What are parents? 'print parent ID' always matches top level

[Chris Murphy <lists@colorremedies.com>]

I'm confused by the use of parent in this context. Do we have more
than one kind of "parent"?

Example below shows four subvolumes (not snapshots), and the last one
is a snapshot of the first.

# btrfs sub list -p /mnt
ID 257 gen 1414 parent 5 top level 5 path root
ID 258 gen 1351 parent 5 top level 5 path home
ID 259 gen 139 parent 5 top level 5 path boot
ID 262 gen 1407 parent 257 top level 257 path root/var/lib/machines
ID 264 gen 1413 parent 5 top level 5 path rootsnap


If this "parent" has anything to do with snapshots, I'd expect the
last line to read:
ID 264 gen 1413 parent 257 top level 5 path rootsnap


And for that matter I don't know what I expect for the fourth line:
ID 262 gen 1407 parent ? top level 257 path root/var/lib/machines

It has no parent, none of the non-snapshot subvolumes do. I don't
think it's OK to repurpose "parent" if it has two contextual meanings.


Chris Murphy