Allow disabling of IPV6_ONLY for AsyncUDPSocket

Summary: It's useful to be able to use the v4-mapped addresses sometimes.

Reviewed By: avasylev

Differential Revision: D24371807

fbshipit-source-id: 74fe13fa4bef9c77cc51a18137559facda9bdbeb

folly/io/async/AsyncUDPSocket.cpp

@@ -94,7 +94,7 @@ AsyncUDPSocket::~AsyncUDPSocket() {
   }
 }
 
-void AsyncUDPSocket::init(sa_family_t family) {
+void AsyncUDPSocket::init(sa_family_t family, BindOptions bindOptions) {
   NetworkSocket socket =
       netops::socket(family, SOCK_DGRAM, family != AF_UNIX ? IPPROTO_UDP : 0);
   if (socket == NetworkSocket()) {
@@ -184,9 +184,8 @@ void AsyncUDPSocket::init(sa_family_t family) {
     }
   }
 
-  // If we're using IPv6, make sure we don't accept V4-mapped connections
   if (family == AF_INET6) {
-    int flag = 1;
+    int flag = static_cast<int>(bindOptions.bindV6Only);
     if (netops::setsockopt(
             socket, IPPROTO_IPV6, IPV6_V6ONLY, &flag, sizeof(flag))) {
       throw AsyncSocketException(
@@ -203,8 +202,10 @@ void AsyncUDPSocket::init(sa_family_t family) {
   EventHandler::changeHandlerFD(fd_);
 }
 
-void AsyncUDPSocket::bind(const folly::SocketAddress& address) {
-  init(address.getFamily());
+void AsyncUDPSocket::bind(
+    const folly::SocketAddress& address,
+    BindOptions bindOptions) {
+  init(address.getFamily(), bindOptions);
 
   // bind to the address
   sockaddr_storage addrStorage;
@@ -227,7 +228,7 @@ void AsyncUDPSocket::bind(const folly::SocketAddress& address) {
 void AsyncUDPSocket::connect(const folly::SocketAddress& address) {
   // not bound yet
   if (fd_ == NetworkSocket()) {
-    init(address.getFamily());
+    init(address.getFamily(), BindOptions());
   }
 
   sockaddr_storage addrStorage;

folly/io/async/AsyncUDPSocket.h

@@ -160,13 +160,24 @@ class AsyncUDPSocket : public EventHandler {
     return localAddress_;
   }
 
+  /**
+   * Contains options to pass to bind.
+   */
+  struct BindOptions {
+    constexpr BindOptions() noexcept {}
+    // Whether IPV6_ONLY should be set on the socket.
+    bool bindV6Only{true};
+  };
+
   /**
    * Bind the socket to the following address. If port is not
    * set in the `address` an ephemeral port is chosen and you can
    * use `address()` method above to get it after this method successfully
-   * returns.
+   * returns. The parameter BindOptions contains parameters for the bind.
    */
-  virtual void bind(const folly::SocketAddress& address);
+  virtual void bind(
+      const folly::SocketAddress& address,
+      BindOptions options = BindOptions());
 
   /**
    * Connects the UDP socket to a remote destination address provided in
@@ -475,7 +486,7 @@ class AsyncUDPSocket : public EventHandler {
   AsyncUDPSocket(const AsyncUDPSocket&) = delete;
   AsyncUDPSocket& operator=(const AsyncUDPSocket&) = delete;
 
-  void init(sa_family_t family);
+  void init(sa_family_t family, BindOptions bindOptions);
 
   // EventHandler
   void handlerReady(uint16_t events) noexcept override;

folly/io/async/test/MockAsyncUDPSocket.h

@@ -27,7 +27,9 @@ struct MockAsyncUDPSocket : public AsyncUDPSocket {
   ~MockAsyncUDPSocket() override {}
 
   MOCK_CONST_METHOD0(address, const SocketAddress&());
-  MOCK_METHOD1(bind, void(const SocketAddress&));
+  MOCK_METHOD2(
+      bind,
+      void(const SocketAddress&, AsyncUDPSocket::BindOptions bindOptions));
   MOCK_METHOD2(setFD, void(NetworkSocket, AsyncUDPSocket::FDOwnership));
   MOCK_METHOD2(
       write,