guix system: Add 'extension-graph' command.

* guix/scripts/system.scm (service-node-label, service-node-type,
  export-extension-graph): New procedures.
  (guix-system)[parse-sub-command]: Add 'extension-graph'.
  Honor it.
  (show-help): Add 'extension-graph'.
* doc/guix.texi (Invoking guix system): Document it.
  (Service Composition): Add cross-reference.

doc/guix.texi

@@ -6983,6 +6983,30 @@ KVM kernel module should be loaded, and the @file{/dev/kvm} device node
 must exist and be readable and writable by the user and by the daemon's
 build users.
 
+The @command{guix system} command has even more to offer!  The following
+sub-commands allow you to visualize how your system services relate to
+each other:
+
+@anchor{system-extension-graph}
+@table @code
+
+@item extension-graph
+Emit in Dot/Graphviz format to standard output the @dfn{service
+extension graph} of the operating system defined in @var{file}
+(@pxref{Service Composition}, for more information on service
+extensions.)
+
+The command:
+
+@example
+$ guix system extension-graph @var{file} | dot -Tpdf > services.pdf
+@end example
+
+produces a PDF file showing the extension relations among services.
+
+@end table
+
+
 @node Defining Services
 @subsection Defining Services
 
@@ -7015,6 +7039,7 @@ collects device management rules and makes them available to the eudev
 daemon; the @file{/etc} service populates the system's @file{/etc}
 directory.
 
+@cindex service extensions
 GuixSD services are connected by @dfn{extensions}.  For instance, the
 secure shell service @emph{extends} dmd---GuixSD's initialization system,
 running as PID@tie{}1---by giving it the command lines to start and stop
@@ -7035,6 +7060,9 @@ as arrows, a typical system might provide something like this:
 
 At the bottom, we see the @dfn{boot service}, which produces the boot
 script that is executed at boot time from the initial RAM disk.
+@xref{system-extension-graph, the @command{guix system extension-graph}
+command}, for information on how to generate this representation for a
+particular operating system definition.
 
 @cindex service types
 Technically, developers can define @dfn{service types} to express these

guix/scripts/system.scm

@@ -28,12 +28,14 @@
   #:use-module (guix profiles)
   #:use-module (guix scripts)
   #:use-module (guix scripts build)
+  #:use-module (guix scripts graph)
   #:use-module (guix build utils)
   #:use-module (gnu build install)
   #:use-module (gnu system)
   #:use-module (gnu system file-systems)
   #:use-module (gnu system vm)
   #:use-module (gnu system grub)
+  #:use-module (gnu services)
   #:use-module (gnu packages grub)
   #:use-module (srfi srfi-1)
   #:use-module (srfi srfi-19)
@@ -278,6 +280,38 @@ it atomically, and then run OS's activation script."
                        systems)))
     (filter-map system->grub-entry systems numbers times)))
 
+
+;;;
+;;; Graph.
+;;;
+
+(define (service-node-label service)
+  "Return a label to represent SERVICE."
+  (let ((type  (service-kind service))
+        (value (service-parameters service)))
+    (string-append (symbol->string (service-type-name type))
+                   (cond ((or (number? value) (symbol? value))
+                          (string-append " " (object->string value)))
+                         ((string? value)
+                          (string-append " " value))
+                         ((file-system? value)
+                          (string-append " " (file-system-mount-point value)))
+                         (else
+                          "")))))
+
+(define (service-node-type services)
+  "Return a node type for SERVICES.  Since <service> instances are not
+self-contained (they express dependencies on service types, not on services),
+we have to create the 'edges' procedure dynamically as a function of the full
+list of services."
+  (node-type
+   (name "service")
+   (description "the DAG of services")
+   (identifier (lift1 object-address %store-monad))
+   (label service-node-label)
+   (edges (lift1 (service-back-edges services) %store-monad))))
+
+
 
 ;;;
 ;;; Action.
@@ -366,6 +400,16 @@ building anything."
              ;; All we had to do was to build SYS.
              (return (derivation->output-path sys))))))))
 
+(define (export-extension-graph os port)
+  "Export the service extension graph of OS to PORT."
+  (let* ((services (operating-system-services os))
+         (boot     (find (lambda (service)
+                           (eq? (service-kind service) boot-service-type))
+                         services)))
+    (export-graph (list boot) (current-output-port)
+                  #:node-type (service-node-type services)
+                  #:reverse-edges? #t)))
+
 
 ;;;
 ;;; Options.
@@ -388,7 +432,9 @@ Build the operating system declared in FILE according to ACTION.\n"))
   (display (_ "\
    disk-image       build a disk image, suitable for a USB stick\n"))
   (display (_ "\
-   init             initialize a root file system to run GNU.\n"))
+   init             initialize a root file system to run GNU\n"))
+  (display (_ "\
+   extension-graph  emit the service extension graph in Dot format\n"))
 
   (show-build-options-help)
   (display (_ "
@@ -496,7 +542,8 @@ Build the operating system declared in FILE according to ACTION.\n"))
         (alist-cons 'argument arg result)
         (let ((action (string->symbol arg)))
           (case action
-            ((build vm vm-image disk-image reconfigure init)
+            ((build vm vm-image disk-image reconfigure init
+              extension-graph)
              (alist-cons 'action action result))
             (else (leave (_ "~a: unknown action~%") action))))))
 
@@ -561,6 +608,10 @@ Build the operating system declared in FILE according to ACTION.\n"))
       (run-with-store store
         (mbegin %store-monad
           (set-guile-for-build (default-guile))
+          (case action
+            ((extension-graph)
+             (export-extension-graph os (current-output-port)))
+            (else
              (perform-action action os
                              #:dry-run? dry?
                              #:derivations-only? (assoc-ref opts
@@ -574,7 +625,7 @@ Build the operating system declared in FILE according to ACTION.\n"))
                                                       (_ #f))
                                                     opts)
                              #:grub? grub?
-                          #:target target #:device device))
+                             #:target target #:device device))))
         #:system system))))
 
 ;;; system.scm ends here