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.

guix/packages.scm

@@ -168,6 +168,9 @@ (define-module (guix packages)
             package-error-invalid-license
             &package-input-error
             package-input-error?
+            &package-cyclic-dependency-error
+            package-cyclic-dependency-error?
+            package-error-dependency-cycle
             package-error-invalid-input
             &package-cross-build-system-error
             package-cross-build-system-error?
@@ -806,6 +809,10 @@ (define-condition-type &package-input-error &package-error
   package-input-error?
   (input package-error-invalid-input))
 
+(define-condition-type &package-cyclic-dependency-error &package-error
+  package-cyclic-dependency-error?
+  (cycle package-error-dependency-cycle))
+
 (define-condition-type &package-cross-build-system-error &package-error
   package-cross-build-system-error?)
 
@@ -1317,8 +1324,20 @@ (define package-transitive-supported-systems
   (let ()
     (define (supported-systems-procedure system)
       (define supported-systems
+        ;; The VISITED parameter allows for cycle detection.  This is a pretty
+        ;; strategic place to do that: most commands call it upfront, yet it's
+        ;; not on the hot path of 'package->derivation'.  The downside is that
+        ;; only package-level cycles are detected.
+        (let ((visited (make-parameter (setq))))
           (mlambdaq (package)
-          (parameterize ((%current-system system))
+            (when (set-contains? (visited) package)
+              (raise (condition
+                      (&package-cyclic-dependency-error
+                       (package package)
+                       (cycle (set->list (visited)))))))
+
+            (parameterize ((visited (set-insert package (visited)))
+                           (%current-system system))
               (fold (lambda (input systems)
                       (match input
                         ((label (? package? package) . _)
@@ -1327,7 +1346,7 @@ (define supported-systems
                         (_
                          systems)))
                     (package-supported-systems package)
-                  (bag-direct-inputs (package->bag package system #f))))))
+                    (bag-direct-inputs (package->bag package system #f)))))))
 
       supported-systems)
 

guix/ui.scm

@@ -722,6 +722,15 @@ (define (port-filename* port)
                 (leave (G_ "~a:~a:~a: package `~a' has an invalid input: ~s~%")
                        file line column
                        (package-full-name package) input)))
+             ((package-cyclic-dependency-error? c)
+              (let ((package (package-error-package c)))
+                (leave (package-location package)
+                       (G_ "~a: dependency cycle detected:
+  ~a~{ -> ~a~}~%")
+                       (package-full-name package)
+                       (package-full-name package)
+                       (map package-full-name
+                            (package-error-dependency-cycle c)))))
              ((package-cross-build-system-error? c)
               (let* ((package (package-error-package c))
                      (loc     (package-location package))

tests/packages.scm

@@ -368,6 +368,23 @@ (define read-at
           (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'.