[Auth] Added note to getStoredUser(forAccessGroup:) documentation about tvOS keychain sharing issues (#9244)

* Add note about tvOS keychain sharing issue

* Update Auth CHANGELOG

* Review

* Log error when running on tvOS and sharing auth state

* Update API docs

* Update changelog

* Style

* Improve message clarity

* Style

* Review 2

* Review 3

Author: ncooke3

FirebaseAuth/CHANGELOG.md

@@ -1,3 +1,6 @@
+# v8.12.0
+- [added] Added documentation note and error logging to `getStoredUser(forAccessGroup:)` regarding tvOS keychain sharing issues. (#8878)
+
 # v8.11.0
 - [changed] Added a `X-Firebase-GMPID` header to network requests. (#9046)
 - [fixed] Added multi-tenancy support to generic OAuth providers. (#7990)

FirebaseAuth/Sources/Auth/FIRAuth.m

@@ -2261,6 +2261,16 @@ - (nullable FIRUser *)getStoredUserForAccessGroup:(NSString *_Nullable)accessGro
 #endif  // TARGET_OS_WATCH
     user = [unarchiver decodeObjectOfClass:[FIRUser class] forKey:userKey];
   } else {
+#if TARGET_OS_TV
+    if (self.shareAuthStateAcrossDevices) {
+      FIRLogError(kFIRLoggerAuth, @"I-AUT000001",
+                  @"Getting a stored user for a given access group is not supported "
+                  @"on tvOS when `shareAuthStateAcrossDevices` is set to `true` (#8878)."
+                  @"This case will return `nil`.");
+      return nil;
+    }
+#endif  // TARGET_OS_TV
+
     user = [self.storedUserManager getStoredUserForAccessGroup:accessGroup
                                    shareAuthStateAcrossDevices:self.shareAuthStateAcrossDevices
                                              projectIdentifier:self.app.options.APIKey

FirebaseAuth/Sources/Public/FirebaseAuth/FIRAuth.h

@@ -857,6 +857,9 @@ NS_SWIFT_NAME(Auth)
 
 /** @fn getStoredUserForAccessGroup:error:
     @brief Get the stored user in the given accessGroup.
+    @note This API is not supported on tvOS when `shareAuthStateAcrossDevices` is set to `true`.
+        This case will return `nil`.
+        Please refer to https://github.com/firebase/firebase-ios-sdk/issues/8878 for details.
  */
 - (nullable FIRUser *)getStoredUserForAccessGroup:(NSString *_Nullable)accessGroup
                                             error:(NSError *_Nullable *_Nullable)outError;