ui: Gracefully deal with zero-output derivations.

* guix/ui.scm (show-what-to-build)[built-or-substitutable?]: New
  procedure.  Check whether OUT is #f.
  Use it.
* tests/ui.scm ("show-what-to-build, zero outputs"): New test.

1 files changed, 12 insertions, 0 deletions

diff --git a/tests/ui.scm b/tests/ui.scm
index 886223ef54..4bf7a779c5 100644
--- a/tests/ui.scm
+++ b/tests/ui.scm
@@ -19,6 +19,8 @@
 
 (define-module (test-ui)
   #:use-module (guix ui)
+  #:use-module (guix store)
+  #:use-module (guix derivations)
   #:use-module (srfi srfi-1)
   #:use-module (srfi srfi-19)
   #:use-module (srfi srfi-64))
@@ -189,6 +191,16 @@ interface, and powerful string processing.")
     (lambda args
       #t)))
 
+(test-equal "show-what-to-build, zero outputs"
+  ""
+  (with-store store
+    (let ((drv (derivation store "zero" "/bin/sh" '()
+                           #:outputs '())))
+      (with-error-to-string
+       (lambda ()
+         ;; This should print nothing.
+         (show-what-to-build store (list drv)))))))
+
 (test-end "ui")