services: Add 'package-database' service.

* gnu/services/admin.scm (%default-package-database-update-schedule):
New variable.
(<package-database-configuration>): New record type.
(package-database-mcron-jobs): New procedure.
(package-database-service-type): New variable.
* doc/guix.texi (File Search Services): Document it.

doc/guix.texi

@@ -4481,7 +4481,9 @@ database, usually under @file{/var/cache/guix/locate}; if it does not
 exist or is too old, it falls back to the per-user database, by default
 under @file{~/.cache/guix/locate}.  On a multi-user system,
 administrators may want to periodically update the system-wide database
-so that all users can benefit from it.
+so that all users can benefit from it, for instance by setting up
+@code{package-database-service-type} (@pxref{File Search Services,
+@code{package-database-service-type}}).
 
 The general syntax is:
 
@@ -25000,6 +25002,51 @@ GNU@tie{}Findutils}).
 @end table
 @end deftp
 
+The second service, @code{package-database-service-type}, builds the
+database used by @command{guix locate}, which lets you search for
+packages that contain a given file (@pxref{Invoking guix locate}).  The
+service periodically updates a system-wide database, which will be
+readily available to anyone running @command{guix locate} on the system.
+To use this service with its default settings, add this snippet to your
+service list:
+
+@lisp
+(service package-database-service-type)
+@end lisp
+
+This will run @command{guix locate --update} once a week.
+
+@defvar package-database-service-type
+This is the service type for periodic @command{guix locate} updates
+(@pxref{Invoking guix locate}).  Its value must be a
+@code{package-database-configuration} record, as shown below.
+@end defvar
+
+@deftp {Data Type} package-database-configuration
+Data type to configure periodic package database updates.  It has the
+following fields:
+
+@table @asis
+@item @code{package} (default: @code{guix})
+The Guix package to use.
+
+@item @code{schedule} (default: @code{%default-package-database-update-schedule})
+String or G-exp denoting an mcron schedule for the periodic
+@command{guix locate --update} job (@pxref{Guile Syntax,,, mcron,
+GNU@tie{}mcron}).
+
+@item @code{method} (default: @code{'store})
+Indexing method for @command{guix locate}.  The default value,
+@code{'store}, yields a more complete database but is relatively
+expensive in terms of CPU and input/output.
+
+@item @code{channels} (default: @code{#~%default-channels})
+G-exp denoting the channels to use when updating the database
+(@pxref{Channels}).
+@end table
+@end deftp
+
+
 @node Database Services
 @subsection Database Services
 

gnu/services/admin.scm

@@ -29,6 +29,8 @@
   #:use-module (gnu services configuration)
   #:use-module (gnu services mcron)
   #:use-module (gnu services shepherd)
+  #:use-module (gnu system accounts)
+  #:use-module ((gnu system shadow) #:select (account-service-type))
   #:use-module ((guix store) #:select (%store-prefix))
   #:use-module (guix gexp)
   #:use-module (guix modules)
@@ -69,6 +71,14 @@
             %default-file-database-update-schedule
             %default-file-database-excluded-directories
 
+            package-database-service-type
+            package-database-configuration
+            package-database-configuration?
+            package-database-configuration-package
+            package-database-configuration-schedule
+            package-database-configuration-method
+            package-database-configuration-channels
+
             unattended-upgrade-service-type
             unattended-upgrade-configuration
             unattended-upgrade-configuration?
@@ -338,6 +348,49 @@ which lets you search for files by name.  The database is created by running
 the @command{updatedb} command.")
    (default-value (file-database-configuration))))
 
+(define %default-package-database-update-schedule
+  ;; Default mcron schedule for the periodic 'guix locate --update' job: once
+  ;; every Monday.
+  "10 23 * * 1")
+
+(define-configuration/no-serialization package-database-configuration
+  (package (file-like guix)
+           "The Guix package to use.")
+  (schedule (string-or-gexp
+             %default-package-database-update-schedule)
+            "String or G-exp denoting an mcron schedule for the periodic
+@command{guix locate --update} job (@pxref{Guile Syntax,,, mcron,
+GNU@tie{}mcron}).")
+  (method    (symbol 'store)
+             "Indexing method for @command{guix locate}.  The default value,
+@code{'store}, yields a more complete database but is relatively expensive in
+terms of CPU and input/output.")
+  (channels (gexp #~%default-channels)
+            "G-exp denoting the channels to use when updating the database
+(@pxref{Channels})."))
+
+(define (package-database-mcron-jobs configuration)
+  (match-record configuration <package-database-configuration>
+    (package schedule method channels)
+    (let ((channels (scheme-file "channels.scm" channels)))
+      (list #~(job #$schedule
+                   ;; XXX: The whole thing's running as "root" just because it
+                   ;; needs write access to /var/cache/guix/locate.
+                   (string-append #$(file-append package "/bin/guix")
+                                  " time-machine -C " #$channels
+                                  " -- locate --update --method="
+                                  #$(symbol->string method)))))))
+
+(define package-database-service-type
+  (service-type
+   (name 'package-database)
+   (extensions (list (service-extension mcron-service-type
+                                        package-database-mcron-jobs)))
+   (description
+    "Periodically update the package database used by the @code{guix locate} command,
+which lets you search for packages that provide a given file.")
+   (default-value (package-database-configuration))))
+
 
 ;;;
 ;;; Unattended upgrade.