empty directory from previous subvolume in a snapshot is not sent|received

[Christoph Anton Mitterer <calestyo@scientia.org>]

Hey.

Not sure whether this is a bug or expected.

On my btrfs I have subvolumes like:
  data/
  2023-06-21/
where e.g. data/ contains the root filesystem and 2023-06-21/ is a ro-
snapshot thereof.


When I created 2023-06-21/ from data/, the latter contained another
(rw-)subvolume data/pictures/, which I've deleted (actually: moved out
the files back to data/ and rmdir-ed the now empty subvol... or maybe I
did btrfs subvolume delete - not sure anymore) again after creating the
snapshot.


Now 2023-06-21/ contains an empty (non-subvolume) 2023-06-21/pictures/,
which is expected.


Today I've send|received 2023-06-21/ to another btrfs (at that point,
the original data/pictures/ subvolume was already gone), and diff -qr -
-no-dereference-ed the two afterwards.

Outcome (apart from "differing" files/sockets/block/char special files)
is that the target doesn't contain the empty pictures/ dir.


Not a big problem for me,... but is this expected or some kind of
strange bug?


Thanks,
Chris.