RENAME of a mailbox under its own child returns OK but stores it at a third name #873

maddy 0.9.6 accepts RENAME of a mailbox to a name under itself, then
the mailbox is at neither the old name nor the requested name.

After LOGIN as the mailbox owner:

a1 CREATE w58d
a1 OK CREATE completed

a2 APPEND w58d {22+}
Subject: cyclemarker

m
a2 OK APPEND completed

a3 RENAME w58d w58d.child
a3 OK RENAME completed

a4 SELECT w58d
a4 NO No such mailbox

a4 SELECT w58d.child
a4 NO No such mailbox

a4 SELECT w58d.child.child
* 1 EXISTS
a4 OK [READ-WRITE] SELECT completed

The message is still there, under w58d.child.child, a name the client
never asked for. Dovecot 2.3.19.1 rejects the same RENAME with
"NO Can't rename mailbox under its own child".

The untagged "* BAD imap: cannot parse command: no enough fields" in
my transcript is from a NOOP I sent while reading the APPEND reply.
RENAME itself returned OK.

Please reject a target that is under the source (source is a prefix of
the target, followed by the hierarchy delimiter) with NO. A successful
RENAME should leave the mailbox at the requested name.

Drnbnonono