udp: Polish udp_vu_sock_info() and remove from vu specific code

udp_vu_sock_info() uses MSG_PEEK to look ahead at the next datagram to be
received and gets its source address.  Currently we only use it in the
vhost-user path, but there's nothing inherently vhost-user specific about
it.  We have upcoming uses for it elsewhere so rename and move to udp.c.

While we're there, polish its error reporting a litle.

Signed-off-by: David Gibson <david@gibson.dropbear.id.au>
[sbrivio: Drop excess newline before udp_sock_recv()]
Signed-off-by: Stefano Brivio <sbrivio@redhat.com>

udp.c

@@ -629,6 +629,30 @@ static int udp_sock_errs(const struct ctx *c, union epoll_ref ref)
 	return n_err;
 }
 
+/**
+ * udp_peek_addr() - Get source address for next packet
+ * @s:		Socket to get information from
+ * @src:	Socket address (output)
+ *
+ * Return: 0 on success, -1 otherwise
+ */
+int udp_peek_addr(int s, union sockaddr_inany *src)
+{
+	struct msghdr msg = {
+		.msg_name = src,
+		.msg_namelen = sizeof(*src),
+	};
+	int rc;
+
+	rc = recvmsg(s, &msg, MSG_PEEK | MSG_DONTWAIT);
+	if (rc < 0) {
+		if (errno != EAGAIN && errno != EWOULDBLOCK)
+			warn_perror("Error peeking at socket address");
+		return rc;
+	}
+	return 0;
+}
+
 /**
  * udp_sock_recv() - Receive datagrams from a socket
  * @c:		Execution context

udp_internal.h

@@ -30,5 +30,6 @@ size_t udp_update_hdr4(struct iphdr *ip4h, struct udp_payload_t *bp,
 size_t udp_update_hdr6(struct ipv6hdr *ip6h, struct udp_payload_t *bp,
                        const struct flowside *toside, size_t dlen,
 		       bool no_udp_csum);
+int udp_peek_addr(int s, union sockaddr_inany *src);
 
 #endif /* UDP_INTERNAL_H */

udp_vu.c

@@ -57,23 +57,6 @@ static size_t udp_vu_hdrlen(bool v6)
 	return hdrlen;
 }
 
-/**
- * udp_vu_sock_info() - get socket information
- * @s:		Socket to get information from
- * @s_in:	Socket address (output)
- *
- * Return: 0 if socket address can be read, -1 otherwise
- */
-static int udp_vu_sock_info(int s, union sockaddr_inany *s_in)
-{
-	struct msghdr msg = {
-		.msg_name = s_in,
-		.msg_namelen = sizeof(union sockaddr_inany),
-	};
-
-	return recvmsg(s, &msg, MSG_PEEK | MSG_DONTWAIT);
-}
-
 /**
  * udp_vu_sock_recv() - Receive datagrams from socket into vhost-user buffers
  * @c:		Execution context
@@ -230,7 +213,7 @@ void udp_vu_listen_sock_data(const struct ctx *c, union epoll_ref ref,
 		int iov_used;
 		bool v6;
 
-		if (udp_vu_sock_info(ref.fd, &s_in) < 0)
+		if (udp_peek_addr(ref.fd, &s_in) < 0)
 			break;
 
 		sidx = udp_flow_from_sock(c, ref, &s_in, now);