mount: make sure got into MOUNT_DEAD state after a successful umount (#3444)

Without this code the following can happen:
1. Open a file to keep a mount busy
2. Try to stop the corresponding mount unit with systemctl
   -> umount fails and the failure is remembered in mount->result
3. Close the file and umount the filesystem manually
   -> mount_dispatch_io() calls "mount_enter_dead(mount, MOUNT_SUCCESS)"
   -> Old error in mount->result is reused and the mount unit enters a
      failed state

Clear the old error result when 'mountinfo' reports a successful umount to
fix this.

1 files changed, 1 insertions, 0 deletions

diff --git a/src/core/mount.c b/src/core/mount.c
index 665a60bb55..8290ac859d 100644
--- a/src/core/mount.c
+++ b/src/core/mount.c
@@ -1706,6 +1706,7 @@ static int mount_dispatch_io(sd_event_source *source, int fd, uint32_t revents,
                                 /* This has just been unmounted by
                                  * somebody else, follow the state
                                  * change. */
+                                mount->result = MOUNT_SUCCESS;
                                 mount_enter_dead(mount, MOUNT_SUCCESS);
                                 break;