services: guix: Generalize extensions.

* gnu/services/base.scm (<guix-extension>): New record type.
(guix-extension-merge): New procedure.
(guix-service-type): Honor extensions.
* doc/guix.texi (Base Services): Document it.

Signed-off-by: Ludovic Courtès <ludo@gnu.org>

1 files changed, 20 insertions, 0 deletions

diff --git a/doc/guix.texi b/doc/guix.texi
index 2aef0380fb..967145a627 100644
--- a/doc/guix.texi
+++ b/doc/guix.texi
@@ -103,6 +103,7 @@ Copyright @copyright{} 2022 Remco van 't Veer@*
 Copyright @copyright{} 2022 Aleksandr Vityazev@*
 Copyright @copyright{} 2022 Philip M@sup{c}Grath@*
 Copyright @copyright{} 2022 Karl Hallsby@*
+Copyright @copyright{} 2022 Justin Veilleux@*
 
 Permission is granted to copy, distribute and/or modify this document
 under the terms of the GNU Free Documentation License, Version 1.3 or
@@ -17729,6 +17730,25 @@ A directory path where the @command{guix-daemon} will perform builds.
 @end table
 @end deftp
 
+@deftp {Data Type} guix-extension
+
+This data type represents the parameters of the Guix build daemon that
+are extendable. This is the type of the object that must be used within
+a guix service extension.
+@xref{Service Composition}, for more information.
+
+@table @asis
+@item @code{authorized-keys} (default: @code{'()})
+A list of file-like objects where each element contains a public key.
+
+@item @code{substitute-urls} (default: @code{'()})
+A list of strings where each element is a substitute URL.
+
+@item @code{chroot-directories} (default: @code{'()})
+A list of file-like objects or strings pointing to additional directories the build daemon can use.
+@end table
+@end deftp
+
 @deffn {Scheme Procedure} udev-service [#:udev @var{eudev} #:rules @code{'()}]
 Run @var{udev}, which populates the @file{/dev} directory dynamically.
 udev rules can be provided as a list of files through the @var{rules}