packages: 'package-transitive-supported-systems' detects cycles.

With this change, commands such as 'guix build' or 'guix package' report
obvious package-level cycles upfront.  Derivation-level cycles are not
detected.

* guix/packages.scm (&package-cyclic-dependency-error): New condition
type.
(package-transitive-supported-systems): Define 'visited', check it, and
parameterize it.
* guix/ui.scm (call-with-error-handling): Handle
'&package-cyclic-dependency-error'.
* tests/packages.scm ("package-transitive-supported-systems detects
cycles"): Add test.

1 files changed, 17 insertions, 0 deletions

diff --git a/tests/packages.scm b/tests/packages.scm
index 5e8eac99dc4..2b7ab01f7dd 100644
--- a/tests/packages.scm
+++ b/tests/packages.scm
@@ -368,6 +368,23 @@
           (package-transitive-supported-systems d)
           (package-transitive-supported-systems e))))
 
+(test-equal "package-transitive-supported-systems detects cycles"
+  '("c" "a" "b" "c")
+  (letrec* ((a (dummy-package "a"
+                 (build-system trivial-build-system)
+                 (native-inputs (list c))))
+            (b (dummy-package "b"
+                 (build-system trivial-build-system)
+                 (inputs (list a))))
+            (c (dummy-package "c"
+                 (build-system trivial-build-system)
+                 (inputs (list b)))))
+    (guard (c ((package-cyclic-dependency-error? c)
+               (map package-name
+                    (cons (package-error-package c)
+                          (package-error-dependency-cycle c)))))
+      (package-transitive-supported-systems c))))
+
 (test-assert "package-development-inputs"
   ;; Note: Due to propagated inputs, 'package-development-inputs' returns a
   ;; couple more inputs, such as 'linux-libre-headers'.