Fix multithreaded _completion access (#6068)

* Fix unguarded multithreaded access

* Update changelog

* fix typo in doc comments

* fix nullability build error

* Serialize via FIRAuthGlobalWorkQueue

* remove unused code

Author: morganchen12

FirebaseAuth/CHANGELOG.md

@@ -1,3 +1,6 @@
+# Unreleased
+- [fixed] Fixed a multithreaded memory access issue on iOS (#5979).
+
 # v6.7.0
 - [changed] Functionally neutral source reorganization for preliminary Swift Package Manager support. (#5856)
 

FirebaseAuth/Sources/Utilities/FIRAuthURLPresenter.m

@@ -39,6 +39,8 @@ @interface FIRAuthURLPresenter () <SFSafariViewControllerDelegate, FIRAuthWebVie
 @implementation FIRAuthURLPresenter {
   /** @var _isPresenting
       @brief Whether or not some web-based content is being presented.
+          Accesses to this property are serialized on the global Auth work queue
+          and thus this variable should not be read or written outside of the work queue.
    */
   BOOL _isPresenting;
 
@@ -63,7 +65,9 @@ @implementation FIRAuthURLPresenter {
   id<FIRAuthUIDelegate> _UIDelegate;
 
   /** @var _completion
-      @brief The completion handler for the current presentaion, if one is active.
+      @brief The completion handler for the current presentation, if one is active.
+          Accesses to this variable are serialized on the global Auth work queue
+          and thus this variable should not be read or written outside of the work queue.
       @remarks This variable is also used as a flag to indicate a presentation is active.
    */
   FIRAuthURLPresentationCompletion _Nullable _completion;
@@ -75,12 +79,16 @@ - (void)presentURL:(NSURL *)URL
          completion:(FIRAuthURLPresentationCompletion)completion {
   if (_isPresenting) {
     // Unable to start a new presentation on top of another.
-    _completion(nil, [FIRAuthErrorUtils webContextAlreadyPresentedErrorWithMessage:nil]);
+    // Invoke the new completion closure and leave the old one as-is
+    // to be invoked when the presentation finishes.
+    dispatch_async(dispatch_get_main_queue(), ^() {
+      completion(nil, [FIRAuthErrorUtils webContextAlreadyPresentedErrorWithMessage:nil]);
+    });
     return;
   }
   _isPresenting = YES;
   _callbackMatcher = callbackMatcher;
-  _completion = completion;
+  _completion = [completion copy];
   dispatch_async(dispatch_get_main_queue(), ^() {
     self->_UIDelegate = UIDelegate ?: [FIRAuthDefaultUIDelegate defaultUIDelegate];
     if ([SFSafariViewController class]) {
@@ -162,8 +170,8 @@ - (void)finishPresentationWithURL:(nullable NSURL *)URL error:(nullable NSError
   _callbackMatcher = nil;
   id<FIRAuthUIDelegate> UIDelegate = _UIDelegate;
   _UIDelegate = nil;
-  FIRAuthURLPresentationCompletion completion = _completion;
-  _completion = nil;
+  FIRAuthURLPresentationCompletion completion = [_completion copy];
+  _completion = NULL;
   void (^finishBlock)(void) = ^() {
     self->_isPresenting = NO;
     completion(URL, error);