Comment 6 for bug 1831010

We can construct tree of mount points, based on their mount-id and parent-id numbers and recursively detach the leaves until we reach the point that we wanted to unmount.

In pseudo python code:

func detach(path, mount_id=None):
   # Get a fresh mountinfo table
   # Due to mount event propagation using a cached copy
   # is probably impossible without correctly re-implementing
   # kernel propagation logic.
   mi = mountinfo()
   # If we don't know the mount_id of the path we want to detach
   # we must scan the mount table in reverse, since the last entry
   # is the most recent one.
   if mount_id is None:
     for mie in reversed(mi.entries):
       if mie.mount_point == path:
          mount_id = mie.mount_id
          break
     else:
       raise Exception(f"{path} is not a mount point")
   # Construct a tree where each mount info entry knows about
   # the children based on parent-id, mount-id association.
   tree = treeify(mi)
   # Find the node we want to detach
   node = tree.find(mount_id=mount_id)
   assert node.mount_point == path
   # Detach each child, those will only be the mount entries
   # that are immediately underneath this mount point.
   for child in node.children():
      detach(child.mount_point, child.mount_id)
   umount2(node.mount_point, MS_DETACH)