Added support for passing an (impure) NIX_BUILD_CORES variable to build expressions.

This patch adds the configuration file variable "build-cores" and the
command line argument "--cores". These settings specify the number of
CPU cores to utilize for parallel building within a job, i.e. by passing
an appropriate "-j" flag to GNU Make. The default value is 1, which
means that parallel building is *disabled*. If the number of build cores
is specified as 0 (synonymously: "guess" or "auto"), then the actual
value is supposed to be auto-detected by builders at run-time, i.e by
calling the nproc(1) utility from coreutils.

The environment variable $NIX_BUILD_CORES is available to builders, but
the contents of that variable does *not* influence the hash that goes
into the $out store path, i.e. the number of build cores to be utilized
can be changed at will without requiring any re-builds.

1 files changed, 3 insertions, 0 deletions

diff --git a/src/libstore/build.cc b/src/libstore/build.cc
index f901c1f7d9..a53c23bf44 100644
--- a/src/libstore/build.cc
+++ b/src/libstore/build.cc
@@ -1411,6 +1411,9 @@ void DerivationGoal::startBuilder()
        in the store or in the build directory). */
     env["NIX_STORE"] = nixStore;
 
+    /* The maximum number of cores to utilize for parallel building. */
+    env["NIX_BUILD_CORES"] = (format("%d") % buildCores).str();
+
     /* Add all bindings specified in the derivation. */
     foreach (StringPairs::iterator, i, drv.env)
         env[i->first] = i->second;