Fixes race condition of Network module (facebook#25156)

Summary:
There exists race condition in `sendRequest:withDelegate:` method, it can do the session creation multiple times, because we don't lock that, which would leads `EXC_BAD_ACCESS` because use and deallocated session concurrently, we can refer to how to create a singleton safely.

Related facebook#25152.

## Changelog

[iOS] [Fixed] - Fixes race condition of Network module
Pull Request resolved: facebook#25156

Differential Revision: D15671734

Pulled By: sammy-SC

fbshipit-source-id: 5021e6cf33c2b55e3f7adf573ab5c8e6a8d82e23

Author: zhongwuzw

Libraries/Network/RCTHTTPRequestHandler.mm

@@ -29,12 +29,12 @@ @implementation RCTHTTPRequestHandler
 
 - (void)invalidate
 {
-  dispatch_async(self->_methodQueue, ^{
-    [self->_session invalidateAndCancel];
-    self->_session = nil;
-  });
+  std::lock_guard<std::mutex> lock(_mutex);
+  [self->_session invalidateAndCancel];
+  self->_session = nil;
 }
 
+// Needs to lock before call this method.
 - (BOOL)isValid
 {
   // if session == nil and delegates != nil, we've been invalidated
@@ -58,6 +58,7 @@ - (BOOL)canHandleRequest:(NSURLRequest *)request
 - (NSURLSessionDataTask *)sendRequest:(NSURLRequest *)request
                          withDelegate:(id<RCTURLRequestDelegate>)delegate
 {
+  std::lock_guard<std::mutex> lock(_mutex);
   // Lazy setup
   if (!_session && [self isValid]) {
     // You can override default NSURLSession instance property allowsCellularAccess (default value YES)
@@ -83,19 +84,12 @@ - (NSURLSessionDataTask *)sendRequest:(NSURLRequest *)request
                                              delegate:self
                                         delegateQueue:callbackQueue];
 
-    std::lock_guard<std::mutex> lock(_mutex);
     _delegates = [[NSMapTable alloc] initWithKeyOptions:NSPointerFunctionsStrongMemory
                                            valueOptions:NSPointerFunctionsStrongMemory
                                                capacity:0];
   }
-  __block NSURLSessionDataTask *task = nil;
-  dispatch_sync(self->_methodQueue, ^{
-    task = [self->_session dataTaskWithRequest:request];
-  });
-  {
-    std::lock_guard<std::mutex> lock(_mutex);
-    [_delegates setObject:delegate forKey:task];
-  }
+  NSURLSessionDataTask *task = [_session dataTaskWithRequest:request];
+  [_delegates setObject:delegate forKey:task];
   [task resume];
   return task;
 }