postfix, dovecot-lda, /run/dovecot/stats-writer socket permision and local user delivery, again

[Michael Tokarev]

Hi!

There are many questions on the 'net about the same theme:
lda(mjt): Error: net_connect_unix(/run/dovecot/stats-writer) failed: Permission denied

dovecot-lda is run from postfix as mailbox_command for local users.

There's even a suggestion on dovecot wiki to make dovecot-lda setgid
(to mail) - for a different reason but people use this trick to shut
this error message up.  See eg the last message at
https://forum.zentyal.org/index.php?topic=35236.0 which points to
https://wiki2.dovecot.org/LDA/Postfix .

But the thing is: mail delivery stops right after making dovecot-lda
setgid, like this:

Feb 22 00:06:46 tsrv dovecot: lda(mjt)<253650><s+nnHub+E2LS3gMA7tuOMA>: \
  Fatal: setresgid(914(dovecot),914(dovecot),8(mail)) failed with euid=1000(mjt): \
  Operation not permitted

why the hell lda tries to setuid to dovecot user?
It will definitely unable to write to the maildir since it is owned by
user mjt, not by user dovecot!

And I *only* made it setgid, not setuid.

How to use dovecot-lda together with postfix, after all?

Thanks,

/mjt