page-xfer: make connect_to_page_server return socket fd

It will used by lazy-pages daemon to enable polling for reception of page
data from remote dump

travis-ci: success for uffd: A new set of improvements
Signed-off-by: Mike Rapoport <rppt@linux.vnet.ibm.com>
Signed-off-by: Pavel Emelyanov <xemul@virtuozzo.com>

criu/cr-dump.c

@@ -1565,7 +1565,7 @@ int cr_pre_dump_tasks(pid_t pid)
 	if (vdso_init())
 		goto err;
 
-	if (connect_to_page_server())
+	if (connect_to_page_server() < 0)
 		goto err;
 
 	if (setup_alarm_handler())
@@ -1761,7 +1761,7 @@ int cr_dump_tasks(pid_t pid)
 			goto err;
 	}
 
-	if (connect_to_page_server())
+	if (connect_to_page_server() < 0)
 		goto err;
 
 	if (setup_alarm_handler())

criu/page-xfer.c

@@ -906,7 +906,7 @@ out:
 	 * on urgent data is the smartest mode ever.
 	 */
 	tcp_cork(page_server_sk, true);
-	return 0;
+	return page_server_sk;
 }
 
 int disconnect_from_page_server(void)

criu/uffd.c

@@ -847,6 +847,17 @@ close_uffd:
 	return -1;
 }
 
+static int prepare_page_server_socket(void)
+{
+	int sk;
+
+	sk = connect_to_page_server();
+	if (sk < 0)
+		return -1;
+
+	return 0;
+}
+
 int cr_lazy_pages(bool daemon)
 {
 	struct epoll_event *events;
@@ -894,7 +905,7 @@ int cr_lazy_pages(bool daemon)
 	if (prepare_uffds(lazy_sk, epollfd))
 		return -1;
 
-	if (connect_to_page_server())
+	if (prepare_page_server_socket())
 		return -1;
 
 	ret = handle_requests(epollfd, events);