Fix for crashes due to dangerous NSMutableDictionary access in AFHTTPRequestSerialiser, added a dispatch queue for safety in multithreaded environments.

AFNetworking/AFURLRequestSerialization.m

@@ -189,6 +189,7 @@ static void *AFHTTPRequestSerializerObserverContext = &AFHTTPRequestSerializerOb
 @interface AFHTTPRequestSerializer ()
 @property (readwrite, nonatomic, strong) NSMutableSet *mutableObservedChangedKeyPaths;
 @property (readwrite, nonatomic, strong) NSMutableDictionary *mutableHTTPRequestHeaders;
+@property (readwrite, nonatomic, strong) dispatch_queue_t requestHeaderModificationQueue;
 @property (readwrite, nonatomic, assign) AFHTTPRequestQueryStringSerializationStyle queryStringSerializationStyle;
 @property (readwrite, nonatomic, copy) AFQueryStringSerializationBlock queryStringSerialization;
 @end
@@ -208,6 +209,7 @@ static void *AFHTTPRequestSerializerObserverContext = &AFHTTPRequestSerializerOb
     self.stringEncoding = NSUTF8StringEncoding;
 
     self.mutableHTTPRequestHeaders = [NSMutableDictionary dictionary];
+    self.requestHeaderModificationQueue = dispatch_queue_create("requestHeaderModificationQueue", DISPATCH_QUEUE_CONCURRENT);
 
     // Accept-Language HTTP Header; see http://www.w3.org/Protocols/rfc2616/rfc2616-sec14.html#sec14.4
     NSMutableArray *acceptLanguagesComponents = [NSMutableArray array];
@@ -306,17 +308,27 @@ static void *AFHTTPRequestSerializerObserverContext = &AFHTTPRequestSerializerOb
 #pragma mark -
 
 - (NSDictionary *)HTTPRequestHeaders {
-    return [NSDictionary dictionaryWithDictionary:self.mutableHTTPRequestHeaders];
+    NSDictionary __block *value;
+    dispatch_sync(self.requestHeaderModificationQueue, ^{
+        value = [NSDictionary dictionaryWithDictionary:self.mutableHTTPRequestHeaders];
+    });
+    return value;
 }
 
 - (void)setValue:(NSString *)value
 forHTTPHeaderField:(NSString *)field
 {
-	[self.mutableHTTPRequestHeaders setValue:value forKey:field];
+    dispatch_barrier_async(self.requestHeaderModificationQueue, ^{
+        [self.mutableHTTPRequestHeaders setValue:value forKey:field];
+    });
 }
 
 - (NSString *)valueForHTTPHeaderField:(NSString *)field {
-    return [self.mutableHTTPRequestHeaders valueForKey:field];
+    NSString __block *value;
+    dispatch_sync(self.requestHeaderModificationQueue, ^{
+        value = [self.mutableHTTPRequestHeaders valueForKey:field];
+    });
+    return value;
 }
 
 - (void)setAuthorizationHeaderFieldWithUsername:(NSString *)username
@@ -328,7 +340,9 @@ forHTTPHeaderField:(NSString *)field
 }
 
 - (void)clearAuthorizationHeader {
-	[self.mutableHTTPRequestHeaders removeObjectForKey:@"Authorization"];
+    dispatch_barrier_async(self.requestHeaderModificationQueue, ^{
+        [self.mutableHTTPRequestHeaders removeObjectForKey:@"Authorization"];
+    });
 }
 
 #pragma mark -
@@ -560,7 +574,9 @@ forHTTPHeaderField:(NSString *)field
 }
 
 - (void)encodeWithCoder:(NSCoder *)coder {
-    [coder encodeObject:self.mutableHTTPRequestHeaders forKey:NSStringFromSelector(@selector(mutableHTTPRequestHeaders))];
+    dispatch_sync(self.requestHeaderModificationQueue, ^{
+        [coder encodeObject:self.mutableHTTPRequestHeaders forKey:NSStringFromSelector(@selector(mutableHTTPRequestHeaders))];
+    });
     [coder encodeInteger:self.queryStringSerializationStyle forKey:NSStringFromSelector(@selector(queryStringSerializationStyle))];
 }
 
@@ -568,7 +584,9 @@ forHTTPHeaderField:(NSString *)field
 
 - (instancetype)copyWithZone:(NSZone *)zone {
     AFHTTPRequestSerializer *serializer = [[[self class] allocWithZone:zone] init];
-    serializer.mutableHTTPRequestHeaders = [self.mutableHTTPRequestHeaders mutableCopyWithZone:zone];
+    dispatch_sync(self.requestHeaderModificationQueue, ^{
+        serializer.mutableHTTPRequestHeaders = [self.mutableHTTPRequestHeaders mutableCopyWithZone:zone];
+    });
     serializer.queryStringSerializationStyle = self.queryStringSerializationStyle;
     serializer.queryStringSerialization = self.queryStringSerialization;