services: unattended-upgrade: Add 'operating-system-expression' field.

* gnu/services/admin.scm (<unattended-upgrade-configuration>)
[operating-system-expression]: New field.
(unattended-upgrade-mcron-jobs): Honor it.
* doc/guix.texi (Unattended Upgrades): Document it.

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

1 files changed, 13 insertions, 1 deletions

diff --git a/doc/guix.texi b/doc/guix.texi
index 11a4ca18fb..cd0f561f71 100644
--- a/doc/guix.texi
+++ b/doc/guix.texi
@@ -22,7 +22,7 @@
 @set SUBSTITUTE-URLS https://@value{SUBSTITUTE-SERVER-1} https://@value{SUBSTITUTE-SERVER-2}
 
 @copying
-Copyright @copyright{} 2012-2022 Ludovic Courtès@*
+Copyright @copyright{} 2012-2023 Ludovic Courtès@*
 Copyright @copyright{} 2013, 2014, 2016 Andreas Enge@*
 Copyright @copyright{} 2013 Nikita Karetnikov@*
 Copyright @copyright{} 2014, 2015, 2016 Alex Kost@*
@@ -110,6 +110,7 @@ Copyright @copyright{} 2022 Simon Streit@*
 Copyright @copyright{} 2022 (@*
 Copyright @copyright{} 2022 John Kehayias@*
 Copyright @copyright{} 2022 Ivan Vilata-i-Balaguer@*
+Copyright @copyright{} 2023 Giacomo Leidi@*
 
 Permission is granted to copy, distribute and/or modify this document
 under the terms of the GNU Free Documentation License, Version 1.3 or
@@ -21362,6 +21363,17 @@ Therefore, uses of @code{local-file} within @file{config.scm} will work
 as expected.  @xref{G-Expressions}, for information about
 @code{local-file} and @code{file-append}.
 
+@item @code{operating-system-expression} (default: @code{#f})
+This field specifies an expression that evaluates to the operating
+system to use for the upgrade.  If no value is provided the
+@code{operating-system-file} field value is used.
+
+@lisp
+(unattended-upgrade-configuration
+  (operating-system-expression
+    #~(@@ (guix system install) installation-os)))
+@end lisp
+
 @item @code{services-to-restart} (default: @code{'(mcron)})
 This field specifies the Shepherd services to restart when the upgrade
 completes.