fix: prevent race condition crash in FPRTraceBackgroundActivityTracker

Replace async notification observer registration with synchronous registration
to eliminate timing-dependent crashes when objects are deallocated quickly.

Fixes #14273

Author: JesusRojass

FirebasePerformance/Sources/AppActivity/FPRTraceBackgroundActivityTracker.m

@@ -35,21 +35,29 @@ - (instancetype)init {
     } else {
       _traceBackgroundState = FPRTraceStateForegroundOnly;
     }
-    dispatch_async(dispatch_get_main_queue(), ^{
-      [[NSNotificationCenter defaultCenter] addObserver:self
-                                               selector:@selector(applicationDidBecomeActive:)
-                                                   name:UIApplicationDidBecomeActiveNotification
-                                                 object:[UIApplication sharedApplication]];
-
-      [[NSNotificationCenter defaultCenter] addObserver:self
-                                               selector:@selector(applicationDidEnterBackground:)
-                                                   name:UIApplicationDidEnterBackgroundNotification
-                                                 object:[UIApplication sharedApplication]];
-    });
+    if ([NSThread isMainThread]) {
+      [self registerNotificationObservers];
+    } else {
+      dispatch_sync(dispatch_get_main_queue(), ^{
+        [self registerNotificationObservers];
+      });
+    }
   }
   return self;
 }
 
+- (void)registerNotificationObservers {
+  [[NSNotificationCenter defaultCenter] addObserver:self
+                                           selector:@selector(applicationDidBecomeActive:)
+                                               name:UIApplicationDidBecomeActiveNotification
+                                             object:[UIApplication sharedApplication]];
+
+  [[NSNotificationCenter defaultCenter] addObserver:self
+                                           selector:@selector(applicationDidEnterBackground:)
+                                               name:UIApplicationDidEnterBackgroundNotification
+                                             object:[UIApplication sharedApplication]];
+}
+
 - (void)dealloc {
   // Remove all the notification observers registered.
   [[NSNotificationCenter defaultCenter] removeObserver:self];

FirebasePerformance/Tests/Unit/FPRTraceBackgroundActivityTrackerTest.m

@@ -61,4 +61,74 @@ - (void)testBackgroundTracking {
                                }];
 }
 
+/** Tests that synchronous observer registration works correctly and observers are immediately available. */
+- (void)testObservers_synchronousRegistrationAddsObserver {
+  NSNotificationCenter *notificationCenter = [NSNotificationCenter defaultCenter];
+  FPRTraceBackgroundActivityTracker *tracker = [[FPRTraceBackgroundActivityTracker alloc] init];
+  XCTAssertNotNil(tracker);
+  
+  [notificationCenter postNotificationName:UIApplicationDidBecomeActiveNotification
+                                   object:[UIApplication sharedApplication]];
+  XCTAssertEqual(tracker.traceBackgroundState, FPRTraceStateForegroundOnly);
+  
+  tracker = nil;
+  XCTAssertNil(tracker);
+  XCTAssertNoThrow([notificationCenter postNotificationName:UIApplicationDidBecomeActiveNotification
+                                                     object:[UIApplication sharedApplication]]);
+  XCTAssertNoThrow([notificationCenter postNotificationName:UIApplicationDidEnterBackgroundNotification
+                                                     object:[UIApplication sharedApplication]]);
+}
+
+/** Tests rapid creation and deallocation to verify race condition fix. */
+- (void)testRapidCreationAndDeallocation_noRaceCondition {
+  for (int i = 0; i < 100; i++) {
+    @autoreleasepool {
+      FPRTraceBackgroundActivityTracker *tracker = [[FPRTraceBackgroundActivityTracker alloc] init];
+      XCTAssertNotNil(tracker);
+      
+      [[NSNotificationCenter defaultCenter] postNotificationName:UIApplicationDidBecomeActiveNotification
+                                                           object:[UIApplication sharedApplication]];
+    }
+  }
+  
+  XCTAssertNoThrow([[NSNotificationCenter defaultCenter] postNotificationName:UIApplicationDidBecomeActiveNotification
+                                                                        object:[UIApplication sharedApplication]]);
+  XCTAssertNoThrow([[NSNotificationCenter defaultCenter] postNotificationName:UIApplicationDidEnterBackgroundNotification
+                                                                        object:[UIApplication sharedApplication]]);
+}
+
+/** Tests that observers are registered immediately after init on main thread. */
+- (void)testObservers_immediateRegistrationOnMainThread {
+  XCTAssertTrue([NSThread isMainThread]);
+  
+  FPRTraceBackgroundActivityTracker *tracker = [[FPRTraceBackgroundActivityTracker alloc] init];
+  
+  [[NSNotificationCenter defaultCenter] postNotificationName:UIApplicationDidBecomeActiveNotification
+                                                       object:[UIApplication sharedApplication]];
+  
+  XCTAssertEqual(tracker.traceBackgroundState, FPRTraceStateForegroundOnly);
+}
+
+/** Tests observer registration when created from background thread. */
+- (void)testObservers_registrationFromBackgroundThread {
+  XCTestExpectation *expectation = [self expectationWithDescription:@"Background thread creation"];
+  
+  dispatch_async(dispatch_get_global_queue(DISPATCH_QUEUE_PRIORITY_DEFAULT, 0), ^{
+    FPRTraceBackgroundActivityTracker *tracker = [[FPRTraceBackgroundActivityTracker alloc] init];
+    XCTAssertNotNil(tracker);
+    
+    dispatch_async(dispatch_get_main_queue(), ^{
+      [[NSNotificationCenter defaultCenter] postNotificationName:UIApplicationDidBecomeActiveNotification
+                                                           object:[UIApplication sharedApplication]];
+      
+      XCTAssertEqual(tracker.traceBackgroundState, FPRTraceStateForegroundOnly);
+      [expectation fulfill];
+    });
+  });
+  
+  [self waitForExpectationsWithTimeout:5.0 handler:^(NSError *error) {
+    XCTAssertNil(error, @"Test timed out");
+  }];
+}
+
 @end