Synchronise getter and setter of the cancelled property of the SDWebImageCombinedOperation in order to eliminate data race.

SDWebImage/Core/SDWebImageManager.m

@@ -17,7 +17,9 @@
 static id<SDImageCache> _defaultImageCache;
 static id<SDImageLoader> _defaultImageLoader;
 
-@interface SDWebImageCombinedOperation ()
+@interface SDWebImageCombinedOperation () {
+    SD_LOCK_DECLARE(_cancelledLock); // a lock to keep the access to `cancelled` thread-safe
+}
 
 @property (assign, nonatomic, getter = isCancelled) BOOL cancelled;
 @property (strong, nonatomic, readwrite, nullable) id<SDWebImageOperation> loaderOperation;
@@ -803,12 +805,28 @@ static id<SDImageLoader> _defaultImageLoader;
 
 @implementation SDWebImageCombinedOperation
 
+- (instancetype)init {
+    if (self = [super init]) {
+        SD_LOCK_INIT(_cancelledLock);
+    }
+
+    return self;
+}
+
+- (BOOL)isCancelled {
+    BOOL isCancelled = NO;
+    SD_LOCK(_cancelledLock);
+    isCancelled = _cancelled;
+    SD_UNLOCK(_cancelledLock);
+    return isCancelled;
+}
+
 - (void)cancel {
-    @synchronized(self) {
-        if (self.isCancelled) {
+    SD_LOCK(_cancelledLock);
+    if (_cancelled) {
         return;
     }
-        self.cancelled = YES;
+    _cancelled = YES;
     if (self.cacheOperation) {
         [self.cacheOperation cancel];
         self.cacheOperation = nil;
@@ -818,7 +836,7 @@ static id<SDImageLoader> _defaultImageLoader;
         self.loaderOperation = nil;
     }
     [self.manager safelyRemoveOperationFromRunning:self];
-    }
+    SD_UNLOCK(_cancelledLock);
 }
 
 @end