Call onChildrenChanged to close the legacy subscription error path

When receiving an error from a legacy `MediaBrowserService` after
having successfully subscribed to a given `parentId`, the callback
needs to be asked to load the children again to receive an error
from the service.

Before this change such an error was dropped as a no-op by Media3
without the `MediaBrowser` giving a chance to react on such an error
being sent by the legacy service.

#cherrypick

PiperOrigin-RevId: 686052969

RELEASENOTES.md

@@ -134,6 +134,9 @@
         `MediaBrowserCompat` when connecting to a legacy `MediaBrowserCompat`.
         The service can receive the connection hints passed in as root hints
         with the first call to `onGetRoot()`.
+    *   Fix bug where a `MediaBrowser` connected to a legacy browser service,
+        didn't receive an error sent by the service after the browser has
+        subscribed to a `parentid`.
 *   UI:
     *   Make the stretched/cropped video in
         `PlayerView`-in-Compose-`AndroidView` workaround opt-in, due to issues

libraries/session/src/main/java/androidx/media3/session/MediaBrowserImplLegacy.java

@@ -151,15 +151,16 @@ import org.checkerframework.checker.initialization.qual.UnderInitialization;
     if (browserCompat == null) {
       return Futures.immediateFuture(LibraryResult.ofError(ERROR_SESSION_DISCONNECTED));
     }
+    Bundle options = createOptionsForSubscription(params);
     SettableFuture<LibraryResult<Void>> future = SettableFuture.create();
-    SubscribeCallback callback = new SubscribeCallback(future);
+    SubscribeCallback callback = new SubscribeCallback(parentId, options, future);
     List<SubscribeCallback> list = subscribeCallbacks.get(parentId);
     if (list == null) {
       list = new ArrayList<>();
       subscribeCallbacks.put(parentId, list);
     }
     list.add(callback);
-    browserCompat.subscribe(parentId, createOptions(params), callback);
+    browserCompat.subscribe(parentId, options, callback);
     return future;
   }
 
@@ -199,7 +200,7 @@ import org.checkerframework.checker.initialization.qual.UnderInitialization;
     }
 
     SettableFuture<LibraryResult<ImmutableList<MediaItem>>> future = SettableFuture.create();
-    Bundle options = createOptions(params, page, pageSize);
+    Bundle options = createOptionsWithPagingInfo(params, page, pageSize);
     browserCompat.subscribe(parentId, options, new GetChildrenCallback(future, parentId));
     return future;
   }
@@ -295,7 +296,7 @@ import org.checkerframework.checker.initialization.qual.UnderInitialization;
     }
 
     SettableFuture<LibraryResult<ImmutableList<MediaItem>>> future = SettableFuture.create();
-    Bundle options = createOptions(params, page, pageSize);
+    Bundle options = createOptionsWithPagingInfo(params, page, pageSize);
     options.putInt(MediaBrowserCompat.EXTRA_PAGE, page);
     options.putInt(MediaBrowserCompat.EXTRA_PAGE_SIZE, pageSize);
     browserCompat.search(
@@ -367,12 +368,13 @@ import org.checkerframework.checker.initialization.qual.UnderInitialization;
     return browserCompats.get(extras);
   }
 
-  private static Bundle createOptions(@Nullable LibraryParams params) {
+  private static Bundle createOptionsForSubscription(@Nullable LibraryParams params) {
     return params == null ? new Bundle() : new Bundle(params.extras);
   }
 
-  private static Bundle createOptions(@Nullable LibraryParams params, int page, int pageSize) {
+  private static Bundle createOptionsWithPagingInfo(
-    Bundle options = createOptions(params);
+      @Nullable LibraryParams params, int page, int pageSize) {
+    Bundle options = createOptionsForSubscription(params);
     options.putInt(MediaBrowserCompat.EXTRA_PAGE, page);
     options.putInt(MediaBrowserCompat.EXTRA_PAGE_SIZE, pageSize);
     return options;
@@ -465,26 +467,33 @@ import org.checkerframework.checker.initialization.qual.UnderInitialization;
 
   private class SubscribeCallback extends SubscriptionCallback {
 
+    private final String subscriptionParentId;
+    private final Bundle subscriptionOptions;
     private final SettableFuture<LibraryResult<Void>> future;
 
-    public SubscribeCallback(SettableFuture<LibraryResult<Void>> future) {
+    public SubscribeCallback(
+        String subscriptionParentId,
+        Bundle subscriptionOptions,
+        SettableFuture<LibraryResult<Void>> future) {
+      this.subscriptionParentId = subscriptionParentId;
+      this.subscriptionOptions = subscriptionOptions;
       this.future = future;
     }
 
     @Override
     public void onError(@Nullable String parentId) {
-      onErrorInternal();
+      onError(subscriptionParentId, subscriptionOptions);
     }
 
     @Override
     public void onError(@Nullable String parentId, @Nullable Bundle options) {
-      onErrorInternal();
+      onErrorInternal(subscriptionParentId, subscriptionOptions);
     }
 
     @Override
     public void onChildrenLoaded(
         @Nullable String parentId, @Nullable List<MediaBrowserCompat.MediaItem> children) {
-      onChildrenLoadedInternal(parentId, children);
+      onChildrenLoadedInternal(subscriptionParentId, children);
     }
 
     @Override
@@ -492,17 +501,31 @@ import org.checkerframework.checker.initialization.qual.UnderInitialization;
         @Nullable String parentId,
         @Nullable List<MediaBrowserCompat.MediaItem> children,
         @Nullable Bundle options) {
-      onChildrenLoadedInternal(parentId, children);
+      onChildrenLoadedInternal(subscriptionParentId, children);
     }
 
-    private void onErrorInternal() {
+    private void onErrorInternal(String parentId, Bundle options) {
+      if (future.isDone()) {
+        // Delegate to the browser by calling `onChildrenChanged` that makes the app call
+        // `getChildren()` for which the service can return the appropriate error code. This makes a
+        // redundant call to `subscribe` that can be expected to be not expensive as it just returns
+        // an exception.
+        getInstance()
+            .notifyBrowserListener(
+                listener ->
+                    listener.onChildrenChanged(
+                        getInstance(),
+                        parentId,
+                        Integer.MAX_VALUE,
+                        new LibraryParams.Builder().setExtras(options).build()));
+      }
       // Don't need to unsubscribe here, because MediaBrowserServiceCompat can notify children
       // changed after the initial failure and MediaBrowserCompat could receive the changes.
       future.set(LibraryResult.ofError(ERROR_UNKNOWN));
     }
 
     private void onChildrenLoadedInternal(
-        @Nullable String parentId, @Nullable List<MediaBrowserCompat.MediaItem> children) {
+        String parentId, @Nullable List<MediaBrowserCompat.MediaItem> children) {
       if (TextUtils.isEmpty(parentId)) {
         Log.w(TAG, "SubscribeCallback.onChildrenLoaded(): Ignoring empty parentId");
         return;
@@ -526,7 +549,6 @@ import org.checkerframework.checker.initialization.qual.UnderInitialization;
       getInstance()
           .notifyBrowserListener(
               listener -> {
-                // TODO(b/193193565): Cache children result for later getChildren() calls.
                 listener.onChildrenChanged(getInstance(), parentId, itemCount, params);
               });
       future.set(LibraryResult.ofVoid());

libraries/test_session_common/src/main/java/androidx/media3/test/session/common/MediaBrowserConstants.java

@@ -50,6 +50,8 @@ public class MediaBrowserConstants {
       "parent_auth_expired_error_non_fatal";
   public static final String PARENT_ID_AUTH_EXPIRED_ERROR_KEY_ERROR_RESOLUTION_ACTION_LABEL =
       "parent_auth_expired_error_label";
+  public static final String PARENT_ID_ALLOW_FIRST_ON_GET_CHILDREN =
+      "parent_allow_first_on_get_children";
 
   public static final List<String> GET_CHILDREN_RESULT = new ArrayList<>();
   public static final int CHILDREN_COUNT = 100;

libraries/test_session_common/src/main/java/androidx/media3/test/session/common/MediaBrowserServiceCompatConstants.java

@@ -32,6 +32,8 @@ public class MediaBrowserServiceCompatConstants {
   public static final String TEST_SEND_CUSTOM_COMMAND = "sendCustomCommand";
   public static final String TEST_MEDIA_ITEMS_WITH_BROWSE_ACTIONS =
       "getLibraryRoot_withBrowseActions";
+  public static final String TEST_SUBSCRIBE_THEN_REJECT_ON_LOAD_CHILDREN =
+      "subscribe_thenRejectOnLoadChildren";
 
   private MediaBrowserServiceCompatConstants() {}
 }

libraries/test_session_current/src/androidTest/java/androidx/media3/session/MediaBrowserCompatWithMediaLibraryServiceTest.java

@@ -36,6 +36,7 @@ import static androidx.media3.test.session.common.MediaBrowserConstants.CHILDREN
 import static androidx.media3.test.session.common.MediaBrowserConstants.CONNECTION_HINTS_KEY_LIBRARY_ERROR_REPLICATION_MODE;
 import static androidx.media3.test.session.common.MediaBrowserConstants.CUSTOM_ACTION;
 import static androidx.media3.test.session.common.MediaBrowserConstants.CUSTOM_ACTION_EXTRAS;
+import static androidx.media3.test.session.common.MediaBrowserConstants.EXTRAS_KEY_NOTIFY_CHILDREN_CHANGED_MEDIA_ID;
 import static androidx.media3.test.session.common.MediaBrowserConstants.GET_CHILDREN_RESULT;
 import static androidx.media3.test.session.common.MediaBrowserConstants.LONG_LIST_COUNT;
 import static androidx.media3.test.session.common.MediaBrowserConstants.MEDIA_ID_GET_BROWSABLE_ITEM;
@@ -43,6 +44,7 @@ import static androidx.media3.test.session.common.MediaBrowserConstants.MEDIA_ID
 import static androidx.media3.test.session.common.MediaBrowserConstants.MEDIA_ID_GET_ITEM_WITH_METADATA;
 import static androidx.media3.test.session.common.MediaBrowserConstants.MEDIA_ID_GET_PLAYABLE_ITEM;
 import static androidx.media3.test.session.common.MediaBrowserConstants.PARENT_ID;
+import static androidx.media3.test.session.common.MediaBrowserConstants.PARENT_ID_ALLOW_FIRST_ON_GET_CHILDREN;
 import static androidx.media3.test.session.common.MediaBrowserConstants.PARENT_ID_AUTH_EXPIRED_ERROR;
 import static androidx.media3.test.session.common.MediaBrowserConstants.PARENT_ID_AUTH_EXPIRED_ERROR_DEPRECATED;
 import static androidx.media3.test.session.common.MediaBrowserConstants.PARENT_ID_AUTH_EXPIRED_ERROR_KEY_ERROR_RESOLUTION_ACTION_LABEL;
@@ -670,7 +672,7 @@ public class MediaBrowserCompatWithMediaLibraryServiceTest
   @Test
   public void getChildren_emptyResult() throws Exception {
     String testParentId = PARENT_ID_NO_CHILDREN;
-    connectAndWait(/* rootHints= */ Bundle.EMPTY);
+    connectAndWait(/* rootHints= */ null);
     CountDownLatch latch = new CountDownLatch(1);
     AtomicReference<List<MediaItem>> childrenRef = new AtomicReference<>();
 
@@ -712,8 +714,7 @@ public class MediaBrowserCompatWithMediaLibraryServiceTest
   }
 
   @Test
-  public void getChildren_browserNotifyChildrenChanged_callsOnChildrenLoadedTwice()
+  public void subscribe_browserNotifyChildrenChanged_callsOnChildrenLoadedTwice() throws Exception {
-      throws Exception {
     String testParentId = SUBSCRIBE_PARENT_ID_2;
     connectAndWait(/* rootHints= */ Bundle.EMPTY);
     CountDownLatch latch = new CountDownLatch(2);
@@ -743,6 +744,53 @@ public class MediaBrowserCompatWithMediaLibraryServiceTest
     assertThat(childrenList.get(1)).hasSize(12);
   }
 
+  @Test
+  public void subscribe_onChildrenChangedWithMaxValue_convertedToOnError() throws Exception {
+    String testParentId = PARENT_ID_ALLOW_FIRST_ON_GET_CHILDREN;
+    connectAndWait(/* rootHints= */ Bundle.EMPTY);
+    CountDownLatch latch = new CountDownLatch(2);
+    List<String> parentIds = new ArrayList<>();
+    List<Bundle> optionsList = new ArrayList<>();
+    List<List<MediaItem>> childrenList = new ArrayList<>();
+    Bundle requestNotifyChildrenWithDelayBundle =
+        createNotifyChildrenChangedBundle(
+            testParentId,
+            /* itemCount= */ Integer.MAX_VALUE,
+            /* delayMs= */ 100L,
+            /* broadcast= */ false);
+    requestNotifyChildrenWithDelayBundle.putInt(MediaBrowserCompat.EXTRA_PAGE_SIZE, 12);
+
+    browserCompat.subscribe(
+        testParentId,
+        requestNotifyChildrenWithDelayBundle,
+        new SubscriptionCallback() {
+          @Override
+          public void onChildrenLoaded(String parentId, List<MediaItem> children, Bundle options) {
+            parentIds.add(parentId);
+            childrenList.add(children);
+            optionsList.add(options);
+            latch.countDown();
+          }
+
+          @Override
+          public void onError(String parentId, Bundle options) {
+            parentIds.add(parentId);
+            optionsList.add(options);
+            latch.countDown();
+          }
+        });
+
+    assertThat(latch.await(TIMEOUT_MS, MILLISECONDS)).isTrue();
+    assertThat(parentIds).containsExactly(testParentId, testParentId);
+    assertThat(childrenList).hasSize(1);
+    assertThat(childrenList.get(0)).hasSize(12);
+    assertThat(optionsList).hasSize(2);
+    assertThat(optionsList.get(0).getString(EXTRAS_KEY_NOTIFY_CHILDREN_CHANGED_MEDIA_ID))
+        .isEqualTo(PARENT_ID_ALLOW_FIRST_ON_GET_CHILDREN);
+    assertThat(optionsList.get(1).getString(EXTRAS_KEY_NOTIFY_CHILDREN_CHANGED_MEDIA_ID))
+        .isEqualTo(PARENT_ID_ALLOW_FIRST_ON_GET_CHILDREN);
+  }
+
   @Test
   public void getChildren_broadcastNotifyChildrenChanged_callsOnChildrenLoadedTwice()
       throws Exception {

libraries/test_session_current/src/androidTest/java/androidx/media3/session/MediaBrowserListenerWithMediaBrowserServiceCompatTest.java

@@ -35,6 +35,7 @@ import static androidx.media3.test.session.common.MediaBrowserServiceCompatConst
 import static androidx.media3.test.session.common.MediaBrowserServiceCompatConstants.TEST_MEDIA_ITEMS_WITH_BROWSE_ACTIONS;
 import static androidx.media3.test.session.common.MediaBrowserServiceCompatConstants.TEST_ON_CHILDREN_CHANGED_SUBSCRIBE_AND_UNSUBSCRIBE;
 import static androidx.media3.test.session.common.MediaBrowserServiceCompatConstants.TEST_SEND_CUSTOM_COMMAND;
+import static androidx.media3.test.session.common.MediaBrowserServiceCompatConstants.TEST_SUBSCRIBE_THEN_REJECT_ON_LOAD_CHILDREN;
 import static androidx.media3.test.session.common.TestUtils.TIMEOUT_MS;
 import static com.google.common.truth.Truth.assertThat;
 import static com.google.common.util.concurrent.MoreExecutors.directExecutor;
@@ -65,6 +66,7 @@ import java.util.ArrayList;
 import java.util.List;
 import java.util.concurrent.CountDownLatch;
 import java.util.concurrent.ExecutionException;
+import java.util.concurrent.atomic.AtomicBoolean;
 import java.util.concurrent.atomic.AtomicReference;
 import org.junit.After;
 import org.junit.Before;
@@ -140,6 +142,7 @@ public class MediaBrowserListenerWithMediaBrowserServiceCompatTest {
   @Test
   public void connect_useConnectionHints_connectionHintsPassedToLegacyServerOnGetRootAsRootHints()
       throws Exception {
+    remoteService.setProxyForTest(TEST_GET_CHILDREN);
     Bundle connectionHints = new Bundle();
     connectionHints.putBoolean(EXTRAS_KEY_SEND_ROOT_HINTS_AS_SESSION_EXTRAS, true);
     CountDownLatch latch = new CountDownLatch(/* count= */ 1);
@@ -610,4 +613,47 @@ public class MediaBrowserListenerWithMediaBrowserServiceCompatTest {
     assertThat(sessionResultRef.get().resultCode).isEqualTo(SessionResult.RESULT_ERROR_UNKNOWN);
     assertThat(sessionResultRef.get().extras.getString("key-1")).isEqualTo("error-from-service");
   }
+
+  @Test
+  public void
+      subscribe_thenNullListOnLoadChildren_exceptionConvertedToOnChildrenChangedIntegerMaxValue()
+          throws Exception {
+    remoteService.setProxyForTest(TEST_SUBSCRIBE_THEN_REJECT_ON_LOAD_CHILDREN);
+    CountDownLatch onChildrenChangedLatch = new CountDownLatch(2);
+    AtomicBoolean onErrorCalled = new AtomicBoolean();
+    List<String> changedParentIds = new ArrayList<>();
+    List<Integer> changedItemCounts = new ArrayList<>();
+    MediaBrowser browser =
+        createBrowser(
+            new MediaBrowser.Listener() {
+              @Override
+              public void onChildrenChanged(
+                  MediaBrowser browser,
+                  String parentId,
+                  int itemCount,
+                  @Nullable LibraryParams params) {
+                changedParentIds.add(parentId);
+                changedItemCounts.add(itemCount);
+                onChildrenChangedLatch.countDown();
+              }
+
+              @Override
+              public void onError(MediaController controller, SessionError sessionError) {
+                onErrorCalled.set(true);
+              }
+            });
+
+    ListenableFuture<LibraryResult<Void>> future =
+        threadTestRule
+            .getHandler()
+            .postAndSync(() -> browser.subscribe("parentId", new LibraryParams.Builder().build()));
+    // Trigger calling onLoadChildren that is rejected.
+    remoteService.notifyChildrenChanged("parentId");
+
+    assertThat(future.get().resultCode).isEqualTo(RESULT_SUCCESS);
+    assertThat(onChildrenChangedLatch.await(TIMEOUT_MS, MILLISECONDS)).isTrue();
+    assertThat(changedParentIds).containsExactly("parentId", "parentId");
+    assertThat(changedItemCounts).containsExactly(2, Integer.MAX_VALUE).inOrder();
+    assertThat(onErrorCalled.get()).isFalse();
+  }
 }

libraries/test_session_current/src/main/java/androidx/media3/session/MockMediaBrowserServiceCompat.java

@@ -35,6 +35,7 @@ import static androidx.media3.test.session.common.MediaBrowserServiceCompatConst
 import static androidx.media3.test.session.common.MediaBrowserServiceCompatConstants.TEST_MEDIA_ITEMS_WITH_BROWSE_ACTIONS;
 import static androidx.media3.test.session.common.MediaBrowserServiceCompatConstants.TEST_ON_CHILDREN_CHANGED_SUBSCRIBE_AND_UNSUBSCRIBE;
 import static androidx.media3.test.session.common.MediaBrowserServiceCompatConstants.TEST_SEND_CUSTOM_COMMAND;
+import static androidx.media3.test.session.common.MediaBrowserServiceCompatConstants.TEST_SUBSCRIBE_THEN_REJECT_ON_LOAD_CHILDREN;
 import static java.lang.Math.min;
 
 import android.content.Intent;
@@ -100,7 +101,6 @@ public class MockMediaBrowserServiceCompat extends MediaBrowserServiceCompat {
     sessionCompat.setCallback(new MediaSessionCompat.Callback() {});
     sessionCompat.setActive(true);
     setSessionToken(sessionCompat.getSessionToken());
-
     testBinder = new RemoteMediaBrowserServiceCompatStub(sessionCompat);
   }
 
@@ -299,6 +299,9 @@ public class MockMediaBrowserServiceCompat extends MediaBrowserServiceCompat {
         case TEST_MEDIA_ITEMS_WITH_BROWSE_ACTIONS:
           setProxyForMediaItemsWithBrowseActions(session);
           break;
+        case TEST_SUBSCRIBE_THEN_REJECT_ON_LOAD_CHILDREN:
+          setProxyForSubscribeAndRejectGetChildren();
+          break;
         default:
           throw new IllegalArgumentException("Unknown testName: " + testName);
       }
@@ -456,6 +459,45 @@ public class MockMediaBrowserServiceCompat extends MediaBrowserServiceCompat {
           });
     }
 
+    private void setProxyForSubscribeAndRejectGetChildren() {
+      setMediaBrowserServiceProxy(
+          new MockMediaBrowserServiceCompat.Proxy() {
+
+            private boolean isSubscribed;
+
+            @Override
+            public void onLoadChildren(String parentId, Result<List<MediaItem>> result) {
+              onLoadChildren(parentId, result, new Bundle());
+            }
+
+            @Override
+            public void onLoadChildren(
+                String parentId, Result<List<MediaItem>> result, Bundle options) {
+              if (isSubscribed) {
+                // Accept the first call that a Media3 browser interprets as a successful
+                // subscription. Then reject any further access to onLoadChildren().
+                result.sendResult(null);
+                return;
+              }
+              isSubscribed = true;
+              result.sendResult(
+                  ImmutableList.of(
+                      new MediaItem(
+                          new MediaDescriptionCompat.Builder()
+                              .setMediaUri(Uri.parse("http://www.example.com/1"))
+                              .setMediaId("mediaId1")
+                              .build(),
+                          MediaItem.FLAG_PLAYABLE),
+                      new MediaItem(
+                          new MediaDescriptionCompat.Builder()
+                              .setMediaUri(Uri.parse("http://www.example.com/2"))
+                              .setMediaId("mediaId2")
+                              .build(),
+                          MediaItem.FLAG_PLAYABLE)));
+            }
+          });
+    }
+
     private void getChildren_authenticationError_receivesPlaybackException(
         MediaSessionCompat session, boolean isFatal) {
       setMediaBrowserServiceProxy(

libraries/test_session_current/src/main/java/androidx/media3/session/MockMediaLibraryService.java

@@ -42,6 +42,7 @@ import static androidx.media3.test.session.common.MediaBrowserConstants.MEDIA_ID
 import static androidx.media3.test.session.common.MediaBrowserConstants.MEDIA_ID_GET_ITEM_WITH_METADATA;
 import static androidx.media3.test.session.common.MediaBrowserConstants.MEDIA_ID_GET_PLAYABLE_ITEM;
 import static androidx.media3.test.session.common.MediaBrowserConstants.PARENT_ID;
+import static androidx.media3.test.session.common.MediaBrowserConstants.PARENT_ID_ALLOW_FIRST_ON_GET_CHILDREN;
 import static androidx.media3.test.session.common.MediaBrowserConstants.PARENT_ID_AUTH_EXPIRED_ERROR;
 import static androidx.media3.test.session.common.MediaBrowserConstants.PARENT_ID_AUTH_EXPIRED_ERROR_DEPRECATED;
 import static androidx.media3.test.session.common.MediaBrowserConstants.PARENT_ID_AUTH_EXPIRED_ERROR_KEY_ERROR_RESOLUTION_ACTION_LABEL;
@@ -227,6 +228,7 @@ public class MockMediaLibraryService extends MediaLibraryService {
       playlistAddExtras.putString("key-1", "playlist_add");
       Bundle radioExtras = new Bundle();
       radioExtras.putString("key-1", "radio");
+      Log.d("notifyChildrenChanged", "new TestLibrarySessionCallback()");
       session =
           new MediaLibrarySession.Builder(
                   MockMediaLibraryService.this,
@@ -284,6 +286,8 @@ public class MockMediaLibraryService extends MediaLibraryService {
 
   private class TestLibrarySessionCallback implements MediaLibrarySession.Callback {
 
+    private int getChildrenCallCount = 0;
+
     @Override
     public MediaSession.ConnectionResult onConnect(
         MediaSession session, ControllerInfo controller) {
@@ -351,6 +355,7 @@ public class MockMediaLibraryService extends MediaLibraryService {
       }
       switch (mediaId) {
         case MEDIA_ID_GET_BROWSABLE_ITEM:
+        case PARENT_ID_ALLOW_FIRST_ON_GET_CHILDREN:
         case SUBSCRIBE_PARENT_ID_2:
           return Futures.immediateFuture(
               LibraryResult.ofItem(createBrowsableMediaItem(mediaId), /* params= */ null));
@@ -380,6 +385,7 @@ public class MockMediaLibraryService extends MediaLibraryService {
         int page,
         int pageSize,
         @Nullable LibraryParams params) {
+      getChildrenCallCount++;
       assertLibraryParams(params);
       if (Objects.equals(parentId, PARENT_ID_NO_CHILDREN)) {
         return Futures.immediateFuture(LibraryResult.ofItemList(ImmutableList.of(), params));
@@ -399,6 +405,15 @@ public class MockMediaLibraryService extends MediaLibraryService {
         errorBundle.putString("key", "value");
         return Futures.immediateFuture(
             LibraryResult.ofError(new SessionError(ERROR_BAD_VALUE, "error message", errorBundle)));
+      } else if (Objects.equals(parentId, PARENT_ID_ALLOW_FIRST_ON_GET_CHILDREN)) {
+        return getChildrenCallCount == 1
+            ? Futures.immediateFuture(
+                LibraryResult.ofItemList(
+                    getPaginatedResult(GET_CHILDREN_RESULT, page, pageSize), params))
+            : Futures.immediateFuture(
+                LibraryResult.ofError(
+                    LibraryResult.RESULT_ERROR_SESSION_AUTHENTICATION_EXPIRED,
+                    new LibraryParams.Builder().build()));
       } else if (Objects.equals(parentId, PARENT_ID_AUTH_EXPIRED_ERROR)
           || Objects.equals(parentId, PARENT_ID_AUTH_EXPIRED_ERROR_DEPRECATED)
           || Objects.equals(parentId, PARENT_ID_SKIP_LIMIT_REACHED_ERROR)) {