[Nix-dev] running a daemon as a user

[Mathijs Kwik]

Hi all,

I can use some hints on running a daemon as a user.
Nixos creates the user, and the job.preStart makes sure the directory
below /var/lib is owned by that user.

The daemon itself doesn't have support for specifying a user/group, so I use:
su username -c "mydaemon"

This works fine, but as a consequence, I have 1 extra process (su)
running the whole time.
So I tried:
su username -c "mydaemon &"

Which takes care of the extra process, but now, my daemon process is
the result of 3 forks, so upstart isn't able to track it and ends up
with the wrong pid.

I think this is somewhat strange, because without the &, the daemon
runs as the first fork (first process is "su", next is my daemon). So
I don't fully understand why & causes 2 extra forks. Probably it
causes some inbetween shell process.

Anyway, I don't think su is the best way to do this. I can try to
setUID the binary, but that means writing to the nix store, which is
probably bad as well.

Any hints?
Thanks,
Mathijs