userdb prefetch doesn't work with imapc (2.2.18)

[Nagy, Attila]

Hi,

I have the following config in dovecot 2.2.18:
# doveconf -n userdb passdb
userdb {
   driver = prefetch
}
passdb {
   args = host=127.0.0.1 port=1430
   default_fields = userdb_imapc_user=%u userdb_imapc_password=%w
   driver = imap
}

I guess it should cause a single login to the backend IMAP server when 
issuing a LOGIN to dovecot.
Yet, dovecot opens two TCP sessions. In the first, it issues a LOGIN and 
in the second it logs in again (LOGIN) and issues LIST "" "".

At the end (after LOGIN succeeds to dovecot), only one of the two 
connections remain open to the backend server.

Any ideas about this?

Thanks,