Discard backbuffer before playback gets stuck.

If the back buffer is using too much memory, there is a risk
playback could get stuck because LoadControl refuses to load
further data. This eventually results in a stuck-buffering
playback error.

We can detect this case, clear the back buffer and then ask
the LoadControl again to avoid failing playback in such a case.

PiperOrigin-RevId: 472679797

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

@@ -167,6 +167,11 @@ import java.util.concurrent.atomic.AtomicBoolean;
    * to load it.
    */
   private static final long PLAYBACK_STUCK_AFTER_MS = 4000;
+  /**
+   * Threshold under which a buffered duration is assumed to be empty. We cannot use zero to account
+   * for buffers currently hold but not played by the renderer.
+   */
+  private static final long PLAYBACK_BUFFER_EMPTY_THRESHOLD_US = 500_000;
 
   private final Renderer[] renderers;
   private final Set<Renderer> renderersToReset;
@@ -1050,7 +1055,7 @@ import java.util.concurrent.atomic.AtomicBoolean;
         }
       }
       if (!playbackInfo.isLoading
-          && playbackInfo.totalBufferedDurationUs < 500_000
+          && playbackInfo.totalBufferedDurationUs < PLAYBACK_BUFFER_EMPTY_THRESHOLD_US
           && isLoadingPossible()) {
         // The renderers are not ready, there is more media available to load, and the LoadControl
         // is refusing to load it (indicated by !playbackInfo.isLoading). This could be because the
@@ -2306,8 +2311,23 @@ import java.util.concurrent.atomic.AtomicBoolean;
             ? loadingPeriodHolder.toPeriodTime(rendererPositionUs)
             : loadingPeriodHolder.toPeriodTime(rendererPositionUs)
                 - loadingPeriodHolder.info.startPositionUs;
-    return loadControl.shouldContinueLoading(
-        playbackPositionUs, bufferedDurationUs, mediaClock.getPlaybackParameters().speed);
+    boolean shouldContinueLoading =
+        loadControl.shouldContinueLoading(
+            playbackPositionUs, bufferedDurationUs, mediaClock.getPlaybackParameters().speed);
+    if (!shouldContinueLoading
+        && bufferedDurationUs < PLAYBACK_BUFFER_EMPTY_THRESHOLD_US
+        && (backBufferDurationUs > 0 || retainBackBufferFromKeyframe)) {
+      // LoadControl doesn't want to continue loading despite no buffered data. Clear back buffer
+      // and try again in case it's blocked on memory usage of the back buffer.
+      queue
+          .getPlayingPeriod()
+          .mediaPeriod
+          .discardBuffer(playbackInfo.positionUs, /* toKeyframe= */ false);
+      shouldContinueLoading =
+          loadControl.shouldContinueLoading(
+              playbackPositionUs, bufferedDurationUs, mediaClock.getPlaybackParameters().speed);
+    }
+    return shouldContinueLoading;
   }
 
   private boolean isLoadingPossible() {

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

@@ -12085,6 +12085,26 @@ public final class ExoPlayerTest {
     verify(listener, atLeast(2)).onDeviceVolumeChanged(anyInt(), anyBoolean());
   }
 
+  @Test
+  public void loadControlBackBuffer_withInsufficientMemoryLimits_stillContinuesPlayback()
+      throws Exception {
+    DefaultLoadControl loadControl =
+        new DefaultLoadControl.Builder()
+            .setTargetBufferBytes(500_000)
+            .setBackBuffer(
+                /* backBufferDurationMs= */ 1_000_000, /* retainBackBufferFromKeyframe= */ true)
+            .build();
+
+    ExoPlayer player = new TestExoPlayerBuilder(context).setLoadControl(loadControl).build();
+    player.setMediaItem(
+        MediaItem.fromUri("asset:///media/mp4/sample_with_increasing_timestamps_360p.mp4"));
+    player.prepare();
+    player.play();
+    runUntilPlaybackState(player, Player.STATE_ENDED);
+
+    // Assert that playing works without getting stuck due to the memory used by the back buffer.
+  }
+
   // Internal methods.
 
   private static ActionSchedule.Builder addSurfaceSwitch(ActionSchedule.Builder builder) {