home: Add redshift service.

* gnu/home/services/desktop.scm: New file.
* gnu/local.mk (GNU_SYSTEM_MODULES): Add it.
* doc/guix.texi (Desktop Home Services): New node.

doc/guix.texi

@@ -37598,6 +37598,7 @@ services)}.
 * Shells: Shells Home Services.          POSIX shells, Bash, Zsh.
 * Mcron: Mcron Home Service.             Scheduled User's Job Execution.
 * Shepherd: Shepherd Home Service.       Managing User's Daemons.
+* Desktop: Desktop Home Services.        Services for graphical environments.
 @end menu
 @c In addition to that Home Services can provide
 
@@ -37985,6 +37986,85 @@ mechanism instead (@pxref{Shepherd Services}).
 @end table
 @end deftp
 
+@node Desktop Home Services
+@subsection Desktop Home Services
+
+The @code{(gnu home services desktop)} module provides services that you
+may find useful on ``desktop'' systems running a graphical user
+environment such as Xorg.
+
+@defvr {Scheme Variable} home-redshift-service-type
+This is the service type for @uref{https://github.com/jonls/redshift,
+Redshift}, a program that adjusts the display color temperature
+according to the time of day.  Its associated value must be a
+@code{home-redshift-configuration} record, as shown below.
+
+A typical configuration, where we manually specify the latitude and
+longitude, might look like this:
+
+@lisp
+(service home-redshift-service-type
+         (home-redshift-configuration
+          (location-provider 'manual)
+          (latitude 35.81)    ;northern hemisphere
+          (longitude -0.80))) ;west of Greenwich
+@end lisp
+@end defvr
+
+@deftp {Data Type} home-redshift-configuration
+Available @code{home-redshift-configuration} fields are:
+
+@table @asis
+@item @code{redshift} (default: @code{redshift}) (type: file-like)
+Redshift package to use.
+
+@item @code{location-provider} (default: @code{geoclue2}) (type: symbol)
+Geolocation provider---@code{'manual} or @code{'geoclue2}.  In the
+former case, you must also specify the @code{latitude} and
+@code{longitude} fields so Redshift can determine daytime at your place.
+In the latter case, the Geoclue system service must be running; it will
+be queried for location information.
+
+@item @code{adjustment-method} (default: @code{randr}) (type: symbol)
+Color adjustment method.
+
+@item @code{daytime-temperature} (default: @code{6500}) (type: integer)
+Daytime color temperature (kelvins).
+
+@item @code{nighttime-temperature} (default: @code{4500}) (type: integer)
+Nighttime color temperature (kelvins).
+
+@item @code{daytime-brightness} (default: @code{disabled}) (type: maybe-inexact-number)
+Daytime screen brightness, between 0.1 and 1.0.
+
+@item @code{nighttime-brightness} (default: @code{disabled}) (type: maybe-inexact-number)
+Nighttime screen brightness, between 0.1 and 1.0.
+
+@item @code{latitude} (default: @code{disabled}) (type: maybe-inexact-number)
+Latitude, when @code{location-provider} is @code{'manual}.
+
+@item @code{longitude} (default: @code{disabled}) (type: maybe-inexact-number)
+Longitude, when @code{location-provider} is @code{'manual}.
+
+@item @code{dawn-time} (default: @code{disabled}) (type: maybe-string)
+Custom time for the transition from night to day in the
+morning---@code{"HH:MM"} format.  When specified, solar elevation is not
+used to determine the daytime/nighttime period.
+
+@item @code{dusk-time} (default: @code{disabled}) (type: maybe-string)
+Likewise, custom time for the transition from day to night in the
+evening.
+
+@item @code{extra-content} (default: @code{""}) (type: raw-configuration-string)
+Extra content appended as-is to the Redshift configuration file.  Run
+@command{man redshift} for more information about the configuration file
+format.
+
+@end table
+
+@end deftp
+
+
 @node Invoking guix home
 @section Invoking @code{guix home}
 

gnu/home/services/desktop.scm

@@ -0,0 +1,174 @@
+;;; GNU Guix --- Functional package management for GNU
+;;; Copyright © 2022 Ludovic Courtès <ludo@gnu.org>
+;;;
+;;; This file is part of GNU Guix.
+;;;
+;;; GNU Guix is free software; you can redistribute it and/or modify it
+;;; under the terms of the GNU General Public License as published by
+;;; the Free Software Foundation; either version 3 of the License, or (at
+;;; your option) any later version.
+;;;
+;;; GNU Guix is distributed in the hope that it will be useful, but
+;;; WITHOUT ANY WARRANTY; without even the implied warranty of
+;;; MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+;;; GNU General Public License for more details.
+;;;
+;;; You should have received a copy of the GNU General Public License
+;;; along with GNU Guix.  If not, see <http://www.gnu.org/licenses/>.
+
+(define-module (gnu home services desktop)
+  #:use-module (gnu home services)
+  #:use-module (gnu home services shepherd)
+  #:use-module (gnu services configuration)
+  #:autoload   (gnu packages xdisorg) (redshift)
+  #:use-module (guix records)
+  #:use-module (guix gexp)
+  #:use-module (srfi srfi-1)
+  #:use-module (ice-9 match)
+  #:export (home-redshift-configuration
+            home-redshift-configuration?
+
+            home-redshift-service-type))
+
+
+;;;
+;;; Redshift.
+;;;
+
+(define (serialize-integer field value)
+  (string-append (match field
+                   ('daytime-temperature "temp-day")
+                   ('nighttime-temperature "temp-night")
+                   ('daytime-brightness "brightness-day")
+                   ('nighttime-brightness "brightness-night")
+                   ('latitude "lat")
+                   ('longitude "lon")
+                   (_ (symbol->string field)))
+                 "=" (number->string value) "\n"))
+
+(define (serialize-symbol field value)
+  (string-append (symbol->string field)
+                 "=" (symbol->string value) "\n"))
+
+(define (serialize-string field value)
+  (string-append (symbol->string field)
+                 "=" value "\n"))
+
+(define serialize-inexact-number serialize-integer)
+
+(define (inexact-number? n)
+  (and (number? n) (inexact? n)))
+(define-maybe inexact-number)
+(define-maybe string)
+
+(define (serialize-raw-configuration-string field value)
+  value)
+(define raw-configuration-string? string?)
+
+(define-configuration home-redshift-configuration
+  (redshift
+   (file-like redshift)
+   "Redshift package to use.")
+
+  (location-provider
+   (symbol 'geoclue2)
+   "Geolocation provider---@code{'manual} or @code{'geoclue2}.
+
+In the former case, you must also specify the @code{latitude} and
+@code{longitude} fields so Redshift can determine daytime at your place.  In
+the latter case, the Geoclue system service must be running; it will be
+queried for location information.")
+  (adjustment-method
+   (symbol 'randr)
+   "Color adjustment method.")
+
+  ;; Default values from redshift(1).
+  (daytime-temperature
+   (integer 6500)
+   "Daytime color temperature (kelvins).")
+  (nighttime-temperature
+   (integer 4500)
+   "Nighttime color temperature (kelvins).")
+
+  (daytime-brightness
+   (maybe-inexact-number 'disabled)
+   "Daytime screen brightness, between 0.1 and 1.0.")
+  (nighttime-brightness
+   (maybe-inexact-number 'disabled)
+   "Nighttime screen brightness, between 0.1 and 1.0.")
+
+  (latitude
+   (maybe-inexact-number 'disabled)
+   "Latitude, when @code{location-provider} is @code{'manual}.")
+  (longitude
+   (maybe-inexact-number 'disabled)
+   "Longitude, when @code{location-provider} is @code{'manual}.")
+
+  (dawn-time
+   (maybe-string 'disabled)
+   "Custom time for the transition from night to day in the
+morning---@code{\"HH:MM\"} format.  When specified, solar elevation is not
+used to determine the daytime/nighttime period.")
+  (dusk-time
+   (maybe-string 'disabled)
+   "Likewise, custom time for the transition from day to night in the
+evening.")
+
+  (extra-content
+   (raw-configuration-string "")
+   "Extra content appended as-is to the Redshift configuration file.  Run
+@command{man redshift} for more information about the configuration file
+format."))
+
+(define (serialize-redshift-configuration config)
+  (define location-fields
+    '(latitude longitude))
+
+  (define (location-field? field)
+    (memq (configuration-field-name field) location-fields))
+
+  (define (secondary-field? field)
+    (or (location-field? field)
+        (memq (configuration-field-name field)
+              '(redshift extra-content))))
+
+  #~(string-append
+     "[redshift]\n"
+     #$(serialize-configuration config
+                                (remove secondary-field?
+                                        home-redshift-configuration-fields))
+
+     #$(home-redshift-configuration-extra-content config)
+
+     "\n[manual]\n"
+     #$(serialize-configuration config
+                                (filter location-field?
+                                        home-redshift-configuration-fields))))
+
+(define (redshift-shepherd-service config)
+  (define config-file
+    (computed-file "redshift.conf"
+                   #~(call-with-output-file #$output
+                       (lambda (port)
+                         (display #$(serialize-redshift-configuration config)
+                                  port)))))
+
+  (list (shepherd-service
+         (documentation "Redshift program.")
+         (provision '(redshift))
+         ;; FIXME: This fails to start if Home is first activated from a
+         ;; non-X11 session.
+         (start #~(make-forkexec-constructor
+                   (list #$(file-append redshift "/bin/redshift")
+                         "-c" #$config-file)))
+         (stop #~(make-kill-destructor)))))
+
+(define home-redshift-service-type
+  (service-type
+   (name 'home-redshift)
+   (extensions (list (service-extension home-shepherd-service-type
+                                        redshift-shepherd-service)))
+   (default-value (home-redshift-configuration))
+   (description
+    "Run Redshift, a program that adjusts the color temperature of display
+according to time of day.")))

gnu/local.mk

@@ -79,6 +79,7 @@ GNU_SYSTEM_MODULES =				\
   %D%/ci.scm					\
   %D%/home.scm					\
   %D%/home/services.scm			\
+  %D%/home/services/desktop.scm			\
   %D%/home/services/symlink-manager.scm		\
   %D%/home/services/fontutils.scm		\
   %D%/home/services/shells.scm			\