Btrfs uuid snapshots: orphaned parent_uuid after deleting intermediate subvol

[Eric Wheeler <btrfs@lists.ewheeler.net>]

Hello all,

If I create three subvolumes like so:

# btrfs subvolume create a
# btrfs subvolume snapshot a b
# btrfs subvolume snapshot b c

I get a parent-child relationship which can be determined like so:

# btrfs subvolume list -uq /home/ |grep [abc]$
parent_uuid - uuid 0e5f473a-d9e5-144a-8f49-1899af7320ad path a
parent_uuid 0e5f473a-d9e5-144a-8f49-1899af7320ad uuid cb4768eb-98e3-5e4c-935d-14f1b97b0de2 path b
parent_uuid cb4768eb-98e3-5e4c-935d-14f1b97b0de2 uuid 5ee8de35-2bab-d642-b5c2-f619e46f65c2 path c

Now if I delete 'b', the parent_uuid of 'c' doesn't change to point at 'a':

# btrfs subvolume delete b
# btrfs subvolume list -uq /home/ |grep [abc]$
parent_uuid - uuid 0e5f473a-d9e5-144a-8f49-1899af7320ad path a
parent_uuid cb4768eb-98e3-5e4c-935d-14f1b97b0de2 uuid 5ee8de35-2bab-d642-b5c2-f619e46f65c2 path c

Notice that 'c' still points at b's UUID, but 'b' is missing and the 
parent_uuid for 'c' wasn't set to '-' as if it were a root node (like 'a').

Is this an inconsistency?  Child parent_uuid's it be updated on delete?

It would be nice to know that 'c' is actually a descendent of 'a', even 
after having deleted 'b'.  Is a way to look that up somehow?


This is running 4.1.15, so its a bit behind.  If this is fixed in a later 
version then please let me know that too.  Thanks!


--
Eric Wheeler