profiles: Do away with 'manifest=?'.

* guix/profiles.scm (manifest=?): Remove.
* guix/scripts/package.scm (readlink*): New procedure.
  (guix-package)[process-actions]: Use 'readlink*' and compare the
  profile to be built, PROF, with PROFILE to determine whether there's
  nothing to be done.

guix/profiles.scm

@@ -48,7 +48,6 @@
             manifest-remove
             manifest-installed?
             manifest-matching-entries
-            manifest=?
 
             profile-manifest
             profile-derivation
@@ -196,13 +195,6 @@ must be a manifest-pattern."
 
   (filter matches? (manifest-entries manifest)))
 
-(define (manifest=? m1 m2)
-  "Return #t if manifests M1 and M2 are equal.  This differs from 'equal?' in
-that the 'inputs' field is ignored for the comparison, since it is know to
-have no effect on the manifest contents."
-  (equal? (manifest->sexp m1)
-          (manifest->sexp m2)))
-
 
 ;;;
 ;;; Profiles.

guix/scripts/package.scm

@@ -750,6 +750,16 @@ removed from MANIFEST."
   (unless (string=? profile %current-profile)
     (add-indirect-root store (canonicalize-path profile))))
 
+(define (readlink* file)
+  "Call 'readlink' until the result is not a symlink."
+  (catch 'system-error
+    (lambda ()
+      (readlink* (readlink file)))
+    (lambda args
+      (if (= EINVAL (system-error-errno args))
+          file
+          (apply throw args)))))
+
 
 ;;;
 ;;; Entry point.
@@ -921,9 +931,9 @@ more information.~%"))
              (when (equal? profile %current-profile)
                (ensure-default-profile))
 
-             (if (manifest=? new manifest)
-                 (format (current-error-port) (_ "nothing to be done~%"))
-                 (let ((prof-drv (profile-derivation (%store) new))
+             (unless (and (null? install) (null? remove))
+               (let* ((prof-drv (profile-derivation (%store) new))
+                      (prof     (derivation->output-path prof-drv))
                       (remove   (manifest-matching-entries manifest remove)))
                  (show-what-to-remove/install remove install dry-run?)
                  (show-what-to-build (%store) (list prof-drv)
@@ -931,9 +941,13 @@ more information.~%"))
                                      (assoc-ref opts 'substitutes?)
                                      #:dry-run? dry-run?)
 
-                   (or dry-run?
-                       (let* ((prof   (derivation->output-path prof-drv))
-                              (number (generation-number profile))
+                 (cond
+                  (dry-run? #t)
+                  ((and (file-exists? profile)
+                        (and=> (readlink* profile) (cut string=? prof <>)))
+                   (format (current-error-port) (_ "nothing to be done~%")))
+                  (else
+                   (let* ((number (generation-number profile))
 
                           ;; Always use NUMBER + 1 for the new profile,
                           ;; possibly overwriting a "previous future
@@ -950,7 +964,7 @@ more information.~%"))
                                            count)
                                     count)
                             (display-search-paths entries
-                                                      profile)))))))))))
+                                                  profile))))))))))))
 
   (define (process-query opts)
     ;; Process any query specified by OPTS.  Return #t when a query was