derivations: Add #:substitutable?, distinguished from #:local-build?.

Fixes <http://bugs.gnu.org/18747>.

* guix/derivations.scm (substitutable-derivation?): Rewrite to check for
  "allowSubstitutes".
  (derivation): Add #:substitutable? parameter.
  [user+system-env-vars]: Honor it.
  (build-expression->derivation): Add #:substitutable? and honor it.
* guix/gexp.scm (gexp->derivation): Likewise.
* tests/derivations.scm ("derivation-prerequisites-to-build and substitutes,
  non-substitutable build"): Use #:substitutable? instead of #:local-build?.
  ("substitutable-derivation?", "derivation-prerequisites-to-build and
  substitutes, local build"): New tests.
* guix/download.scm (url-fetch): Adjust comment.
* guix/git-download.scm (git-fetch): Likewise.
* guix/build-system/gnu.scm (gnu-build, gnu-cross-build): Use #:substitutable?
  instead of #:local-build?.
* doc/guix.texi (Derivations, G-Expressions): Adjust accordingly.

5 files changed, 28 insertions, 21 deletions

diff --git a/guix/build-system/gnu.scm b/guix/build-system/gnu.scm
index 05b6e6f680..8702c6c915 100644
--- a/guix/build-system/gnu.scm
+++ b/guix/build-system/gnu.scm
@@ -367,10 +367,7 @@ are allowed to refer to."
                                 #:inputs input-drvs
                                 #:outputs outputs
                                 #:modules imported-modules
-
-                                ;; XXX: Update when
-                                ;; <http://bugs.gnu.org/18747> is fixed.
-                                #:local-build? (not substitutable?)
+                                #:substitutable? substitutable?
 
                                 #:allowed-references
                                 (and allowed-references
@@ -513,10 +510,7 @@ platform."
                                 #:inputs (append native-drvs target-drvs)
                                 #:outputs outputs
                                 #:modules imported-modules
-
-                                ;; XXX: Update when
-                                ;; <http://bugs.gnu.org/18747> is fixed.
-                                #:local-build? (not substitutable?)
+                                #:substitutable? substitutable?
 
                                 #:allowed-references
                                 (and allowed-references
diff --git a/guix/derivations.scm b/guix/derivations.scm
index 1056caa70a..f12127f0ed 100644
--- a/guix/derivations.scm
+++ b/guix/derivations.scm
@@ -223,10 +223,13 @@ result is the set of prerequisites of DRV not already in valid."
     (("preferLocalBuild" . "1") #f)
     (_ #t)))
 
-(define substitutable-derivation?
-  ;; Return #t if the derivation can be substituted.  Currently the two are
-  ;; synonymous, see <http://bugs.gnu.org/18747>.
-  offloadable-derivation?)
+(define (substitutable-derivation? drv)
+  "Return #t if DRV can be substituted."
+  (match (assoc "allowSubstitutes"
+                (derivation-builder-environment-vars drv))
+    (("allowSubstitutes" . value)
+     (string=? value "1"))
+    (_ #t)))
 
 (define (derivation-output-paths drv sub-drvs)
   "Return the output paths of outputs SUB-DRVS of DRV."
@@ -692,7 +695,8 @@ HASH-ALGO, of the derivation NAME.  RECURSIVE? has the same meaning as for
                      (inputs '()) (outputs '("out"))
                      hash hash-algo recursive?
                      references-graphs allowed-references
-                     leaked-env-vars local-build?)
+                     leaked-env-vars local-build?
+                     (substitutable? #t))
   "Build a derivation with the given arguments, and return the resulting
 <derivation> object.  When HASH and HASH-ALGO are given, a
 fixed-output derivation is created---i.e., one whose result is known in
@@ -715,7 +719,10 @@ as \"http_proxy\" to be passed to derivations that download files.
 
 When LOCAL-BUILD? is true, declare that the derivation is not a good candidate
 for offloading and should rather be built locally.  This is the case for small
-derivations where the costs of data transfers would outweigh the benefits."
+derivations where the costs of data transfers would outweigh the benefits.
+
+When SUBSTITUTABLE? is false, declare that substitutes of the derivation's
+output should not be used."
   (define (add-output-paths drv)
     ;; Return DRV with an actual store path for each of its output and the
     ;; corresponding environment variable.
@@ -753,6 +760,9 @@ derivations where the costs of data transfers would outweigh the benefits."
     (let ((env-vars `(,@(if local-build?
                             `(("preferLocalBuild" . "1"))
                             '())
+                      ,@(if (not substitutable?)
+                            `(("allowSubstitutes" . "0"))
+                            '())
                       ,@(if allowed-references
                             `(("allowedReferences"
                                . ,(string-join allowed-references)))
@@ -1173,7 +1183,7 @@ applied."
                                        guile-for-build
                                        references-graphs
                                        allowed-references
-                                       local-build?)
+                                       local-build? (substitutable? #t))
   "Return a derivation that executes Scheme expression EXP as a builder
 for derivation NAME.  INPUTS must be a list of (NAME DRV-PATH SUB-DRV)
 tuples; when SUB-DRV is omitted, \"out\" is assumed.  MODULES is a list
@@ -1193,7 +1203,7 @@ EXP is built using GUILE-FOR-BUILD (a derivation).  When GUILE-FOR-BUILD is
 omitted or is #f, the value of the `%guile-for-build' fluid is used instead.
 
 See the `derivation' procedure for the meaning of REFERENCES-GRAPHS,
-ALLOWED-REFERENCES, and LOCAL-BUILD?."
+ALLOWED-REFERENCES, LOCAL-BUILD?, and SUBSTITUTABLE?."
   (define guile-drv
     (or guile-for-build (%guile-for-build)))
 
@@ -1319,7 +1329,8 @@ ALLOWED-REFERENCES, and LOCAL-BUILD?."
                 #:outputs outputs
                 #:references-graphs references-graphs
                 #:allowed-references allowed-references
-                #:local-build? local-build?)))
+                #:local-build? local-build?
+                #:substitutable? substitutable?)))
 
 
 ;;;
diff --git a/guix/download.scm b/guix/download.scm
index 3f7f7badce..9a80af7de6 100644
--- a/guix/download.scm
+++ b/guix/download.scm
@@ -277,8 +277,8 @@ in the store."
                             ;; In general, offloading downloads is not a good idea.
                             ;;#:local-build? #t
                             ;; FIXME: The above would also disable use of
-                            ;; substitutes, so comment it out; see
-                            ;; <https://bugs.gnu.org/18747>.
+                            ;; substitutes on old daemons, so comment it out;
+                            ;; see <https://bugs.gnu.org/18747>.
                             )))))
 
 (define* (download-to-store store url #:optional (name (basename url))
diff --git a/guix/gexp.scm b/guix/gexp.scm
index 09b51b3936..49dcc99ac3 100644
--- a/guix/gexp.scm
+++ b/guix/gexp.scm
@@ -320,7 +320,7 @@ names and file names suitable for the #:allowed-references argument to
                            references-graphs
                            allowed-references
                            leaked-env-vars
-                           local-build?)
+                           local-build? (substitutable? #t))
   "Return a derivation NAME that runs EXP (a gexp) with GUILE-FOR-BUILD (a
 derivation) on SYSTEM.  When TARGET is true, it is used as the
 cross-compilation target triplet for packages referred to by EXP.
@@ -439,7 +439,8 @@ The other arguments are as for 'derivation'."
                       #:references-graphs (and=> graphs graphs-file-names)
                       #:allowed-references allowed
                       #:leaked-env-vars leaked-env-vars
-                      #:local-build? local-build?))))
+                      #:local-build? local-build?
+                      #:substitutable? substitutable?))))
 
 (define* (gexp-inputs exp #:key native?)
   "Return the input list for EXP.  When NATIVE? is true, return only native
diff --git a/guix/git-download.scm b/guix/git-download.scm
index f4b48d7a6b..0f2218c13e 100644
--- a/guix/git-download.scm
+++ b/guix/git-download.scm
@@ -90,6 +90,7 @@ HASH-ALGO (a symbol).  Use NAME as the file name, or a generic name if #f."
     (gexp->derivation (or name "git-checkout") build
                       #:system system
                       ;; FIXME: See <https://bugs.gnu.org/18747>.
+                      ;; Uncomment when fixed daemons are widely deployed.
                       ;;#:local-build? #t
                       #:hash-algo hash-algo
                       #:hash hash