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GNU Shepherd 0.10.0rc1 available for testing!

From: Ludovic Courtès
Subject: GNU Shepherd 0.10.0rc1 available for testing!
Date: Fri, 28 Apr 2023 15:37:12 +0200
User-agent: Gnus/5.13 (Gnus v5.13) Emacs/28.2 (gnu/linux)

Hello Guix!

I am pleased to announce that the first release candidate of version
0.10.0 of the GNU Shepherd is available for testing!

If you’re using Guix System or Guix Home, check out the instructions
below to give it a try.  Please report success or failure!  If you had
problems with previous versions, please check if they’re still there.

  sha256: 1x3lxsi6xhhds4pq30c3shydmhiidkf1wl2l7mxkpklmlycnbqgg

In your operating system configuration (and similarly for your
‘home-environment’), make the following changes:

  (use-modules (gnu packages admin))

  (define shepherd-next
      (inherit shepherd)
      (version "0.10.0rc1")
      (source (origin
                (method url-fetch)
                (uri (string-append
                      version ".tar.gz"))

    ;; …
     (modify-services (operating-system-default-essential-services
        config => (shepherd-configuration
                   (shepherd shepherd-next))))))

You can then reconfigure, reboot, and enjoy!

You can also help with translation.  An updated PO template should soon
be available at the Translation Project:

My goal is for 0.10.x to be the last series before 1.0.  The list of
changes is quite long—see the ‘NEWS’ excerpt below.

Feedback more than welcome!


* Changes in 0.10.0 (yet to be released)

** Distinguish ‘starting’ and ‘stopping’ intermediate service statuses

In previous version, a service would be either “running” or “stopped”.  The
intermediate states “starting” and “stopping” are now properly captured and
you can see them when running ‘herd status’.

** ‘start’ and ‘stop’ block when service is already being started/stopped

With previous version, a client running ‘herd start SERVICE’ while SERVICE is
already being started would cause shepherd to attempt to start a second
instance of that service, ultimately resulting in confusion, disappointment,
and frustration.

This is no longer the case: when a service is already being started/stopped,
additional invocation of ‘herd start’ or ‘herd stop’ now block until the
service is running/stopped.

** ‘shepherd’ starts services in parallel

Services started with ‘start-in-the-background’ and more generally service
dependencies get started in parallel.  This can reduce startup times in case
of a “wide” service dependency graph with some services that take a while to

** ‘shepherd’ keeps track of failures and status change times

For each service, shepherd maintains an event log including the time of recent
status changes as well as the time of startup failures, if any.  The ‘herd
status SERVICE’ command now shows the time when the service entered its
current status and whether it failed to start; ‘herd status’ also prominently
lists services that failed to start.

** New ‘herd log’ command

Related to the previous item, the new ‘herd log’ command displays an aggregate
of the service event logs, showing the time at which each service changed

** New ‘herd graph’ command

The new ‘herd graph’ command emits a Graphviz/Dot representation of the
service dependency graph, which can be viewed for example with ‘xdot’:

  herd graph | xdot -

Guix System users get similar information with ‘guix system shepherd-graph’
(and likewise for Guix Home).  The difference here is that this reflects the
current system status, showing transient services, services that failed to
start, and so on.

** ‘herd’ output is colorized

At long last!  We hope you’ll enjoy a little bit of coloring to highlight
important bits in the output of various commands.

** New services shipped: ‘monitoring’ and ‘repl’

The Shepherd now ships with optional services—see “Service Collection” in the
manual.  The ‘monitoring’ service logs resource usage of the ‘shepherd’
process itself.  The ‘repl’ service runs a read-eval-print loop (REPL) in the
‘shepherd’ so you can hack it live—enjoy it, but handle it with care!

** Socket-actived, systemd-style services can now be started eagerly

The ‘make-systemd-constructor’ procedure has a new #:lazy-start? parameter.
It defaults to #true, meaning that the process is started lazily, on the first
connection to one of its sockets, as was the case in 0.9.x.  Passing
#:lazy-start? #false instructs shepherd to instead start the process eagerly,
as soon as the listening sockets are ready.

This is useful for services that require socket activation as a startup
synchronization mechanism, yet are expected to run as soon as possible.  An
example is ‘guix publish --advertise’: it should be started eagerly so it can
start advertising itself via Avahi.

** Each registered name maps to exactly one service

There used to be a fuzzy notion of “conflicting services”, when a given
service name could potentially refer to more than one service.  This has
proved to be confusing more than anything else; now, each registered service
name refers to exactly one service.  The interface related to that feature,
such as the ‘conflicts-with’ method, is done.

** For systemd and inetd services, retry ‘bind’ upon EADDRINUSE

Services started with ‘make-systemd-constructor’ and ‘make-inetd-constructor’
will now retry several times when ‘bind’ returns EADDRINUSE (“Address already
in use”) for their listening socket(s).

** ‘system’ and ‘make-system-constructor’ are now non-blocking

In versions up to 0.9.3, calling Guile’s ‘system’ procedure (which is what
‘make-system-constructor’ does) would block the ‘shepherd’ process until the
shell spawned by ‘system’ has terminated.  This is no longer the case.

** GOOPS interface is deprecated

When it was created in 2002, the Shepherd (née dmd) embraced GOOPS, Guile’s
object-oriented programming system, then a brand new and promising approach
for 21st century programs.  In hindsight, while there were a couple of classes
and a bunch of methods, the code base was not really making much use of GOOPS.
The current maintainer deemed it unnecessary and encouraging a programming
style at odds with the shiny horizon of purely functional, actor-style

The GOOPS interface is still available in 0.10.0; for example, you can still
write ~(make <service> #:provides …)~ in your configuration file.  However,
GOOPS support will be removed in the next major series, most likely labeled

A new interface has been defined.  Check out the “Legacy GOOPS Interface”
section of the manual for more information, and email if
you have any questions or concerns.

** Interfaces removed and changed

Several obscure or undocumented interfaces were removed:

  - support for the ‘unknown’ service;
  - support for “persistency” (sic);
  - the ‘cd’ action of the ‘root’ service;
  - the ‘launch-service’ procedure of (shepherd service).

New deprecations:

  - ‘make-actions’ is deprecated in favor of ‘actions’;
  - calling ‘register-services’ with an arbitrary number of arguments is now
    deprecated; you should now call it with a single argument, the list of
    services to register.

** Major internal overhaul

As you can guess from the list of user-visible changes above, the Shepherd has
undergone a major internal overhaul.  The 0.9.x series introduced the use of
Fibers, Guile’s lightweight concurrent facility; shepherd took advantage of it
notably with the introduction of systemd-style and inetd-style services.  This
new stable series takes it further.

In particular, each <service> record has an associated fiber called the
“service controller”.  Following the actor model, each of these fibers reacts
to messages it receives, be they event notification—e.g., process
termination—or user requests—e.g., querying the service status, requesting
that the service be stopped.  Other noteworthy actors include the “process
monitor” and the “service registry”.

This has allowed us to address a number of race conditions while also leading
to clearer code with linear flows that one can more easily reason about.
Overall, it makes the code base much more pleasant to work with and certainly
easier to hack than other implementations mired in the “callback hell”.

Documentation has been overhauled as well to reflect all these changes.  Check
out the new subsections under “Services” for more information.

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