Guix

GNU Shepherd user services

Efraim Flashner — 2020年05月17日

One of the things which sets Guix apart from other GNU/Linux distributions is that it uses GNU Shepherd instead of the now ubiquitous systemd. A side effect of this is that user systemd units do not work on Guix System. Love, hate or extremely ambivalent toward systemd, this means that users cannot rely on already written systemd unit files for their regular user-level services.

There are a couple of benefits to using GNU Shepherd, and not all of them are due to it already being installed on Guix. Becoming comfortable with using Shepherd and understanding how to write and edit Shepherd service configurations makes the transition from other GNU/Linux distributions to Guix System easier. More complex services with their own logic tree, using the full power of GNU Guile, are also possible. This means you can have one service that behaves differently if it's running on a different system or architecture without needing to call out to shell scripts or using minimally different service definitions.

The GNU Shepherd manual suggests putting all the services inside a monolithic init.scm file, located by default at $XDG_CONFIG_DIR/shepherd/init.scm. While this does make it easy to keep everything in one place, it does create one glaring issue: any changes to the file mean that all the services need to be stopped and restarted in order for any changes to take place.

Luckily there's a nice function called scandir hiding in ice-9 ftw which returns a list of all files in a specified directory (with options for narrowing down the list or sorting it). This means that our init.scm can contain a minimum of code and all actual services can be loaded from individual files.

First the minimal init.scm:

(use-modules (shepherd service)
             ((ice-9 ftw) #:select (scandir)))

;; Load all the files in the directory 'init.d' with a suffix '.scm'.
(for-each
  (lambda (file)
    (load (string-append "init.d/" file)))
  (scandir (string-append (dirname (current-filename)) "/init.d")
           (lambda (file)
             (string-suffix? ".scm" file))))

;; Send shepherd into the background
(action 'shepherd 'daemonize)

Let's take a sample service for running syncthing, as defined in $XDG_CONFIG_DIR/shepherd/init.d/syncthing.scm:

(define syncthing
  (make <service>
    #:provides '(syncthing)
    #:docstring "Run `syncthing' without calling the browser"
    #:start (make-forkexec-constructor
              '("syncthing" "-no-browser")
              #:log-file (string-append (getenv "HOME")
                                        "/log/syncthing.log"))
    #:stop (make-kill-destructor)
    #:respawn? #t))
(register-services syncthing)

(start syncthing)

As with any other shepherd service it is defined and registered, and in this case it will start automatically. When the file is loaded by shepherd after being discovered by scandir everything works exactly as though the service definition were located directly inside the init.scm.

Now lets make a change. Since syncthing already has a -logfile flag and it has built-in log rotation that sounds better than using shepherd's #:log-file option. First we'll make our changes to the service:

(define syncthing
  (make <service>
    #:provides '(syncthing)
    #:docstring "Run `syncthing' without calling the browser"
    #:start (make-forkexec-constructor
              '("syncthing" "-no-browser"
                "-logflags=3" ; prefix with date & time
                "-logfile=/home/user/log/syncthing.log"))
    #:stop (make-kill-destructor)
    #:respawn? #t))
(register-services syncthing)

(start syncthing)

Now we stop syncthing:

$ herd stop syncthing

And we load the new service:

$ herd load root ~/.config/shepherd/init.d/syncthing.scm

This allows for quickly iterating on services without needing to stop all the services! Let's take a look at another service:

(define fccache
  (make <service>
    #:provides '(fccache)
    #:docstring "Run 'fc-cache -frv'"
    #:start (make-forkexec-constructor
              '("guix" "environment" "--ad-hoc" "fontconfig" "--"
                "fc-cache" "-frv")
              #:log-file (string-append (getenv "HOME")
                                        "/log/fccache.log"))
    #:one-shot? #t))

(register-services fccache)

In this example I want to refresh my font cache but I don't want to actually install fontconfig either system-wide or in my profile.

$ which fc-cache
which: no fc-cache in (/home/user/.config/guix/current/bin:/home/user/.guix-profile/bin:/home/user/.guix-profile/sbin:/run/setuid-programs:/run/current-system/profile/bin:/run/current-system/profile/sbin)
$ herd start fccache
Service fccache has been started.

Of course we can import other modules and leverage the code already written there. In this case, instead of using the string "guix environment --ad-hoc fontutils -- fc-cache -frv" let's use the guix environment function already available in guix scripts environment:

(use-modules (guix scripts environment))

(define fccache
  (make <service>
    #:provides '(fccache)
    #:docstring "Run 'fc-cache -frv'"
    #:start (lambda () ; Don't run immediately when registered!
              (guix-environment "--ad-hoc" "fontconfig" "--" "fc-cache" "-frv"))
    #:one-shot? #t))

(register-services fccache)

$ herd load root ~/.config/shepherd/init.d/fccache.scm
Loading /home/user/.config/shepherd/init.d/fccache.scm.
$ herd start fccache
/gnu/store/hbqlzgd8hcf6ndcmx7q7miqrsxb4dmkk-gs-fonts-8.11/share/fonts: caching, new cache contents: 0 fonts, 1 dirs
/gnu/store/hbqlzgd8hcf6ndcmx7q7miqrsxb4dmkk-gs-fonts-8.11/share/fonts/type1: caching, new cache contents: 0 fonts, 1 dirs
/gnu/store/hbqlzgd8hcf6ndcmx7q7miqrsxb4dmkk-gs-fonts-8.11/share/fonts/type1/ghostscript: caching, new cache contents: 35 fonts, 0 dirs
/home/user/.guix-profile/share/fonts: caching, new cache contents: 0 fonts, 7 dirs
/home/user/.guix-profile/share/fonts/opentype: caching, new cache contents: 8 fonts, 0 dirs
/home/user/.guix-profile/share/fonts/otf: caching, new cache contents: 12 fonts, 0 dirs
/home/user/.guix-profile/share/fonts/terminus: caching, new cache contents: 18 fonts, 0 dirs
/home/user/.guix-profile/share/fonts/truetype: caching, new cache contents: 58 fonts, 0 dirs
/home/user/.guix-profile/share/fonts/ttf: caching, new cache contents: 12 fonts, 0 dirs
/home/user/.guix-profile/share/fonts/type1: caching, new cache contents: 0 fonts, 1 dirs
/home/user/.guix-profile/share/fonts/type1/ghostscript: caching, new cache contents: 35 fonts, 0 dirs
/home/user/.guix-profile/share/fonts/woff: caching, new cache contents: 1 fonts, 0 dirs
/run/current-system/profile/share/fonts: skipping, no such directory
/home/user/.local/share/fonts: skipping, no such directory
/home/user/.fonts: skipping, no such directory
/gnu/store/hbqlzgd8hcf6ndcmx7q7miqrsxb4dmkk-gs-fonts-8.11/share/fonts/type1: skipping, looped directory detected
/home/user/.guix-profile/share/fonts/opentype: skipping, looped directory detected
/home/user/.guix-profile/share/fonts/otf: skipping, looped directory detected
/home/user/.guix-profile/share/fonts/terminus: skipping, looped directory detected
/home/user/.guix-profile/share/fonts/truetype: skipping, looped directory detected
/home/user/.guix-profile/share/fonts/ttf: skipping, looped directory detected
/home/user/.guix-profile/share/fonts/type1: skipping, looped directory detected
/home/user/.guix-profile/share/fonts/woff: skipping, looped directory detected
/gnu/store/hbqlzgd8hcf6ndcmx7q7miqrsxb4dmkk-gs-fonts-8.11/share/fonts/type1/ghostscript: skipping, looped directory detected
/home/user/.guix-profile/share/fonts/type1/ghostscript: skipping, looped directory detected
/var/cache/fontconfig: not cleaning unwritable cache directory
/home/user/.cache/fontconfig: cleaning cache directory
/home/user/.fontconfig: not cleaning non-existent cache directory
fc-cache: succeeded
herd: exception caught while executing 'start' on service 'fccache':
Throw to key `quit' with args `(0)'.

The problem with this approach is that guix-environment returns the exit code of the programs it calls and #:start expects a constructor to return #t or #f so there's some work to be done here.

This was just a quick peek into what's possible with GNU Shepherd when run as a user. Next time we'll take a look at integrating mcron to replicate some of systemd's timer functionality.

About GNU Guix

GNU Guix is a transactional package manager and an advanced distribution of the GNU system that respects user freedom. Guix can be used on top of any system running the kernel Linux, or it can be used as a standalone operating system distribution for i686, x86_64, ARMv7, and AArch64 machines.

In addition to standard package management features, Guix supports transactional upgrades and roll-backs, unprivileged package management, per-user profiles, and garbage collection. When used as a standalone GNU/Linux distribution, Guix offers a declarative, stateless approach to operating system configuration management. Guix is highly customizable and hackable through Guile programming interfaces and extensions to the Scheme language.