Punt pause task to the end of the queue (#8296)

* Punt pause task to the end of the queue

* allow more generous execution on the work queue before pause

* Use one frame delay?

* Fix timing issue

* style

Author: morganchen12

FirebaseStorage/Sources/FIRStorage.m

@@ -166,6 +166,7 @@ - (instancetype)initWithApp:(FIRApp *)app
     _scheme = kFIRStorageScheme;
     _port = @(kFIRStoragePort);
     _fetcherServiceForApp = nil;  // Configured in `ensureConfigured()`
+    // Must be a serial queue.
     _dispatchQueue = dispatch_queue_create("com.google.firebase.storage", DISPATCH_QUEUE_SERIAL);
     _maxDownloadRetryTime = 600.0;
     _maxDownloadRetryInterval =

FirebaseStorage/Sources/FIRStorageDownloadTask.m

@@ -165,13 +165,23 @@ - (void)cancelWithError:(NSError *)error {
 - (void)pause {
   __weak FIRStorageDownloadTask *weakSelf = self;
   [self dispatchAsync:^() {
-    weakSelf.state = FIRStorageTaskStatePausing;
-    [weakSelf.fetcher stopFetching];
-    // Give the resume callback a chance to run (if scheduled)
-    [weakSelf.fetcher waitForCompletionWithTimeout:0.001];
-    weakSelf.state = FIRStorageTaskStatePaused;
-    FIRStorageTaskSnapshot *snapshot = weakSelf.snapshot;
-    [weakSelf fireHandlersForStatus:FIRStorageTaskStatusPause snapshot:snapshot];
+    __strong FIRStorageDownloadTask *strongSelf = weakSelf;
+    if (!strongSelf || strongSelf.state == FIRStorageTaskStatePaused ||
+        strongSelf.state == FIRStorageTaskStatePausing) {
+      return;
+    }
+    strongSelf.state = FIRStorageTaskStatePausing;
+    // Use the resume callback to confirm pause status since it always runs after the last
+    // NSURLSession update.
+    [strongSelf.fetcher setResumeDataBlock:^(NSData *data) {
+      // Silence compiler warning about retain cycles
+      __strong __typeof(self) strong = weakSelf;
+      strong->_downloadData = data;
+      strong.state = FIRStorageTaskStatePaused;
+      FIRStorageTaskSnapshot *snapshot = strong.snapshot;
+      [strong fireHandlersForStatus:FIRStorageTaskStatusPause snapshot:snapshot];
+    }];
+    [strongSelf.fetcher stopFetching];
   }];
 }