restore: don't race when closing cg yard

TASK_HELPERs are created with CLONE_FILES, so if we always close the cg yard
here, it will close it for the other helpers and cause problems. Instead, we
close it much later, in code only called by alive tasks, to ensure that there
is no conflict.
Signed-off-by: Tycho Andersen <tycho.andersen@canonical.com>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>

cr-restore.c

@@ -1461,14 +1461,6 @@ static int restore_task_with_children(void *_arg)
 	if (create_children_and_session())
 		goto err_fini_mnt;
 
-	/*
-	 * This must be done after forking to allow child
-	 * to get the cgroup fd so it can move into the
-	 * correct /tasks file if it is in a different cgroup
-	 * set than its parent
-	 */
-	close_service_fd(CGROUP_YARD);
-
 	if (restore_task_mnt_ns(current))
 		goto err_fini_mnt;
 

files.c

@@ -949,6 +949,13 @@ int prepare_fds(struct pstree_item *me)
 
 	pr_info("Opening fdinfo-s\n");
 
+	/*
+	 * This must be done after forking to allow child
+	 * to get the cgroup fd so it can move into the
+	 * correct /tasks file if it is in a different cgroup
+	 * set than its parent
+	 */
+	close_service_fd(CGROUP_YARD);
 	close_pid_proc(); /* flush any proc cached fds we may have */
 
 	if (rsti(me)->fdt) {