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.

4 files changed, 145 insertions, 61 deletions

diff --git a/guix/derivations.scm b/guix/derivations.scm
index 2243d2ba46..cf329819c4 100644
--- a/guix/derivations.scm
+++ b/guix/derivations.scm
@@ -48,6 +48,7 @@
             derivation-input?
             derivation-input-path
             derivation-input-sub-derivations
+            derivation-input-output-paths
 
             fixed-output-derivation?
             derivation-hash
@@ -99,6 +100,14 @@ download with a fixed hash (aka. `fetchurl')."
      #t)
     (_ #f)))
 
+(define (derivation-input-output-paths input)
+  "Return the list of output paths corresponding to INPUT, a
+<derivation-input>."
+  (match input
+    (($ <derivation-input> path sub-drvs)
+     (map (cut derivation-path->output-path path <>)
+          sub-drvs))))
+
 (define (derivation-prerequisites drv)
   "Return the list of derivation-inputs required to build DRV, recursively."
   (let loop ((drv    drv)
@@ -113,47 +122,85 @@ download with a fixed hash (aka. `fetchurl')."
                  inputs)))))
 
 (define* (derivation-prerequisites-to-build store drv
-                                            #:key (outputs
-                                                   (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."
+                                            #:key
+                                            (outputs
+                                             (map
+                                              car
+                                              (derivation-outputs drv)))
+                                            (use-substitutes? #t))
+  "Return two values: the list of derivation-inputs required to build the
+OUTPUTS of DRV and not already available in STORE, recursively, and the list
+of required store paths that can be substituted.  When USE-SUBSTITUTES? is #f,
+that second value is the empty list."
+  (define (derivation-output-paths drv sub-drvs)
+    (match drv
+      (($ <derivation> outputs)
+       (map (lambda (sub-drv)
+              (derivation-output-path (assoc-ref outputs sub-drv)))
+            sub-drvs))))
+
   (define built?
     (cut valid-path? store <>))
 
+  (define substitutable?
+    ;; Return true if the given path is substitutable.  Call
+    ;; `substitutable-paths' upfront, to benefit from parallelism in the
+    ;; substituter.
+    (if use-substitutes?
+        (let ((s (substitutable-paths store
+                                      (append
+                                       (derivation-output-paths drv outputs)
+                                       (append-map
+                                        derivation-input-output-paths
+                                        (derivation-prerequisites drv))))))
+          (cut member <> s))
+        (const #f)))
+
   (define input-built?
-    (match-lambda
-     (($ <derivation-input> path sub-drvs)
-      (let ((out (map (cut derivation-path->output-path path <>)
-                      sub-drvs)))
-        (any built? out)))))
+    (compose (cut any built? <>) derivation-input-output-paths))
+
+  (define input-substitutable?
+    ;; Return true if and only if all of SUB-DRVS are subsitutable.  If at
+    ;; least one is missing, then everything must be rebuilt.
+    (compose (cut every substitutable? <>) derivation-input-output-paths))
 
   (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)))
-                              (derivation-inputs drv))))
-          (fold loop
-                (append inputs result)
-                (map (lambda (i)
-                       (call-with-input-file (derivation-input-path i)
-                         read-derivation))
-                     inputs)
-                (map derivation-input-sub-derivations inputs))))))
+    (every built? (derivation-output-paths drv sub-drvs)))
+
+  (define (derivation-substitutable? drv sub-drvs)
+    (every substitutable? (derivation-output-paths drv sub-drvs)))
+
+  (let loop ((drv        drv)
+             (sub-drvs   outputs)
+             (build      '())
+             (substitute '()))
+    (cond ((derivation-built? drv sub-drvs)
+           (values build substitute))
+          ((derivation-substitutable? drv sub-drvs)
+           (values build
+                   (append (derivation-output-paths drv sub-drvs)
+                           substitute)))
+          (else
+           (let ((inputs (remove (lambda (i)
+                                   (or (member i build) ; XXX: quadratic
+                                       (input-built? i)
+                                       (input-substitutable? i)))
+                                 (derivation-inputs drv))))
+             (fold2 loop
+                    (append inputs build)
+                    (append (append-map (lambda (input)
+                                          (if (and (not (input-built? input))
+                                                   (input-substitutable? input))
+                                              (derivation-input-output-paths
+                                               input)
+                                              '()))
+                                        (derivation-inputs drv))
+                            substitute)
+                    (map (lambda (i)
+                           (call-with-input-file (derivation-input-path i)
+                             read-derivation))
+                         inputs)
+                    (map derivation-input-sub-derivations inputs)))))))
 
 (define (%read-derivation drv-port)
   ;; Actually read derivation from DRV-PORT.
diff --git a/guix/scripts/build.scm b/guix/scripts/build.scm
index 339ad0d06f..f296f3031f 100644
--- a/guix/scripts/build.scm
+++ b/guix/scripts/build.scm
@@ -237,7 +237,9 @@ Build the given PACKAGE-OR-DERIVATION and return their output paths.\n"))
                                    (_ #f))
                                   opts)))
 
-          (show-what-to-build (%store) drv (assoc-ref opts 'dry-run?))
+          (show-what-to-build (%store) drv
+                              #:use-substitutes? (assoc-ref opts 'substitutes?)
+                              #:dry-run? (assoc-ref opts 'dry-run?))
 
           ;; TODO: Add more options.
           (set-build-options (%store)
diff --git a/guix/scripts/package.scm b/guix/scripts/package.scm
index 5b340c6ab7..f83c0573e7 100644
--- a/guix/scripts/package.scm
+++ b/guix/scripts/package.scm
@@ -674,7 +674,9 @@ Install, remove, or upgrade PACKAGES in a single transaction.\n"))
             (ensure-default-profile))
 
           (show-what-to-remove/install remove* install* dry-run?)
-          (show-what-to-build (%store) drv dry-run?)
+          (show-what-to-build (%store) drv
+                              #:use-substitutes? (assoc-ref opts 'substitutes?)
+                              #:dry-run? dry-run?)
 
           (or dry-run?
               (and (build-derivations (%store) drv)
diff --git a/guix/ui.scm b/guix/ui.scm
index dfb6418a10..db0711bb61 100644
--- a/guix/ui.scm
+++ b/guix/ui.scm
@@ -144,33 +144,66 @@ error."
           (leave (_ "expression `~s' does not evaluate to a package~%")
                  exp)))))
 
-(define* (show-what-to-build store drv #:optional dry-run?)
+(define* (show-what-to-build store drv
+                             #:key dry-run? (use-substitutes? #t))
   "Show what will or would (depending on DRY-RUN?) be built in realizing the
 derivations listed in DRV.  Return #t if there's something to build, #f
-otherwise."
-  (let* ((req  (append-map (lambda (drv-path)
-                             (let ((d (call-with-input-file drv-path
-                                        read-derivation)))
-                               (derivation-prerequisites-to-build
-                                store d)))
-                           drv))
-         (req* (delete-duplicates
-                (append (remove (compose (cute valid-path? store <>)
-                                         derivation-path->output-path)
-                                drv)
-                        (map derivation-input-path req)))))
+otherwise.  When USE-SUBSTITUTES?, check and report what is prerequisites are
+available for download."
+  (let*-values (((build download)
+                 (fold2 (lambda (drv-path build download)
+                          (let ((drv (call-with-input-file drv-path
+                                       read-derivation)))
+                            (let-values (((b d)
+                                          (derivation-prerequisites-to-build
+                                           store drv
+                                           #:use-substitutes?
+                                           use-substitutes?)))
+                              (values (append b build)
+                                      (append d download)))))
+                        '() '()
+                        drv))
+                ((build)                          ; add the DRV themselves
+                 (delete-duplicates
+                  (append (remove (compose (lambda (out)
+                                             (or (valid-path? store out)
+                                                 (and use-substitutes?
+                                                      (has-substitutes? store
+                                                                        out))))
+                                           derivation-path->output-path)
+                                  drv)
+                          (map derivation-input-path build))))
+                ((download)                   ; add the references of DOWNLOAD
+                 (delete-duplicates
+                  (append download
+                          (remove (cut valid-path? store <>)
+                                  (append-map
+                                   substitutable-references
+                                   (substitutable-path-info store download)))))))
     (if dry-run?
-        (format (current-error-port)
-                (N_ "~:[the following derivation would be built:~%~{   ~a~%~}~;~]"
-                    "~:[the following derivations would be built:~%~{    ~a~%~}~;~]"
-                    (length req*))
-                (null? req*) req*)
-        (format (current-error-port)
-                (N_ "~:[the following derivation will be built:~%~{   ~a~%~}~;~]"
-                    "~:[the following derivations will be built:~%~{    ~a~%~}~;~]"
-                    (length req*))
-                (null? req*) req*))
-    (pair? req*)))
+        (begin
+          (format (current-error-port)
+                  (N_ "~:[the following derivation would be built:~%~{   ~a~%~}~;~]"
+                      "~:[the following derivations would be built:~%~{    ~a~%~}~;~]"
+                      (length build))
+                  (null? build) build)
+          (format (current-error-port)
+                  (N_ "~:[the following file would be downloaded:~%~{   ~a~%~}~;~]"
+                      "~:[the following files would be downloaded:~%~{    ~a~%~}~;~]"
+                      (length download))
+                  (null? download) download))
+        (begin
+          (format (current-error-port)
+                  (N_ "~:[the following derivation will be built:~%~{   ~a~%~}~;~]"
+                      "~:[the following derivations will be built:~%~{    ~a~%~}~;~]"
+                      (length build))
+                  (null? build) build)
+          (format (current-error-port)
+                  (N_ "~:[the following file will be downloaded:~%~{   ~a~%~}~;~]"
+                      "~:[the following files will be downloaded:~%~{    ~a~%~}~;~]"
+                      (length download))
+                  (null? download) download)))
+    (pair? build)))
 
 (define-syntax with-error-handling
   (syntax-rules ()