guix gc: Error out when extra arguments are passed.

Fixes <http://bugs.gnu.org/21817>.
Reported by Petter Berntsen <petter@mykolab.ch>.

* guix/scripts/gc.scm (guix-gc)[assert-no-extra-arguments]: New
  procedure.
  Use it for actions 'collect-garbage', 'optimize', and 'verify'.
* tests/guix-gc.sh: Add tests.

guix/scripts/gc.scm

@@ -182,6 +182,10 @@ Invoke the garbage collector.\n"))
                                (('argument . arg) arg)
                                (_ #f))
                               opts)))
+      (define (assert-no-extra-arguments)
+        (unless (null? paths)
+          (leave (_ "extraneous arguments: ~{~a ~}~%") paths)))
+
       (define (list-relatives relatives)
         (for-each (compose (lambda (path)
                              (for-each (cut simple-format #t "~a~%" <>)
@@ -192,6 +196,7 @@ Invoke the garbage collector.\n"))
 
       (case (assoc-ref opts 'action)
         ((collect-garbage)
+         (assert-no-extra-arguments)
          (let ((min-freed (assoc-ref opts 'min-freed)))
            (if min-freed
                (collect-garbage store min-freed)
@@ -205,8 +210,10 @@ Invoke the garbage collector.\n"))
         ((list-referrers)
          (list-relatives referrers))
         ((optimize)
+         (assert-no-extra-arguments)
          (optimize-store store))
         ((verify)
+         (assert-no-extra-arguments)
          (let ((options (assoc-ref opts 'verify-options)))
            (exit
             (verify-store store

tests/guix-gc.sh

@@ -1,5 +1,5 @@
 # GNU Guix --- Functional package management for GNU
-# Copyright © 2013 Ludovic Courtès <ludo@gnu.org>
+# Copyright © 2013, 2015 Ludovic Courtès <ludo@gnu.org>
 #
 # This file is part of GNU Guix.
 #
@@ -25,6 +25,12 @@ guix gc --version
 trap "rm -f guix-gc-root" EXIT
 rm -f guix-gc-root
 
+# For some operations, passing extra arguments is an error.
+for option in "" "-C 500M" "--verify" "--optimize"
+do
+    if guix gc $option whatever; then false; else true; fi
+done
+
 # Check the references of a .drv.
 drv="`guix build guile-bootstrap -d`"
 out="`guix build guile-bootstrap`"