tools: ynl: use MSG_DONTWAIT for getting notifications

To stick to libmnl wrappers in the past we had to use poll()
to check if there are any outstanding notifications on the socket.
This is no longer necessary, we can use MSG_DONTWAIT.
Acked-by: Nicolas Dichtel <nicolas.dichtel@6wind.com>
Link: https://lore.kernel.org/r/20240227223032.1835527-16-kuba@kernel.orgSigned-off-by: Jakub Kicinski <kuba@kernel.org>

tools/net/ynl/lib/ynl.c

@@ -458,16 +458,20 @@ static int ynl_cb_null(const struct nlmsghdr *nlh, struct ynl_parse_arg *yarg)
 	return YNL_PARSE_CB_ERROR;
 }
 
-static int ynl_sock_read_msgs(struct ynl_parse_arg *yarg, ynl_parse_cb_t cb)
+static int
+__ynl_sock_read_msgs(struct ynl_parse_arg *yarg, ynl_parse_cb_t cb, int flags)
 {
 	struct ynl_sock *ys = yarg->ys;
 	const struct nlmsghdr *nlh;
 	ssize_t len, rem;
 	int ret;
 
-	len = recv(ys->socket, ys->rx_buf, YNL_SOCKET_BUFFER_SIZE, 0);
-	if (len < 0)
+	len = recv(ys->socket, ys->rx_buf, YNL_SOCKET_BUFFER_SIZE, flags);
+	if (len < 0) {
+		if (flags & MSG_DONTWAIT && errno == EAGAIN)
+			return YNL_PARSE_CB_STOP;
 		return len;
+	}
 
 	ret = YNL_PARSE_CB_STOP;
 	for (rem = len; rem > 0; NLMSG_NEXT(nlh, rem)) {
@@ -509,6 +513,11 @@ static int ynl_sock_read_msgs(struct ynl_parse_arg *yarg, ynl_parse_cb_t cb)
 	return ret;
 }
 
+static int ynl_sock_read_msgs(struct ynl_parse_arg *yarg, ynl_parse_cb_t cb)
+{
+	return __ynl_sock_read_msgs(yarg, cb, 0);
+}
+
 static int ynl_recv_ack(struct ynl_sock *ys, int ret)
 {
 	struct ynl_parse_arg yarg = { .ys = ys, };
@@ -797,18 +806,8 @@ int ynl_ntf_check(struct ynl_sock *ys)
 	int err;
 
 	do {
-		/* libmnl doesn't let us pass flags to the recv to make
-		 * it non-blocking so we need to poll() or peek() :|
-		 */
-		struct pollfd pfd = { };
-
-		pfd.fd = ys->socket;
-		pfd.events = POLLIN;
-		err = poll(&pfd, 1, 1);
-		if (err < 1)
-			return err;
-
-		err = ynl_sock_read_msgs(&yarg, ynl_ntf_trampoline);
+		err = __ynl_sock_read_msgs(&yarg, ynl_ntf_trampoline,
+					   MSG_DONTWAIT);
 		if (err < 0)
 			return err;
 	} while (err > 0);