derivations: Add #:local-build? parameter for derivations.

* guix/derivations.scm (derivation): Add #:local-build? parameter and
  honor it.
  (build-expression->derivation): Likewise.
* doc/guix.texi (Derivations): Update documentation of these
  procedures.

2 files changed, 41 insertions, 19 deletions

diff --git a/doc/guix.texi b/doc/guix.texi
index 325467c82d..91fa07f1a8 100644
--- a/doc/guix.texi
+++ b/doc/guix.texi
@@ -1452,7 +1452,11 @@ derivations as Scheme objects, along with procedures to create and
 otherwise manipulate derivations.  The lowest-level primitive to create
 a derivation is the @code{derivation} procedure:
 
-@deffn {Scheme Procedure} derivation @var{store} @var{name} @var{builder} @var{args} [#:outputs '("out")] [#:hash #f] [#:hash-algo #f] [#:hash-mode #f] [#:inputs '()] [#:env-vars '()] [#:system (%current-system)] [#:references-graphs #f]
+@deffn {Scheme Procedure} derivation @var{store} @var{name} @var{builder} @
+  @var{args} [#:outputs '("out")] [#:hash #f] [#:hash-algo #f] @
+  [#:hash-mode #f] [#:inputs '()] [#:env-vars '()] @
+  [#:system (%current-system)] [#:references-graphs #f] @
+  [#:local-build? #f]
 Build a derivation with the given arguments, and return the resulting
 @code{<derivation>} object.
 
@@ -1464,6 +1468,11 @@ When @var{references-graphs} is true, it must be a list of file
 name/store path pairs.  In that case, the reference graph of each store
 path is exported in the build environment in the corresponding file, in
 a simple text format.
+
+When @var{local-build?} is true, declare that the derivation is not a
+good candidate for offloading and should rather be built locally
+(@pxref{Daemon Offload Setup}).  This is the case for small derivations
+where the costs of data transfers would outweigh the benefits.
 @end deffn
 
 @noindent
@@ -1494,7 +1503,7 @@ the caller to directly pass a Guile expression as the build script:
        [#:system (%current-system)] [#:inputs '()] @
        [#:outputs '("out")] [#:hash #f] [#:hash-algo #f] @
        [#:env-vars '()] [#:modules '()] @
-       [#:references-graphs #f] [#:guile-for-build #f]
+       [#:references-graphs #f] [#:local-build? #f] [#:guile-for-build #f]
 Return a derivation that executes Scheme expression @var{exp} as a
 builder for derivation @var{name}.  @var{inputs} must be a list of
 @code{(name drv-path sub-drv)} tuples; when @var{sub-drv} is omitted,
@@ -1516,7 +1525,8 @@ terminates by passing the result of @var{exp} to @code{exit}; thus, when
 @var{guile-for-build} is omitted or is @code{#f}, the value of the
 @code{%guile-for-build} fluid is used instead.
 
-See the @code{derivation} procedure for the meaning of @var{references-graphs}.
+See the @code{derivation} procedure for the meaning of @var{references-graphs}
+and @var{local-build?}.
 @end deffn
 
 @noindent
diff --git a/guix/derivations.scm b/guix/derivations.scm
index 3d9f0affbf..cc8e37c973 100644
--- a/guix/derivations.scm
+++ b/guix/derivations.scm
@@ -1,5 +1,5 @@
 ;;; GNU Guix --- Functional package management for GNU
-;;; Copyright © 2012, 2013 Ludovic Courtès <ludo@gnu.org>
+;;; Copyright © 2012, 2013, 2014 Ludovic Courtès <ludo@gnu.org>
 ;;;
 ;;; This file is part of GNU Guix.
 ;;;
@@ -532,7 +532,8 @@ the derivation called NAME with hash HASH."
                      (system (%current-system)) (env-vars '())
                      (inputs '()) (outputs '("out"))
                      hash hash-algo hash-mode
-                     references-graphs)
+                     references-graphs
+                     local-build?)
   "Build a derivation with the given arguments, and return the resulting
 <derivation> object.  When HASH, HASH-ALGO, and HASH-MODE are given, a
 fixed-output derivation is created---i.e., one whose result is known in
@@ -540,7 +541,11 @@ advance, such as a file download.
 
 When REFERENCES-GRAPHS is true, it must be a list of file name/store path
 pairs.  In that case, the reference graph of each store path is exported in
-the build environment in the corresponding file, in a simple text format."
+the build environment in the corresponding file, in a simple text format.
+
+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."
   (define (add-output-paths drv)
     ;; Return DRV with an actual store path for each of its output and the
     ;; corresponding environment variable.
@@ -571,16 +576,20 @@ the build environment in the corresponding file, in a simple text format."
     ;; Some options are passed to the build daemon via the env. vars of
     ;; derivations (urgh!).  We hide that from our API, but here is the place
     ;; where we kludgify those options.
-    (match references-graphs
-      (((file . path) ...)
-       (let ((value (map (cut string-append <> " " <>)
-                         file path)))
-         ;; XXX: This all breaks down if an element of FILE or PATH contains
-         ;; white space.
-         `(("exportReferencesGraph" . ,(string-join value " "))
-           ,@env-vars)))
-      (#f
-       env-vars)))
+    (let ((env-vars (if local-build?
+                        `(("preferLocalBuild" . "1")
+                          ,@env-vars)
+                        env-vars)))
+      (match references-graphs
+        (((file . path) ...)
+         (let ((value (map (cut string-append <> " " <>)
+                           file path)))
+           ;; XXX: This all breaks down if an element of FILE or PATH contains
+           ;; white space.
+           `(("exportReferencesGraph" . ,(string-join value " "))
+             ,@env-vars)))
+        (#f
+         env-vars))))
 
   (define (env-vars-with-empty-outputs env-vars)
     ;; Return a variant of ENV-VARS where each OUTPUTS is associated with an
@@ -904,7 +913,8 @@ they can refer to each other."
                                        (env-vars '())
                                        (modules '())
                                        guile-for-build
-                                       references-graphs)
+                                       references-graphs
+                                       local-build?)
   "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
@@ -923,7 +933,8 @@ EXP returns #f, the build is considered to have failed.
 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."
+See the `derivation' procedure for the meaning of REFERENCES-GRAPHS and
+LOCAL-BUILD?."
   (define guile-drv
     (or guile-for-build (%guile-for-build)))
 
@@ -1046,4 +1057,5 @@ See the `derivation' procedure for the meaning of REFERENCES-GRAPHS."
 
                 #:hash hash #:hash-algo hash-algo
                 #:outputs outputs
-                #:references-graphs references-graphs)))
+                #:references-graphs references-graphs
+                #:local-build? local-build?)))