scripts: Report what will be substituted.

* guix/derivations.scm (derivation-input-output-paths): New procedure.
  (derivation-prerequisites-to-build): New `use-substitutes?' keyword
  argument.  Change two return the list of substitutable paths as a
  second argument.
* guix/ui.scm (show-what-to-build): Turn `dry-run?' into a keyword
  argument.  New `use-substitutes?' keyword argument.  Use `fold2' and
  adjust to use both return values of
  `derivation-prerequisites-to-build'.  Display what will/would be
  downloaded.
* guix/scripts/build.scm (guix-build): Adjust accordingly.
* guix/scripts/package.scm (guix-package): Likewise.
* tests/derivations.scm ("derivation-prerequisites-to-build and
  substitutes"): New test.

1 files changed, 46 insertions, 0 deletions

diff --git a/tests/derivations.scm b/tests/derivations.scm
index 6012e73216..a50c1af878 100644
--- a/tests/derivations.scm
+++ b/tests/derivations.scm
@@ -32,6 +32,7 @@
   #:use-module (srfi srfi-64)
   #:use-module (rnrs io ports)
   #:use-module (rnrs bytevectors)
+  #:use-module (web uri)
   #:use-module (ice-9 rdelim)
   #:use-module (ice-9 regex)
   #:use-module (ice-9 ftw)
@@ -398,6 +399,51 @@
          ;; prerequisite to build because DRV itself is already built.
          (null? (derivation-prerequisites-to-build %store drv)))))
 
+(test-skip (if (getenv "GUIX_BINARY_SUBSTITUTE_URL") 0 1))
+(test-assert "derivation-prerequisites-to-build and substitutes"
+  (let*-values (((store)
+                 (open-connection))
+                ((drv-path drv)
+                 (build-expression->derivation store "prereq-subst"
+                                               (%current-system)
+                                               (random 1000) '()))
+                ((output)
+                 (derivation-output-path
+                  (assoc-ref (derivation-outputs drv) "out")))
+                ((dir)
+                 (and=> (getenv "GUIX_BINARY_SUBSTITUTE_URL")
+                        (compose uri-path string->uri))))
+    ;; Create fake substituter data, to be read by `substitute-binary'.
+    (call-with-output-file (string-append dir "/nix-cache-info")
+      (lambda (p)
+        (format p "StoreDir: ~a\nWantMassQuery: 0\n"
+                (%store-prefix))))
+    (call-with-output-file (string-append dir "/" (store-path-hash-part output)
+                                          ".narinfo")
+      (lambda (p)
+        (format p "StorePath: ~a
+URL: ~a
+Compression: none
+NarSize: 1234
+References: 
+System: ~a
+Deriver: ~a~%"
+                output                              ; StorePath
+                (string-append dir "/example.nar")  ; URL
+                (%current-system)                   ; System
+                (basename drv-path))))              ; Deriver
+
+    (let-values (((build download)
+                  (derivation-prerequisites-to-build store drv))
+                 ((build* download*)
+                  (derivation-prerequisites-to-build store drv
+                                                     #:use-substitutes? #f)))
+      (pk build download build* download*)
+      (and (null? build)
+           (equal? download (list output))
+           (null? download*)
+           (null? build*)))))
+
 (test-assert "build-expression->derivation with expression returning #f"
   (let* ((builder  '(begin
                       (mkdir %output)