Allow AysncUDPSocket to work without SO_REUSEADDR flag

Summary: Today it's hardcoded to use the flag, which could result
in some problem. We should allow callers to choose.

Reviewed By: @djwatson

Differential Revision: D2345036

folly/io/async/AsyncUDPSocket.cpp

@@ -63,6 +63,7 @@ void AsyncUDPSocket::bind(const folly::SocketAddress& address) {
                               errno);
   }
 
+  if (reuseAddr_) {
     // put the socket in reuse mode
     int value = 1;
     if (setsockopt(socket,
@@ -74,6 +75,7 @@ void AsyncUDPSocket::bind(const folly::SocketAddress& address) {
                                 "failed to put socket in reuse mode",
                                 errno);
     }
+  }
 
   if (reusePort_) {
     // put the socket in port reuse mode

folly/io/async/AsyncUDPSocket.h

@@ -150,6 +150,14 @@ class AsyncUDPSocket : public EventHandler {
   void setReusePort(bool reusePort) {
     reusePort_ = reusePort;
   }
+
+  /**
+   * Set SO_REUSEADDR flag on the socket. Default is ON.
+   */
+  void setReuseAddr(bool reuseAddr) {
+    reuseAddr_ = reuseAddr;
+  }
+
  private:
   AsyncUDPSocket(const AsyncUDPSocket&) = delete;
   AsyncUDPSocket& operator=(const AsyncUDPSocket&) = delete;
@@ -172,6 +180,7 @@ class AsyncUDPSocket : public EventHandler {
   // Non-null only when we are reading
   ReadCallback* readCallback_;
 
+  bool reuseAddr_{true};
   bool reusePort_{false};
 };