowIndex;
       }

libraries/exoplayer/src/test/java/androidx/media3/exoplayer/analytics/DefaultPlaybackSessionManagerTest.java

@@ -967,6 +967,45 @@ public void timelineUpdate_toNewMediaWithMediaPeriodId_finishesOtherSessions() {
     inOrder.verifyNoMoreInteractions();
   }
 
+  @Test
+  public void
+      timelineUpdates_withUncreatedSessionDataForPlaceholderSession_createsRealSessionOnly() {
+    Timeline timeline = new FakeTimeline(/* windowCount= */ 2);
+    MediaPeriodId placeholderMediaPeriodIdWindow0 =
+        new MediaPeriodId(
+            timeline.getUidOfPeriod(/* periodIndex= */ 0),
+            /* windowSequenceNumber= */ C.INDEX_UNSET);
+    MediaPeriodId placeholderMediaPeriodIdWindow1 =
+        new MediaPeriodId(
+            timeline.getUidOfPeriod(/* periodIndex= */ 1),
+            /* windowSequenceNumber= */ C.INDEX_UNSET);
+    EventTime placeholderEventTimeWindow0 =
+        createEventTime(timeline, /* windowIndex= */ 0, placeholderMediaPeriodIdWindow0);
+    EventTime emptyTimelineEventTime =
+        createEventTime(Timeline.EMPTY, /* windowIndex= */ 0, /* mediaPeriodId= */ null);
+
+    // Plant data for a placeholder session with unset sequence number using windowIndex = 1
+    sessionManager.getSessionForMediaPeriodId(timeline, placeholderMediaPeriodIdWindow1);
+    // Simulate timeline update to another placeholder id using windowIndex = 0
+    sessionManager.updateSessionsWithTimelineChange(placeholderEventTimeWindow0);
+    // Clear timeline completely to verify state is cleared up correctly
+    sessionManager.updateSessionsWithTimelineChange(emptyTimelineEventTime);
+
+    InOrder inOrder = inOrder(mockListener);
+    ArgumentCaptor<String> sessionId = ArgumentCaptor.forClass(String.class);
+    inOrder
+        .verify(mockListener)
+        .onSessionCreated(eq(placeholderEventTimeWindow0), sessionId.capture());
+    inOrder.verify(mockListener).onSessionActive(placeholderEventTimeWindow0, sessionId.getValue());
+    inOrder
+        .verify(mockListener)
+        .onSessionFinished(
+            emptyTimelineEventTime,
+            sessionId.getValue(),
+            /* automaticTransitionToNextPlayback= */ false);
+    inOrder.verifyNoMoreInteractions();
+  }
+
   @Test
   public void positionDiscontinuity_withinWindow_doesNotFinishSession() {
     Timeline timeline =