derivations: Raise an error for references to non-existent outputs.

Fixes <http://bugs.gnu.org/19630>.
Reported by Ricardo Wurmus <ricardo.wurmus@mdc-berlin.de>.

* guix/derivations.scm (&derivation-error,
  &derivation-missing-output-error): New error conditions.
  (derivation->output-path): Raise a '&derivation-missing-output-error'
  if OUTPUT is not an output of DRV.
* guix/ui.scm (call-with-error-handling): Add case for
  'derivation-missing-output-error?'.
  (show-what-to-build): Check whether (derivation-outputs drv) is
  empty.
* tests/packages.scm ("reference to non-existent output"): Add test.

1 files changed, 30 insertions, 3 deletions

diff --git a/guix/derivations.scm b/guix/derivations.scm
index 4c34fcb4b8..edf6b57e86 100644
--- a/guix/derivations.scm
+++ b/guix/derivations.scm
@@ -21,6 +21,8 @@
   #:use-module (srfi srfi-9)
   #:use-module (srfi srfi-9 gnu)
   #:use-module (srfi srfi-26)
+  #:use-module (srfi srfi-34)
+  #:use-module (srfi srfi-35)
   #:use-module (rnrs io ports)
   #:use-module (rnrs bytevectors)
   #:use-module (ice-9 match)
@@ -59,6 +61,13 @@
             derivation-input-sub-derivations
             derivation-input-output-paths
 
+            &derivation-error
+            derivation-error?
+            derivation-error-derivation
+            &derivation-missing-output-error
+            derivation-missing-output-error?
+            derivation-missing-output
+
             derivation-name
             derivation-output-names
             fixed-output-derivation?
@@ -98,6 +107,18 @@
   #:replace (build-derivations))
 
 ;;;
+;;; Error conditions.
+;;;
+
+(define-condition-type &derivation-error &nix-error
+  derivation-error?
+  (derivation derivation-error-derivation))
+
+(define-condition-type &derivation-missing-output-error &derivation-error
+  derivation-missing-output-error?
+  (output derivation-missing-output))
+
+;;;
 ;;; Nix derivations, as implemented in Nix's `derivations.cc'.
 ;;;
 
@@ -509,9 +530,15 @@ that form."
         (cut write-derivation drv <>))))))
 
 (define* (derivation->output-path drv #:optional (output "out"))
-  "Return the store path of its output OUTPUT."
-  (let ((outputs (derivation-outputs drv)))
-    (and=> (assoc-ref outputs output) derivation-output-path)))
+  "Return the store path of its output OUTPUT.  Raise a
+'&derivation-missing-output-error' condition if OUTPUT is not an output of
+DRV."
+  (let ((output* (assoc-ref (derivation-outputs drv) output)))
+    (if output*
+        (derivation-output-path output*)
+        (raise (condition (&derivation-missing-output-error
+                           (derivation drv)
+                           (output output)))))))
 
 (define (derivation->output-paths drv)
   "Return the list of name/path pairs of the outputs of DRV."