Unable to open included file even it has permissions?

[Robert Scheck]

On Thu, 27 Feb 2020, Ondrej Zajicek wrote:
> You need group foo to access /var/lib/foo/, that is part of secondary
> groups for bird user (as reported by 'id') and these secondary groups are
> assigned e.g. when 'su - bird' is used.
> 
> But when you run 'bird -u bird -g bird', it only set UID (-u) and GID
> (-g), not secondary groups, so bird daemon does not have access to foo
> group. You can check /proc/29334/status to see value of 'Groups'.

Ah, secondary groups! I guess the current behaviour is intended then? And
if so, is this currently documented somewhere (except in the source code)?


Regards,
  Robert