services: shepherd: Allow custom 'shepherd' package.

* gnu/services/shepherd.scm (<shepherd-configuration>): New record.
(shepherd-boot-gexp, shepherd-root-service-type): Use it.
(scm->go, shepherd-configuration-file): Allow passing custom
shepherd package.
* gnu/system.scm (operating-system-shepherd-service-names): Use the new
record.
* guix/scripts/system.scm (export-shepherd-graph): Adjust accordingly.
* doc/guix.texi (Shepherd Services). Document it.

Co-authored-by: Ludovic Courtès <ludo@gnu.org>

1 files changed, 16 insertions, 1 deletions

diff --git a/doc/guix.texi b/doc/guix.texi
index 9b65be3c9c..405b218289 100644
--- a/doc/guix.texi
+++ b/doc/guix.texi
@@ -32782,9 +32782,24 @@ The service type for the Shepherd ``root service''---i.e., PID@tie{}1.
 
 This is the service type that extensions target when they want to create
 shepherd services (@pxref{Service Types and Services}, for an example).
-Each extension must pass a list of @code{<shepherd-service>}.
+Each extension must pass a list of @code{<shepherd-service>}.  Its
+value must be a @code{shepherd-configuration}, as described below.
 @end defvr
 
+@deftp {Data Type} shepherd-configuration
+This data type represents the Shepherd's configuration.
+
+@table @code
+@item shepherd (default: @code{shepherd})
+The Shepherd package to use.
+
+@item services (default: @code{'()})
+A list of @code{<shepherd-service>} to start.
+You should probably use the service extension
+mechanism instead (@pxref{Shepherd Services}).
+@end table
+@end deftp
+
 @defvr {Scheme Variable} %shepherd-root-service
 This service represents PID@tie{}1.
 @end defvr