services/udev: Allow configuring udev to run in debug mode.

This re-introduces commit dd64f441d3dcff9165927b821da2b69b1fc6a24f, which had
been reverted due to previously causing a system hang when debug? was enabled,
a problem that appears to have been resolved within Shepherd.

* gnu/services/base.scm (<udev-configuration>): <debug?>: New field.
* gnu/services/base.scm (udev-shepherd-service): Use it to add '--debug' to
the command line, if applicable.
* doc/guix.texi (Base Services): Document it.

Change-Id: I88243fb4f321ff0876dd227e3c2b22082d37cfcf

2 files changed, 17 insertions, 6 deletions

diff --git a/doc/guix.texi b/doc/guix.texi
index 7b418a4089..90d90b2e1e 100644
--- a/doc/guix.texi
+++ b/doc/guix.texi
@@ -20384,17 +20384,21 @@ In an @code{operating-system} declaration, this service type can be
 @deftp {Data Type} udev-configuration
 Data type representing the configuration of udev.
 
-@table @asis
-@item @code{udev} (default: @code{eudev}) (type: file-like)
+@table @code
+@item udev (default: @code{eudev}) (type: file-like)
 Package object of the udev service.  This package is used at run-time,
 when compiled for the target system.  In order to generate the
 @file{hwdb.bin} hardware index, it is also used when generating the
 system definition, compiled for the current system.
 
-@item @code{rules} (default: @var{'()}) (type: list-of-file-like)
+@item debug? (default: @code{#f}) (type: boolean)
+Whether to enable debug output.  The debug output is written to the
+system log, @file{/var/log/messages}.
+
+@item rules (default: @code{'()}) (type: list-of-file-like)
 List of file-like objects denoting udev rule files under a sub-directory.
 
-@item @code{hardware} (default: @var{'()}) (type: list-of-file-like)
+@item hardware (default: @code{'()}) (type: list-of-file-like)
 List of file-like objects denoting udev hardware description files under
 a sub-directory.
 
diff --git a/gnu/services/base.scm b/gnu/services/base.scm
index 8c6563c99d..650121be8f 100644
--- a/gnu/services/base.scm
+++ b/gnu/services/base.scm
@@ -15,7 +15,7 @@
 ;;; Copyright © 2020, 2021 Brice Waegeneire <brice@waegenei.re>
 ;;; Copyright © 2021 qblade <qblade@protonmail.com>
 ;;; Copyright © 2021 Hui Lu <luhuins@163.com>
-;;; Copyright © 2021, 2022, 2023 Maxim Cournoyer <maxim.cournoyer@gmail.com>
+;;; Copyright © 2021-2023, 2025 Maxim Cournoyer <maxim.cournoyer@gmail.com>
 ;;; Copyright © 2021, 2025 muradm <mail@muradm.net>
 ;;; Copyright © 2022 Guillaume Le Vaillant <glv@posteo.net>
 ;;; Copyright © 2022 Justin Veilleux <terramorpha@cock.li>
@@ -160,6 +160,8 @@
 
             udev-configuration
             udev-configuration?
+            udev-configuration-udev
+            udev-configuration-debug?
             udev-configuration-rules
             udev-configuration-hardware
             udev-service-type
@@ -2581,6 +2583,8 @@ command that allows you to share pre-built binaries with others over HTTP.")))
   udev-configuration?
   (udev   udev-configuration-udev                 ;file-like
           (default eudev))
+  (debug? udev-configuration-debug?               ;boolean
+          (default #f))
   (rules  udev-configuration-rules                ;list of file-like
           (default '()))
   (hardware  udev-configuration-hardware          ;list of file-like
@@ -2723,7 +2727,10 @@ item of PACKAGES."
                (umask old-umask))
 
              (let ((pid (fork+exec-command
-                         (list udevd)
+                         (list udevd
+                               #$@(if (udev-configuration-debug? config)
+                                      '("--debug")
+                                      '()))
                          #:environment-variables
                          (cons*
                           (string-append "LINUX_MODULE_DIRECTORY="