[gparted] Fix unmount error when unmounting below a bind mount point (!89)

[Mike Fleetwood <mfleetwo src gnome org>]

commit 6f811cfaca4d8b0e8cad1dfd1bd81c336fc8528b
Author: Movie Ma <mawei uniontech com>
Date:   Fri Aug 20 12:33:12 2021 +0000

    Fix unmount error when unmounting below a bind mount point (!89)
    
    Bind mounts duplicate part of the file system hierarchy to an additional
    mount point [1].  When mounting and unmounting a second file system
    below a duplicating bind mount Linux automatically presents this lower
    file system as being mounted multiple times.  GParted displays these
    multiple mount points.  However using GParted to unmount this lower file
    system reports an error that the second mount point is no longer
    mounted, because all were unmounted by the first unmount command.
    
    Setup:
    
    1.  Mount an upper file system
    
        # mkdir /mnt/1
        # mount /dev/sdb1 /mnt/1
        # fgrep sdb /proc/mounts
        /dev/sdb1 /mnt/1 ext4 rw,seclabel,relatime,data=ordered 0 0
    
    2.  Bind mount it to a second directory
    
        # mkdir /mnt/b1
        # mount --bind /mnt/1 /mnt/b1
        # fgrep sdb /proc/mounts
        /dev/sdb1 /mnt/1 ext4 rw,seclabel,relatime,data=ordered 0 0
        /dev/sdb1 /mnt/b1 ext4 rw,seclabel,relatime,data=ordered 0 0
    
    3.  Mount a file system below the first
    
        # mkdir /mnt/1/lower
        # mount /dev/sdb2 /mnt/1/lower
        # fgrep sdb /proc/mounts
        /dev/sdb1 /mnt/1 ext4 rw,seclabel,relatime,data=ordered 0 0
        /dev/sdb1 /mnt/b1 ext4 rw,seclabel,relatime,data=ordered 0 0
        /dev/sdb2 /mnt/1/lower xfs rw,seclabel,relatime,attr2,inode64,noquota 0 0
        /dev/sdb2 /mnt/b1/lower xfs rw,seclabel,relatime,attr2,inode64,noquota 0 0
    
        Two mount records for sdb2 were added to /proc/mounts from one mount
        command.
    
    Then use GParted to unmount sdb2.  It reports this error dialog:
        +-------------------------------------+
        |                                     |
        | Could not unmount /dev/sdb2         |
        |                                     |
        | # umount -v '/mnt/b1/lower'         |
        | umount: /mnt/b1/lower: not mounted. |
        +-------------------------------------+
        |            [  OK  ]                 |
        +-------------------------------------+
    
    Fix by checking that the file system is still mounted before each
    unmount attempt.
    
    [1] mount (8), Description, Bind mount operation
        https://man7.org/linux/man-pages/man8/mount.8.html#DESCRIPTION
    
    Closes !89 - Fix unmount error when unmounting below a bind mount point

 src/Win_GParted.cc | 10 ++++++++--
 1 file changed, 8 insertions(+), 2 deletions(-)
---
diff --git a/src/Win_GParted.cc b/src/Win_GParted.cc
index 75ea445e..fcb086eb 100644
--- a/src/Win_GParted.cc
+++ b/src/Win_GParted.cc
@@ -2847,11 +2847,17 @@ bool Win_GParted::unmount_partition( const Partition & partition, Glib::ustring
                }
                else
                {
+                       Glib::ustring checkmount = "grep -w " + Glib::shell_quote(fs_mountpoints[i]) + " 
/proc/mounts";
                        Glib::ustring cmd = "umount -v " + Glib::shell_quote( fs_mountpoints[i] );
                        Glib::ustring dummy;
                        Glib::ustring umount_error;
-                       if ( Utils::execute_command( cmd, dummy, umount_error ) )
-                               umount_errors.push_back( "# " + cmd + "\n" + umount_error );
+
+                       // Check mount point is still mounted
+                       if (! Utils::execute_command(checkmount, dummy, umount_error))
+                       {
+                               if (Utils::execute_command(cmd, dummy, umount_error))
+                                       umount_errors.push_back("# " + cmd + "\n" + umount_error);
+                       }
                }
        }