Move fd_ to the initializer list in AsyncSocket constructor

Summary: When constructing a new AsyncSocket, we can use an already connected file descriptor, which we assign to the `fd_` member variable. We move this assignment from the constructor body to the constructor initializer list to avoid a situation where an observer is unaware of an attached fd because `fd_` was assigned after the observer was created.

Reviewed By: yfeldblum

Differential Revision: D30261699

fbshipit-source-id: 5adbc744715355fba08f5430a77c9094e246899d

folly/io/async/AsyncSocket.cpp

@@ -571,6 +571,7 @@ AsyncSocket::AsyncSocket(
     uint32_t zeroCopyBufId,
     const SocketAddress* peerAddress)
     : zeroCopyBufId_(zeroCopyBufId),
+      fd_(fd),
       eventBase_(evb),
       writeTimeout_(this, evb),
       ioHandler_(this, evb, fd),
@@ -578,7 +579,6 @@ AsyncSocket::AsyncSocket(
   VLOG(5) << "new AsyncSocket(" << this << ", evb=" << evb << ", fd=" << fd
           << ", zeroCopyBufId=" << zeroCopyBufId << ")";
   init();
-  fd_ = fd;
   disableTransparentFunctions(fd_, noTransparentTls_, noTSocks_);
   setCloseOnExec();
   state_ = StateEnum::ESTABLISHED;
@@ -621,7 +621,6 @@ void AsyncSocket::init() {
   shutdownFlags_ = 0;
   state_ = StateEnum::UNINIT;
   eventFlags_ = EventHandler::NONE;
-  fd_ = NetworkSocket();
   sendTimeout_ = 0;
   maxReadsPerEvent_ = 16;
   connectCallback_ = nullptr;