Fixes appWillTerminate not running on GDTStorage's queue (#4879)

* Fixes appWillTerminate not running on GDTStorage's queue

Fixes #4546

Thanks @plu for the test case!

* Use the correct lifecycle event.

* Additional comments.

Author: mikehaney24

GoogleDataTransport/CHANGELOG.md

@@ -1,3 +1,6 @@
+# v4.0.1
+- Fixes missing a dispatch_sync and on-queue work in appWillTerminate of storage. (#4546)
+
 # v4.0.0
 - Internal restructuring to support a single class implementing several backends.
 

GoogleDataTransport/GDTCORLibrary/GDTCORStorage.m

@@ -278,17 +278,19 @@ - (void)appWillBackground:(GDTCORApplication *)app {
 }
 
 - (void)appWillTerminate:(GDTCORApplication *)application {
-  if (@available(macOS 10.13, iOS 11.0, tvOS 11.0, *)) {
-    NSError *error;
-    NSData *data = [NSKeyedArchiver archivedDataWithRootObject:self
-                                         requiringSecureCoding:YES
-                                                         error:&error];
-    [data writeToFile:[GDTCORStorage archivePath] atomically:YES];
-  } else {
+  dispatch_sync(_storageQueue, ^{
+    if (@available(macOS 10.13, iOS 11.0, tvOS 11.0, *)) {
+      NSError *error;
+      NSData *data = [NSKeyedArchiver archivedDataWithRootObject:self
+                                           requiringSecureCoding:YES
+                                                           error:&error];
+      [data writeToFile:[GDTCORStorage archivePath] atomically:YES];
+    } else {
 #if !TARGET_OS_MACCATALYST && !TARGET_OS_WATCH
-    [NSKeyedArchiver archiveRootObject:self toFile:[GDTCORStorage archivePath]];
+      [NSKeyedArchiver archiveRootObject:self toFile:[GDTCORStorage archivePath]];
 #endif
-  }
+    }
+  });
 }
 
 #pragma mark - NSSecureCoding

GoogleDataTransport/GDTCORTests/Unit/GDTCORStorageTest.m

@@ -442,4 +442,31 @@ - (void)testQoSTierFast {
   });
 }
 
+/** Fuzz tests the storing of events at the same time as a terminate lifecycle notification. This
+ * test can fail if there's simultaneous access to ivars of GDTCORStorage with one access being
+ * off the storage's queue. The terminate lifecycle event should operate on and flush the queue.
+ */
+- (void)testStoringEventsDuringTerminate {
+  int numberOfIterations = 1000;
+  for (int i = 0; i < numberOfIterations; i++) {
+    NSString *testString = [NSString stringWithFormat:@"testString %d", i];
+    GDTCOREvent *event = [[GDTCOREvent alloc] initWithMappingID:@"404" target:target];
+    event.dataObjectTransportBytes = [testString dataUsingEncoding:NSUTF8StringEncoding];
+    event.clockSnapshot = [GDTCORClock snapshot];
+    XCTestExpectation *writtenExpectation = [self expectationWithDescription:@"event written"];
+    XCTAssertNoThrow([[GDTCORStorage sharedInstance] storeEvent:event
+                                                     onComplete:^(BOOL wasWritten, NSError *error) {
+                                                       XCTAssertTrue(wasWritten);
+                                                       [writtenExpectation fulfill];
+                                                     }]);
+    [self waitForExpectationsWithTimeout:10 handler:nil];
+    if (i % 5 == 0) {
+      [[GDTCORStorage sharedInstance] removeEvents:[GDTCORStorage sharedInstance].storedEvents.set];
+    }
+    [NSNotificationCenter.defaultCenter
+        postNotificationName:kGDTCORApplicationWillTerminateNotification
+                      object:nil];
+  }
+}
+
 @end