folly: add freebind to async udp socket

Summary:
This is useful if we want to bind to a nonlocal address or an address that doesn't exist yet.

       IP_FREEBIND (since Linux 2.4)
              If enabled, this boolean option allows binding to an IP
              address that is nonlocal or does not (yet) exist.  This
              permits listening on a socket, without requiring the
              underlying network interface or the specified dynamic IP
              address to be up at the time that the application is
              trying to bind to it.  This option is the per-socket
              equivalent of the ip_nonlocal_bind /proc interface
              described below.

Reviewed By: yfeldblum, mjoras

Differential Revision: D30903919

fbshipit-source-id: 09058cac1c14a4003525b0a7c5381ea301ca28da

folly/io/async/AsyncUDPSocket.cpp

@@ -140,6 +140,26 @@ void AsyncUDPSocket::init(sa_family_t family, BindOptions bindOptions) {
     }
   }
 
+  if (freeBind_) {
+    int optname = 0;
+#if defined(IP_FREEBIND)
+    optname = IP_FREEBIND;
+#endif
+    if (!optname) {
+      throw AsyncSocketException(
+          AsyncSocketException::NOT_OPEN, "IP_FREEBIND is not supported");
+    }
+    // put the socket in free bind mode
+    int value = 1;
+    if (netops::setsockopt(
+            socket, IPPROTO_IP, optname, &value, sizeof(value)) != 0) {
+      throw AsyncSocketException(
+          AsyncSocketException::NOT_OPEN,
+          "failed to put socket in free bind mode",
+          errno);
+    }
+  }
+
   if (busyPollUs_ > 0) {
     int optname = 0;
 #if defined(SO_BUSY_POLL)

folly/io/async/AsyncUDPSocket.h

@@ -336,6 +336,12 @@ class AsyncUDPSocket : public EventHandler {
     return fd_;
   }
 
+  /**
+   * Set IP_FREEBIND to allow binding to an address that is nonlocal or doesn't
+   * exist yet.
+   */
+  virtual void setFreeBind(bool freeBind) { freeBind_ = freeBind; }
+
   /**
    * Set reuse port mode to call bind() on the same address multiple times
    */
@@ -551,6 +557,7 @@ class AsyncUDPSocket : public EventHandler {
 
   bool reuseAddr_{false};
   bool reusePort_{false};
+  bool freeBind_{false};
   int rcvBuf_{0};
   int sndBuf_{0};
   int busyPollUs_{0};