Use a internal method to avoid thread-safe issue for file manager. Remove that checkIOQueue and add sync version exist API

SDWebImage/SDImageCache.h

@@ -171,6 +171,13 @@ typedef void(^SDWebImageCalculateSizeBlock)(NSUInteger fileCount, NSUInteger tot
  */
 - (void)diskImageExistsWithKey:(nullable NSString *)key completion:(nullable SDWebImageCheckCacheCompletionBlock)completionBlock;
 
+/**
+ *  Sync check if image data exists in disk cache already (does not load the image)
+ *
+ *  @param key             the key describing the url
+ */
+- (BOOL)diskImageDataExistsWithKey:(nullable NSString *)key;
+
 /**
  * Operation that queries the cache asynchronously and call the completion when done.
  *

SDWebImage/SDImageCache.m

@@ -106,14 +106,6 @@ FOUNDATION_STATIC_INLINE NSUInteger SDCacheCostForImage(UIImage *image) {
     [[NSNotificationCenter defaultCenter] removeObserver:self];
 }
 
-- (void)checkIfQueueIsIOQueue {
-    const char *currentQueueLabel = dispatch_queue_get_label(DISPATCH_CURRENT_QUEUE_LABEL);
-    const char *ioQueueLabel = dispatch_queue_get_label(self.ioQueue);
-    if (strcmp(currentQueueLabel, ioQueueLabel) != 0) {
-        NSLog(@"This method should be called from the ioQueue");
-    }
-}
-
 #pragma mark - Cache paths
 
 - (void)addReadOnlyCachePath:(nonnull NSString *)path {
@@ -201,7 +193,7 @@ FOUNDATION_STATIC_INLINE NSUInteger SDCacheCostForImage(UIImage *image) {
                     }
                     data = [[SDWebImageCodersManager sharedInstance] encodedDataWithImage:image format:format];
                 }
-                [self storeImageDataToDisk:data forKey:key];
+                [self _storeImageDataToDisk:data forKey:key];
             }
             
             if (completionBlock) {
@@ -221,8 +213,16 @@ FOUNDATION_STATIC_INLINE NSUInteger SDCacheCostForImage(UIImage *image) {
     if (!imageData || !key) {
         return;
     }
+    dispatch_sync(self.ioQueue, ^{
+        [self _storeImageDataToDisk:imageData forKey:key];
+    });
+}
 
-    [self checkIfQueueIsIOQueue];
+// Make sure to call form io queue by caller
+- (void)_storeImageDataToDisk:(nullable NSData *)imageData forKey:(nullable NSString *)key {
+    if (!imageData || !key) {
+        return;
+    }
     
     if (![_fileManager fileExistsAtPath:_diskCachePath]) {
         [_fileManager createDirectoryAtPath:_diskCachePath withIntermediateDirectories:YES attributes:nil error:NULL];
@@ -233,7 +233,7 @@ FOUNDATION_STATIC_INLINE NSUInteger SDCacheCostForImage(UIImage *image) {
     // transform to NSUrl
     NSURL *fileURL = [NSURL fileURLWithPath:cachePathForKey];
     
-    [_fileManager createFileAtPath:cachePathForKey contents:imageData attributes:nil];
+    [imageData writeToURL:fileURL options:self.config.diskCacheWritingOptions error:nil];
     
     // disable iCloud backup
     if (self.config.shouldDisableiCloud) {
@@ -244,7 +244,33 @@ FOUNDATION_STATIC_INLINE NSUInteger SDCacheCostForImage(UIImage *image) {
 #pragma mark - Query and Retrieve Ops
 
 - (void)diskImageExistsWithKey:(nullable NSString *)key completion:(nullable SDWebImageCheckCacheCompletionBlock)completionBlock {
-    dispatch_async(_ioQueue, ^{
+    dispatch_async(self.ioQueue, ^{
+        BOOL exists = [self _diskImageDataExistsWithKey:key];
+        if (completionBlock) {
+            dispatch_async(dispatch_get_main_queue(), ^{
+                completionBlock(exists);
+            });
+        }
+    });
+}
+
+- (BOOL)diskImageDataExistsWithKey:(nullable NSString *)key {
+    if (!key) {
+        return NO;
+    }
+    __block BOOL exists = NO;
+    dispatch_sync(self.ioQueue, ^{
+        exists = [self _diskImageDataExistsWithKey:key];
+    });
+    
+    return exists;
+}
+
+// Make sure to call form io queue by caller
+- (BOOL)_diskImageDataExistsWithKey:(nullable NSString *)key {
+    if (!key) {
+        return NO;
+    }
     BOOL exists = [_fileManager fileExistsAtPath:[self defaultCachePathForKey:key]];
     
     // fallback because of https://github.com/rs/SDWebImage/pull/976 that added the extension to the disk file name
@@ -253,12 +279,7 @@ FOUNDATION_STATIC_INLINE NSUInteger SDCacheCostForImage(UIImage *image) {
         exists = [_fileManager fileExistsAtPath:[self defaultCachePathForKey:key].stringByDeletingPathExtension];
     }
     
-        if (completionBlock) {
-            dispatch_async(dispatch_get_main_queue(), ^{
-                completionBlock(exists);
-            });
-        }
-    });
+    return exists;
 }
 
 - (nullable UIImage *)imageFromMemoryCacheForKey:(nullable NSString *)key {

SDWebImage/SDImageCacheConfig.h

@@ -29,10 +29,16 @@
 
 /**
  * The reading options while reading cache from disk.
- * Defaults to 0. You can set this to mapped file to improve performance.
+ * Defaults to 0. You can set this to `NSDataReadingMappedIfSafe` to improve performance.
  */
 @property (assign, nonatomic) NSDataReadingOptions diskCacheReadingOptions;
 
+/**
+ * The writing options while writing cache to disk.
+ * Defaults to `NSDataWritingAtomic`. You can set this to `NSDataWritingWithoutOverwriting` to prevent overwriting an existing file.
+ */
+@property (assign, nonatomic) NSDataWritingOptions diskCacheWritingOptions;
+
 /**
  * The maximum length of time to keep an image in the cache, in seconds.
  */

SDWebImage/SDImageCacheConfig.m

@@ -18,6 +18,7 @@ static const NSInteger kDefaultCacheMaxCacheAge = 60 * 60 * 24 * 7; // 1 week
         _shouldDisableiCloud = YES;
         _shouldCacheImagesInMemory = YES;
         _diskCacheReadingOptions = 0;
+        _diskCacheWritingOptions = NSDataWritingAtomic;
         _maxCacheAge = kDefaultCacheMaxCacheAge;
         _maxCacheSize = 0;
     }