services: Add 'unattended-upgrade-service-type'.

* gnu/services/admin.scm (<unattended-upgrade-configuration>): New
record type.
(%unattended-upgrade-log-file): New variable.
(unattended-upgrade-mcron-jobs, unattended-upgrade-log-rotations): New
procedures.
(unattended-upgrade-service-type): New variable.
* doc/guix.texi (Service Reference): Add 'provenance-service-type' anchor.
(Unattended Upgrades): New section.

doc/guix.texi

@@ -12926,6 +12926,7 @@ declaration.
 * Scheduled Job Execution::     The mcron service.
 * Log Rotation::                The rottlog service.
 * Networking Services::         Network setup, SSH daemon, etc.
+* Unattended Upgrades::         Automated system upgrades.
 * X Window::                    Graphical display.
 * Printing Services::           Local and remote printer support.
 * Desktop Services::            D-Bus and desktop services.
@@ -15298,6 +15299,117 @@ Use this to add additional options and manage shared secrets out-of-band.
 @end table
 @end deftp
 
+@node Unattended Upgrades
+@subsection Unattended Upgrades
+
+@cindex unattended upgrades
+@cindex upgrades, unattended
+Guix provides a service to perform @emph{unattended upgrades}:
+periodically, the system automatically reconfigures itself from the
+latest Guix.  Guix System has several properties that make unattended
+upgrades safe:
+
+@itemize
+@item
+upgrades are transactional (either the upgrade succeeds or it fails, but
+you cannot end up with an ``in-between'' system state);
+@item
+the upgrade log is kept---you can view it with @command{guix system
+list-generations}---and you can roll back to any previous generation,
+should the upgraded system fail to behave as intended;
+@item
+channel code is authenticated so you know you can only run genuine code
+(@pxref{Channels});
+@item
+@command{guix system reconfigure} prevents downgrades, which makes it
+immune to @dfn{downgrade attacks}.
+@end itemize
+
+To set up unattended upgrades, add an instance of
+@code{unattended-upgrade-service-type} like the one below to the list of
+your operating system services:
+
+@lisp
+(service unattended-upgrade-service-type)
+@end lisp
+
+The defaults above set up weekly upgrades: every Sunday at midnight.
+You do not need to provide the operating system configuration file: it
+uses @file{/run/current-system/configuration.scm}, which ensures it
+always uses your latest configuration---@pxref{provenance-service-type},
+for more information about this file.
+
+There are several things that can be configured, in particular the
+periodicity and services (daemons) to be restarted upon completion.
+When the upgrade is successful, the service takes care of deleting
+system generations older that some threshold, as per @command{guix
+system delete-generations}.  See the reference below for details.
+
+To ensure that upgrades are actually happening, you can run
+@command{guix system describe}.  To investigate upgrade failures, visit
+the unattended upgrade log file (see below).
+
+@defvr {Scheme Variable} unattended-upgrade-service-type
+This is the service type for unattended upgrades.  It sets up an mcron
+job (@pxref{Scheduled Job Execution}) that runs @command{guix system
+reconfigure} from the latest version of the specified channels.
+
+Its value must be a @code{unattended-upgrade-configuration} record (see
+below).
+@end defvr
+
+@deftp {Data Type} unattended-upgrade-configuration
+This data type represents the configuration of the unattended upgrade
+service.  The following fields are available:
+
+@table @asis
+@item @code{schedule} (default: @code{"30 01 * * 0"})
+This is the schedule of upgrades, expressed as a gexp containing an
+mcron job schedule (@pxref{Guile Syntax, mcron job specifications,,
+mcron, GNU@tie{}mcron}).
+
+@item @code{channels} (default: @code{#~%default-channels})
+This gexp specifies the channels to use for the upgrade
+(@pxref{Channels}).  By default, the tip of the official @code{guix}
+channel is used.
+
+@item @code{services-to-restart} (default: @code{'(mcron)})
+This field specifies the Shepherd services to restart when the upgrade
+completes.
+
+Those services are restarted right away upon completion, as with
+@command{herd restart}, which ensures that the latest version is
+running---remember that by default @command{guix system reconfigure}
+only restarts services that are not currently running, which is
+conservative: it minimizes disruption but leaves outdated services
+running.
+
+By default, the @code{mcron} service is restarted.  This ensures that
+the latest version of the unattended upgrade job will be used next time.
+
+@item @code{system-expiration} (default: @code{(* 3 30 24 3600)})
+This is the expiration time in seconds for system generations.  System
+generations older that this amount of time are deleted with
+@command{guix system delete-generations} when an upgrade completes.
+
+@quotation Note
+The unattended upgrade service does not run the garbage collector.  You
+will probably want to set up your own mcron job to run @command{guix gc}
+periodically.
+@end quotation
+
+@item @code{maximum-duration} (default: @code{3600})
+Maximum duration in seconds for the upgrade; past that time, the upgrade
+aborts.
+
+This is primarily useful to ensure the upgrade does not end up
+rebuilding or re-downloading ``the world''.
+
+@item @code{log-file} (default: @code{"/var/log/unattended-upgrade.log"})
+File where unattended upgrades are logged.
+@end table
+@end deftp
+
 @node X Window
 @subsection X Window
 
@@ -29628,6 +29740,7 @@ extend it by passing it lists of packages to add to the system profile.
 @end defvr
 
 @cindex provenance tracking, of the operating system
+@anchor{provenance-service-type}
 @defvr {Scheme Variable} provenance-service-type
 This is the type of the service that records @dfn{provenance meta-data}
 in the system itself.  It creates several files under