I had a conversation about this on OFTC #tor:
Is it intended that tor doesn't set supplementary groups when starting
as root and then doing setgid? This describes my issue:
https://tor.stackexchange.com/questions/23225/tor-cant-read-hiddenserviceport-unix-socket-through-group-permissions-when-s
Permissions are set so tor should be able to access through the
`postfix-test-queue` user:
> $ sudo ls -l /var/spool/postfix-test/public/smtpd
> srw-rw-rw- 1 postfix-test postfix-test 0 █
> /var/spool/postfix-test/public/smtpd
> $ sudo ls -l /var/spool/postfix-test
> # ...
> drwx--x--- 2 pos