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Re: Invoking user shepherd; Was: Re: Defining *user* services in Guix

From: Ludovic Courtès
Subject: Re: Invoking user shepherd; Was: Re: Defining *user* services in Guix
Date: Tue, 13 Jun 2017 10:06:16 +0200
User-agent: Gnus/5.13 (Gnus v5.13) Emacs/25.2 (gnu/linux)


Danny Milosavljevic <address@hidden> skribis:

> For a real user shepherd, it would be nice if when I logged in twice using 
> the same user account (without logging out in-between - so resulting in two 
> sessions of that user) it would still only have one shepherd instance for 
> that user account in total - and if that instance (and remaining user 
> processes for that matter) went away only when I logged out from *all* 
> sessions of that user.
> I thought I could fake that by just trying to invoke shepherd on each session 
> start and have it fail on the second attempt - but apparently we will happily 
> start an infinite number of shepherds for one user.  Is that on purpose?
> The relevant place (in shepherd) is:
> (define (open-server-socket file-name)
>   "Open a socket at FILE-NAME, and listen for connections there."
>   (with-fluids ((%default-port-encoding "UTF-8"))
>     (let ((sock    (socket PF_UNIX SOCK_STREAM 0))
>           (address (make-socket-address AF_UNIX file-name)))
>       (false-if-exception (delete-file file-name)) ;  <===== [dannym: WTF.  
> Would it be better to try to connect first?]
>       (bind sock address)
>       (listen sock 10)
>       sock)))
> Probably not good.

The ‘delete-file’ was to avoid EADDRINUSE but I wonder if it’s really a
good idea.

> Maybe better:
> (define (server-present? file-name)
>   "Open a socket at FILE-NAME, and connect to the server, if any.  Return #t 
> if that worked."
>   (with-fluids ((%default-port-encoding "UTF-8"))
>     (let ((sock    (socket PF_UNIX SOCK_STREAM 0))
>           (address (make-socket-address AF_UNIX file-name)))
>       (false-if-exception (connect sock address))))) ; probably missing a 
> "[catch] close".  How to do that best?

That’s a pretty good test.  Note that (gnu services herd) and (shepherd
comm) already provide an ‘open-connection’ procedure to do that.

>From a shell script, you can also simply run “herd status root” and
check the exit status.

> I'm trying to find the right place to insert my "dbus-daemon" invocation for 
> providing the user bus (note: not session bus)...

What’s the difference between a “user bus” and a “session bus”?  In
general, my understanding is that the user dbus-daemon (session bus?) is
started on demand.

> Should we make a system shepherd service that invokes the user shepherd 
> service on behalf of users?  Would that be that safe?

The “guix user” approach that Mathieu shown is more flexible than having
to do something in the ‘operating-system’ declaration IMO.

> Or should we just expect the user to put a (shepherd with fix) invocation 
> into their HOME startup scripts like .xinitrc ?  Note that if we did that 
> there's some session-specific stuff in the session's environment that 
> shepherd will inherit.  Probably not that bad if invoked early enough.

Currently that’s what I do: my ~/.xinitrc runs shepherd and that’s it.


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