derivations: Fix `derivation-prerequisites-to-build' when outputs are there.

Before it would list inputs not built, even if the outputs of the given
derivation were already available.

* guix/derivations.scm (derivation-prerequisites-to-build): Add
  `outputs' keyword parameter.
  [built?, derivation-built?]: New procedures.
  [loop]: Add `sub-drvs' parameter.  Use `derivation-built?' to check if
  the SUB-DRVS of DRV are built before checking its inputs.

guix/derivations.scm

@@ -112,18 +112,37 @@ download with a fixed hash (aka. `fetchurl')."
                      read-derivation))
                  inputs)))))
 
-(define (derivation-prerequisites-to-build store drv)
+(define* (derivation-prerequisites-to-build store drv
-  "Return the list of derivation-inputs required to build DRV and not already
+                                            #:key (outputs
-available in STORE, recursively."
+                                                   (map
+                                                    car
+                                                    (derivation-outputs drv))))
+  "Return the list of derivation-inputs required to build the OUTPUTS of
+DRV and not already available in STORE, recursively."
+  (define built?
+    (cut valid-path? store <>))
+
   (define input-built?
     (match-lambda
      (($ <derivation-input> path sub-drvs)
       (let ((out (map (cut derivation-path->output-path path <>)
                       sub-drvs)))
-        (any (cut valid-path? store <>) out)))))
+        (any built? out)))))
+
+  (define (derivation-built? drv sub-drvs)
+    (match drv
+      (($ <derivation> outputs)
+       (let ((paths (map (lambda (sub-drv)
+                           (derivation-output-path
+                            (assoc-ref outputs sub-drv)))
+                         sub-drvs)))
+         (every built? paths)))))
 
   (let loop ((drv      drv)
+             (sub-drvs outputs)
              (result   '()))
+    (if (derivation-built? drv sub-drvs)
+        result
         (let ((inputs (remove (lambda (i)
                                 (or (member i result) ; XXX: quadratic
                                     (input-built? i)))
@@ -133,7 +152,8 @@ available in STORE, recursively."
                 (map (lambda (i)
                        (call-with-input-file (derivation-input-path i)
                          read-derivation))
-                 inputs)))))
+                     inputs)
+                (map derivation-input-sub-derivations inputs))))))
 
 (define (read-derivation drv-port)
   "Read the derivation from DRV-PORT and return the corresponding

tests/derivations.scm

@@ -353,6 +353,44 @@
     ;; built.
     (null? (derivation-prerequisites-to-build %store drv))))
 
+(test-assert "derivation-prerequisites-to-build when outputs already present"
+  (let*-values (((builder)
+                 '(begin (mkdir %output) #t))
+                ((input-drv-path input-drv)
+                 (build-expression->derivation %store "input"
+                                               (%current-system)
+                                               builder '()))
+                ((input-path)
+                 (derivation-output-path
+                  (assoc-ref (derivation-outputs input-drv)
+                             "out")))
+                ((drv-path drv)
+                 (build-expression->derivation %store "something"
+                                               (%current-system)
+                                               builder
+                                               `(("i" ,input-drv-path))))
+                ((output)
+                 (derivation-output-path
+                  (assoc-ref (derivation-outputs drv) "out"))))
+    ;; Make sure these things are not already built.
+    (when (valid-path? %store input-path)
+      (delete-paths %store (list input-path)))
+    (when (valid-path? %store output)
+      (delete-paths %store (list output)))
+
+    (and (equal? (map derivation-input-path
+                      (derivation-prerequisites-to-build %store drv))
+                 (list input-drv-path))
+
+         ;; Build DRV and delete its input.
+         (build-derivations %store (list drv-path))
+         (delete-paths %store (list input-path))
+         (not (valid-path? %store input-path))
+
+         ;; Now INPUT-PATH is missing, yet it shouldn't be listed as a
+         ;; prerequisite to build because DRV itself is already built.
+         (null? (derivation-prerequisites-to-build %store drv)))))
+
 (test-assert "build-expression->derivation with expression returning #f"
   (let* ((builder  '(begin
                       (mkdir %output)