pull: Distinguish "already up to date" from "updated".

* guix/ui.scm (show-what-to-build): Return (length req*).
* guix/scripts/pull.scm (guix-pull): Print an "already up to date"
  message when there's nothing to build.

guix/scripts/pull.scm

@@ -216,12 +216,16 @@ Download and deploy the latest version of Guix.\n"))
                         ((source-dir)
                          (derivation-output-path
                           (assoc-ref (derivation-outputs drv) "out"))))
-            (show-what-to-build store (list source))
-            (if (build-derivations store (list source))
-                (let ((latest (string-append config-dir "/latest")))
-                  (add-indirect-root store latest)
-                  (switch-symlinks latest source-dir)
-                  (format #t
-                          (_ "updated ~a successfully deployed under `~a'~%")
-                          %guix-package-name latest)
+            (if (show-what-to-build store (list source))
+                (if (build-derivations store (list source))
+                    (let ((latest (string-append config-dir "/latest")))
+                      (add-indirect-root store latest)
+                      (switch-symlinks latest source-dir)
+                      (format #t
+                              (_ "updated ~a successfully deployed under `~a'~%")
+                              %guix-package-name latest)
+                      #t)
+                    (leave (_ "failed to update Guix, check the build log~%")))
+                (begin
+                  (display (_ "Guix already up to date\n"))
                   #t))))))))

guix/ui.scm

@@ -118,7 +118,8 @@ General help using GNU software: <http://www.gnu.org/gethelp/>"))
 
 (define* (show-what-to-build store drv #:optional dry-run?)
   "Show what will or would (depending on DRY-RUN?) be built in realizing the
-derivations listed in DRV."
+derivations listed in DRV.  Return #t if there's something to build, #f
+otherwise."
   (let* ((req  (append-map (lambda (drv-path)
                              (let ((d (call-with-input-file drv-path
                                         read-derivation)))
@@ -140,7 +141,8 @@ derivations listed in DRV."
                 (N_ "~:[the following derivation will be built:~%~{   ~a~%~}~;~]"
                     "~:[the following derivations will be built:~%~{    ~a~%~}~;~]"
                     (length req*))
-                (null? req*) req*))))
+                (null? req*) req*))
+    (pair? req*)))
 
 (define-syntax with-error-handling
   (syntax-rules ()