bug#60116: ‘herd stop guix-daemon’ doesn’t stop running sessions

["Ludovic Courtès" <ludo@gnu.org>]

Suppose you run a bunch of clients, ‘guix shell’, ‘guix build’,
whatever.  For each of these connections, guix-daemon creates a child
process to handle it.  (You can see them as “SessionPID” in the output
of ‘guix processes’.)

‘herd stop guix-daemon’ won’t stop these guix-daemon child processes,
only the parent process.

This is because those sub-processes put themselves in a separate process
group by calling ‘setsid’ (see ‘daemonLoop’ in ‘nix-daemon.cc’);
shepherd terminates the whole process group, but that group contains
nothing but the parent guix-daemon process.  I traced back this ‘setsid’
call to 2006 in Nix (Nix commit
0130ef88ea280e67037fa76bcedc59db17d9a8ca), but there’s no justification.

I would be tempted to just remove that ‘setsid’ call, but I wonder if
there’s something subtle I’m overlooking.

Thoughts?

Ludo’.