utils: Allow wrap-program to be called multiple times.

* guix/build/utils.scm (wrap-program): Multiple invocations of
  wrap-program for the same file create successive wrappers.  Adjust
  docstring.
* tests/build-utils.scm: Test new wrap-program behavior.
  (%store): New variable.

tests/build-utils.scm

@@ -18,9 +18,24 @@
 
 
 (define-module (test-build-utils)
+  #:use-module (guix tests)
+  #:use-module (guix store)
+  #:use-module (guix derivations)
   #:use-module (guix build utils)
-  #:use-module (srfi srfi-64))
+  #:use-module (guix packages)
+  #:use-module (guix build-system)
+  #:use-module (guix build-system trivial)
+  #:use-module (gnu packages)
+  #:use-module (gnu packages bootstrap)
+  #:use-module (srfi srfi-34)
+  #:use-module (srfi srfi-64)
+  #:use-module (rnrs io ports)
+  #:use-module (ice-9 popen))
 
+(define %store
+  (open-connection-for-tests))
+
+
 (test-begin "build-utils")
 
 (test-equal "alist-cons-before"
@@ -80,6 +95,46 @@
                           port
                           cons)))))
 
+(test-assert "wrap-program, one input, multiple calls"
+  (let* ((p (package
+              (name "test-wrap-program") (version "0") (source #f)
+              (synopsis #f) (description #f) (license #f) (home-page #f)
+              (build-system trivial-build-system)
+              (arguments
+               `(#:guile ,%bootstrap-guile
+                 #:modules ((guix build utils))
+                 #:builder
+                 (let* ((out  (assoc-ref %outputs "out"))
+                        (bash (assoc-ref %build-inputs "bash"))
+                        (foo  (string-append out "/foo")))
+                   (begin
+                     (use-modules (guix build utils))
+                     (mkdir out)
+                     (call-with-output-file foo
+                       (lambda (p)
+                         (format p
+                                 "#!~a~%echo \"${GUIX_FOO} ${GUIX_BAR}\"~%"
+                                 bash)))
+                     (chmod foo #o777)
+                     ;; wrap-program uses `which' to find bash for the wrapper
+                     ;; shebang, but it can't know about the bootstrap bash in
+                     ;; the store, since it's not named "bash".  Help it out a
+                     ;; bit by providing a symlink it this package's output.
+                     (symlink bash (string-append out "/bash"))
+                     (setenv "PATH" out)
+                     (wrap-program foo `("GUIX_FOO" prefix ("hello")))
+                     (wrap-program foo `("GUIX_BAR" prefix ("world")))
+                     #t))))
+              (inputs `(("bash" ,(search-bootstrap-binary "bash"
+                                                          (%current-system)))))))
+         (d (package-derivation %store p)))
+    (and (build-derivations %store (pk 'drv d (list d)))
+         (let* ((p    (derivation->output-path d))
+                (foo  (string-append p "/foo"))
+                (pipe (open-input-pipe foo))
+                (str  (get-string-all pipe)))
+           (equal? str "hello world\n")))))
+
 (test-end)