logind: add missing check for conflicting operation v.s. scheduled shutdown

> We don't want to shutdown while a suspend is running, and vice versa.
> This would be confusing and could lead to data loss in the worst case.

https://bugs.launchpad.net/ubuntu/+source/elogind/+bug/1441253/comments/4

According to the above comment, if the conflicting operation is hung,
we don't want to force things when the admin has not passed a force option.

Similarly if you're not an admin, you probably shouldn't get to sneak
around this check by using a scheduled shutdown instead of an unscheduled
one.  (And no-one so far thought it necessary to add such a permission in
PolKit).

Note that if the conflicting operation was _not_ hung, and we lost the
race with suspend, the system might not have shut down at the scheduled
time anyway.  Which is no good if you were scheduling a power outage.
And scheduling a shutdown for an arbitrary time when the system is resumed,
does not seem a very useful semantic.  More likely, scheduled shutdowns are
useful on systems which do not use suspend, such as multi-user servers.
(In which case even PolKit defaults likely don't let the users trigger
suspend).

1 files changed, 8 insertions, 2 deletions

diff --git a/src/login/logind-dbus.c b/src/login/logind-dbus.c
index d5a6a21a6..3e231ac3d 100644
--- a/src/login/logind-dbus.c
+++ b/src/login/logind-dbus.c
@@ -1993,9 +1993,15 @@ static int manager_scheduled_shutdown_handler(
         else
                 target = SPECIAL_REBOOT_TARGET;
 
-        r = execute_shutdown_or_sleep(m, 0, target, &error);
+        /* Don't allow multiple jobs being executed at the same time */
+        if (m->action_what) {
+                log_error("Scheduled shutdown to %s failed: shutdown or sleep operation already in progress", target);
+                return -EALREADY;
+        }
+
+        r = execute_shutdown_or_sleep(m, INHIBIT_SHUTDOWN, target, &error);
         if (r < 0)
-                return log_error_errno(r, "Unable to execute transition to %s: %m", target);
+                return log_error_errno(r, "Scheduled shutdown to %s failed: %m", target);
 
         return 0;
 }