fix: address Objective-C -O1 ARC lifetime issue in GULNetworkURLSession (#255)

* fix: address Objective-C -O1 ARC lifetime issue in GULNetworkURLSession

* changelog

CHANGELOG.md

@@ -7,6 +7,8 @@
   call read `-[NSBundle appStoreReceiptURL]`, which makes a synchronous XPC call
   to the LaunchServices daemon and could block the calling thread, often the
   main thread, for seconds. (firebase/firebase-ios-sdk#16726)
+- [fixed] Fix an ARC-related deadlock in `GULNetworkURLSession` under -O1
+  optimization. (#255)
 
 # 8.1.4
 - [fixed] `GULMutableDictionary` now ignores a `nil` key (and a `nil` object in

GoogleUtilities/Network/GULNetworkURLSession.m

@@ -727,7 +727,9 @@ static const void *kGULSessionTrackerKey = &kGULSessionTrackerKey;
     return;
   }
 
-  GULSessionDeallocTracker *oldTrackerToReleaseOutsideLock = nil;
+  // Retain the old tracker until the lock is released. If it deallocates inside the lock,
+  // its -dealloc method will attempt to re-acquire the lock, resulting in a deadlock.
+  NS_VALID_UNTIL_END_OF_SCOPE GULSessionDeallocTracker *oldTrackerToReleaseOutsideLock = nil;
 
   [[self sessionIDToFetcherMapReadWriteLock] lock];
   GULNetworkURLSessionWeakHolder *holder =
@@ -759,10 +761,6 @@ static const void *kGULSessionTrackerKey = &kGULSessionTrackerKey;
     [[[self class] sessionIDToFetcherMap] removeObjectForKey:sessionID];
   }
   [[self sessionIDToFetcherMapReadWriteLock] unlock];
-
-  // Retain the old tracker until the lock is released. If it deallocates inside the lock,
-  // its -dealloc method will attempt to re-acquire the lock, resulting in a deadlock.
-  (void)oldTrackerToReleaseOutsideLock;
 }
 
 + (nullable GULNetworkURLSession *)sessionFromFetcherMapForSessionID:(NSString *)sessionID {