listener: ipv6only socket bind support

According to RFC3493 and most Linux distributions, default value is to
work in IPv4-mapped mode. If there is a requirement to bind same port
on same ip addresses but different handlers for both IPv4 and IPv6,
it is required to set IPV6_V6ONLY socket option to be sure that the
code works as expected without affected by bindv6only sysctl setting
in system.

See an example working with this patch:
https://gist.github.com/demirten/023008a63cd966e48b0ebcf9af7fc113

Closes: libevent#640 (cherry-pick)

evutil.c

@@ -385,6 +385,14 @@ evutil_make_listen_socket_reuseable_port(evutil_socket_t sock)
 #endif
 }
 
+int
+evutil_make_listen_socket_ipv6only(evutil_socket_t sock)
+{
+	int one = 1;
+	return setsockopt(sock, IPPROTO_IPV6, IPV6_V6ONLY, (void*) &one,
+	    (ev_socklen_t)sizeof(one));
+}
+
 int
 evutil_make_tcp_listen_socket_deferred(evutil_socket_t sock)
 {

include/event2/listener.h

@@ -97,6 +97,18 @@ typedef void (*evconnlistener_errorcb)(struct evconnlistener *, void *);
  * This is only available on Linux and kernel 3.9+
  */
 #define LEV_OPT_REUSEABLE_PORT		(1u<<7)
+/** Flag: Indicates that the listener wants to work only in IPv6 socket.
+ *
+ * According to RFC3493 and most Linux distributions, default value is to
+ * work in IPv4-mapped mode. If there is a requirement to bind same port
+ * on same ip addresses but different handlers for both IPv4 and IPv6,
+ * it is required to set IPV6_V6ONLY socket option to be sure that the
+ * code works as expected without affected by bindv6only sysctl setting in
+ * system.
+ *
+ * This socket option also supported by Windows.
+ */
+#define LEV_OPT_BIND_IPV6ONLY		(1u<<8)
 
 /**
    Allocate a new evconnlistener object to listen for incoming TCP connections

include/event2/util.h

@@ -426,6 +426,18 @@ int evutil_make_listen_socket_reuseable(evutil_socket_t sock);
 EVENT2_EXPORT_SYMBOL
 int evutil_make_listen_socket_reuseable_port(evutil_socket_t sock);
 
+/** Set ipv6 only bind socket option to make listener work only in ipv6 sockets.
+
+    According to RFC3493 and most Linux distributions, default value for the
+    sockets is to work in IPv4-mapped mode. In IPv4-mapped mode, it is not possible
+    to bind same port from different IPv4 and IPv6 handlers.
+
+    @param sock The socket to make in ipv6only working mode
+    @return 0 on success, -1 on failure
+ */
+EVENT2_EXPORT_SYMBOL
+int evutil_make_listen_socket_ipv6only(evutil_socket_t sock);
+
 /** Do platform-specific operations as needed to close a socket upon a
     successful execution of one of the exec*() functions.
 

listener.c

@@ -245,6 +245,11 @@ evconnlistener_new_bind(struct event_base *base, evconnlistener_cb cb,
 			goto err;
 	}
 
+	if (flags & LEV_OPT_BIND_IPV6ONLY) {
+		if (evutil_make_listen_socket_ipv6only(fd) < 0)
+			goto err;
+	}
+
 	if (sa) {
 		if (bind(fd, sa, socklen)<0)
 			goto err;