Merge pull request #2048 from dreampiggy/fix_cancelBlock_multi_thread_issue

Fix #1941 by placing a lock to avoid multi-thread issue for SDWebImageCombinedOperation cancelBlock

@@ -228,11 +228,14 @@
                     [self safelyRemoveOperationFromRunning:strongOperation];
                 }
             }];
+            @synchronized(operation) {
+                // Need same lock to ensure cancelBlock called because cancel method can be called in different queue
                 operation.cancelBlock = ^{
                     [self.imageDownloader cancel:subOperationToken];
                     __strong __typeof(weakOperation) strongOperation = weakOperation;
                     [self safelyRemoveOperationFromRunning:strongOperation];
                 };
+            }
         } else if (cachedImage) {
             __strong __typeof(weakOperation) strongOperation = weakOperation;
             [self callCompletionBlockForOperation:strongOperation completion:completedBlock image:cachedImage data:cachedData error:nil cacheType:cacheType finished:YES url:url];
@@ -319,6 +322,7 @@
 }
 
 - (void)cancel {
+    @synchronized(self) {
         self.cancelled = YES;
         if (self.cacheOperation) {
             [self.cacheOperation cancel];
@@ -326,11 +330,8 @@
         }
         if (self.cancelBlock) {
             self.cancelBlock();
-        
-        // TODO: this is a temporary fix to #809.
-        // Until we can figure the exact cause of the crash, going with the ivar instead of the setter
-//        self.cancelBlock = nil;
-        _cancelBlock = nil;
+            self.cancelBlock = nil;
+        }
     }
 }