[Dovecot] Ambiguous behavior with prefetch database?

Hello,

Let's say dovecot.conf contains:

mail_uid	= dovemailer
mail_gid	= dovemailer
mail_home	= /some/path/%n
mail_location	= mbox:~/mboxes:INBOX=~/mboxes/inbox

and that the password database query is of the form:

password_query = \
SELECT \
	passwd AS password, \
	nickname AS user, \
	mail_home AS userdb_home, \
	mail_location AS userdb_mail, \
WHERE \
	...

The database initially comes with NULL for both mail_home and mail_location, the goal being to be able to progressively replace legacy settings.

With the above, one gets such entries in the logs upon a pop or imap connection:

auth-worker(11262): Debug: auth(u12345678,127.0.0.1): username changed u12345678 -> john.doe
auth: Debug: auth(u12345678,127.0.0.1,<dsNsasbgRQB/AAAB>): username changed u12345678 -> john.doe
[...]
auth: Debug: prefetch(john.doe,127.0.0.1,<dsNsasbgRQB/AAAB>): passdb didn't return userdb entries, trying the next userdb

and, of course, the userdb_query fails since it isn't supposed to be invoked under such circumstances.

Of course, the userdb_query could be adapted so as to handle pop/imap connections in addition to say, lmtp or doveadm connections, but this would anyway raise the question: why bother with a prefetch database setup?

In fact, it seems that the problem comes from the fact that the password_query returns NULL values (i.e. "do not override dovecot.conf settings") for all userdb_xxx settings even if, technically speaking, it returns such columns.

A slight yet somewhat silly modification of the password_query, such as this one:

password_query = \
SELECT \
	passwd AS password, \
	nickname AS user, \
	'dovemailer' AS userdb_uid, \
	mail_home AS userdb_home, \
	mail_location AS userdb_mail, \
WHERE \
	...

indeed seems to bring back all the expected behavior: now, the "passdb returns userdb entries" and, for example, the config's mail_home expands to the expected value /some/path/john.doe.

Could it be that the case "userdb_xxx columns returned, even if all with NULL values" has been somehow overlooked in the code? Or am I erring with my interpetation of all those matters?

TIA, Axel