Dear Maintainer,

I installed a fresh buster for reproduction.
I tested with cyrus-imapd version "3.0.8" from buster and "3.0.11" from stable-backports.

Create user "imapuser" with password "password"
User created 3 folders:

- INBOX/foo
- INBOX/foo/1
- INBOX/foo-1

imap command '. list "" * ' marks folder "foo" as HasNoChildren.

root@debian-buster:~# telnet localhost 143
Trying ::1...
Connected to localhost.
Escape character is '^]'.
* OK [CAPABILITY IMAP4rev1 LITERAL+ ID ENABLE AUTH=DIGEST-MD5 AUTH=CRAM-MD5 AUTH=NTLM AUTH=LOGIN AUTH=PLAIN SASL-IR] debian-buster Cyrus IMAP 3.0.11-Debian-3.0.11-1~bpo10+1 server ready
. login testuser password
. OK [CAPABILITY IMAP4rev1 LITERAL+ ID ENABLE ACL RIGHTS=kxten QUOTA MAILBOX-REFERRALS NAMESPACE UIDPLUS NO_ATOMIC_RENAME UNSELECT CHILDREN MULTIAPPEND BINARY CATENATE CONDSTORE ESEARCH SEARCH=FUZZY SORT SORT=MODSEQ SORT=DISPLAY SORT=UID THREAD=ORDEREDSUBJECT THREAD=REFERENCES THREAD=REFS ANNOTATEMORE ANNOTATE-EXPERIMENT-1 METADATA LIST-EXTENDED LIST-STATUS LIST-MYRIGHTS LIST-METADATA WITHIN QRESYNC SCAN XLIST XMOVE MOVE SPECIAL-USE CREATE-SPECIAL-USE DIGEST=SHA1 X-REPLICATION URLAUTH URLAUTH=BINARY LOGINDISABLED COMPRESS=DEFLATE X-QUOTA=STORAGE X-QUOTA=MESSAGE X-QUOTA=X-ANNOTATION-STORAGE X-QUOTA=X-NUM-FOLDERS IDLE] User logged in SESSIONID=<cyrus-6552-1567434229-1-698549360105201265>
. create INBOX.foo
. OK Completed
. create INBOX.foo.1
. OK Completed
. create INBOX.foo-1
. OK Completed
. list "" *
* LIST (\HasChildren) "." INBOX
* LIST (\HasNoChildren) "." INBOX.foo
* LIST (\HasNoChildren) "." INBOX.foo-1
* LIST (\HasNoChildren) "." INBOX.foo.1
. OK Completed (0.000 secs 4 calls)
. list "" INBOX.%
* LIST (\HasNoChildren) "." INBOX.foo
* LIST (\HasNoChildren) "." INBOX.foo-1
* LIST (\Noselect \HasChildren) "." INBOX.foo
. OK Completed (0.000 secs 3 calls)
. list "" INBOX.foo
* LIST (\HasChildren) "." INBOX.foo
. OK Completed (0.000 secs 2 calls)

Because of the "\HasNoChildren" some clients fail to select subfolder INBOX.foo.1

The code seems to depend on the order of mailboxes and sets "HasChildren" on the previous Entry if the current entry is a child.

Response from GitHub:



The problem is that the internal mailbox separator is ASCII '.', which
sorts /after/ valid mailbox characters like '-' and space. It's fixed by
enabling the following setting in imapd.conf:

    |improved_mboxlist_sort: 0 If enabled, a special comparator will be
    used which will correctly sort mailbox names that contain characters
    such as ' ' and '-'. Note that this option SHOULD NOT be changed on
    a live system. The mail‐ boxes database should be dumped
    (ctl_mboxlist) before the option is changed, removed, and then
    undumped after changing the option. When not using flat files for
    the subscriptions databases the same has to be done (cyr_dbtool) for
    each subscription database See improved_mboxlist_sort.html. |

Ideally this setting would default to enabled, or indeed, be the only
behaviour (with no option at all). But it has ramifications for existing
deployments, so we can't just change it. I assume that it's not simply
enabled in the imapd.conf that the Debian package ships for much the
same reason.

The "improved_mboxlist_sort.html" referenced in the man page seems to be
https://www.cyrusimap.org/imap/developer/thoughts/improved_mboxlist_sort.html

The problem they're seeing (as well as other similar issues between
similarly-named folders that differ by a character that sorts before
ASCII '.') are unfixable in stable releases without enabling this option.

I don't understand all the ramifications of an admin changing the
improved_mboxlist_sort option. It might be as "simple" as the
documentation suggests (dumping and undumping mailboxes and subscription
databases) or it might be more complicated these days. I'd suggest they
join the info-cyrus list
<https://lists.andrew.cmu.edu/mailman/listinfo/info-cyrus> and see if
anyone has been through this process recently and has guidance to offer.