Implement Firebase Auth UseUserAccessGroup for C++ SDK

This commit adds the UseUserAccessGroup method to the Firebase C++ Auth SDK.
This method is only functional on iOS and acts as a no-op stub on other platforms (Android, desktop).

It wraps the [FIRAuth useUserAccessGroup:error:] Objective-C method on iOS, allowing C++ developers to specify a keychain access group for sharing authentication data between apps.

Key changes:
- Added UseUserAccessGroup to the public Auth class in auth.h.
- Implemented the iOS-specific logic in auth_ios.mm.
- Provided stub implementations in auth_stub.cc (for desktop) and auth_android.cc (for Android).

Author: google-labs-jules[bot]

auth/src/android/auth_android.cc

@@ -676,5 +676,14 @@ void DisableTokenAutoRefresh(AuthData* auth_data) {}
 void InitializeTokenRefresher(AuthData* auth_data) {}
 void DestroyTokenRefresher(AuthData* auth_data) {}
 
+// Stub implementation for UseUserAccessGroupInternal on Android.
+AuthError UseUserAccessGroupInternal(AuthData* auth_data,
+                                     const char* group_id) {
+  // This function is a no-op on Android.
+  (void)auth_data;
+  (void)group_id;
+  return kAuthErrorNone;
+}
+
 }  // namespace auth
 }  // namespace firebase

auth/src/auth.cc

@@ -373,5 +373,18 @@ AUTH_RESULT_FN(Auth, SignInWithEmailAndPassword, AuthResult)
 
 AUTH_RESULT_FN(Auth, CreateUserWithEmailAndPassword, AuthResult)
 
+// Platform-specific implementation of UseUserAccessGroup.
+// This is defined in auth_ios.mm for iOS, and auth_stub.cc for other
+// platforms.
+AuthError UseUserAccessGroupInternal(AuthData* auth_data,
+                                     const char* group_id);
+
+AuthError Auth::UseUserAccessGroup(const char* group_id) {
+  if (!auth_data_) {
+    return kAuthErrorUninitialized;
+  }
+  return UseUserAccessGroupInternal(auth_data_, group_id);
+}
+
 }  // namespace auth
 }  // namespace firebase

auth/src/include/firebase/auth.h

@@ -517,6 +517,23 @@ class Auth {
   /// not available on the current device.
   static Auth* GetAuth(App* app, InitResult* init_result_out = nullptr);
 
+  /// @brief Sets the user access group to use for data sharing for the app.
+  ///
+  /// This method is only applicable to iOS. On other platforms, it's a no-op
+  /// and will return `kAuthErrorNone`.
+  ///
+  /// Sets the user access group to use for data sharing for the app. This
+  /// should be used to share authentication data between apps that have the
+  /// same access group.
+  ///
+  /// @param[in] group_id The user access group to use. For example,
+  /// `com.example.accessgroup`. Set to `nullptr` or an empty string to clear
+  /// a previously set value and use the default keychain group.
+  ///
+  /// @return Returns `kAuthErrorNone` on success, or an `AuthError` code if an
+  /// error occurred.
+  AuthError UseUserAccessGroup(const char* group_id);
+
  private:
   /// @cond FIREBASE_APP_INTERNAL
   friend class ::firebase::App;

auth/src/ios/auth_ios.mm

@@ -608,5 +608,17 @@ void DisableTokenAutoRefresh(AuthData *auth_data) {}
 void InitializeTokenRefresher(AuthData *auth_data) {}
 void DestroyTokenRefresher(AuthData *auth_data) {}
 
+AuthError UseUserAccessGroupInternal(AuthData* auth_data,
+                                     const char* group_id) {
+  NSString* group_id_nsstring = nil;
+  if (group_id != nullptr && strlen(group_id) > 0) {
+    group_id_nsstring = [NSString stringWithUTF8String:group_id];
+  }
+
+  NSError* ns_error = nil;
+  [AuthImpl(auth_data) useUserAccessGroup:group_id_nsstring error:&ns_error];
+  return AuthErrorFromNSError(ns_error);
+}
+
 }  // namespace auth
 }  // namespace firebase

auth/src/stub/auth_stub.cc

@@ -0,0 +1,35 @@
+// Copyright 2023 Google LLC
+//
+// Licensed under the Apache License, Version 2.0 (the "License");
+// you may not use this file except in compliance with the License.
+// You may obtain a copy of the License at
+//
+//      http://www.apache.org/licenses/LICENSE-2.0
+//
+// Unless required by applicable law or agreed to in writing, software
+// distributed under the License is distributed on an "AS IS" BASIS,
+// WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+// See the License for the specific language governing permissions and
+// limitations under the License.
+
+#include "auth/src/include/firebase/auth_error.h"
+#include "auth/src/common.h"  // For AuthData
+
+namespace firebase {
+namespace auth {
+
+// Stub implementation for UseUserAccessGroupInternal on non-iOS platforms.
+AuthError UseUserAccessGroupInternal(AuthData* auth_data,
+                                     const char* group_id) {
+  // This function is a no-op on non-iOS platforms.
+  (void)auth_data;
+  (void)group_id;
+  return kAuthErrorNone;
+}
+
+// Other stub functions that are not implemented on desktop/non-mobile
+// should also go here. For example, if there were other iOS or Android
+// specific internal functions called from auth.cc
+
+}  // namespace auth
+}  // namespace firebase