libstore+nix-build: add load-limit setting and use its value for NIX_LOAD_LIMIT en var

src/libstore/build/local-derivation-goal.cc

@@ -1112,6 +1112,9 @@ void LocalDerivationGoal::initEnv()
     /* The maximum number of cores to utilize for parallel building. */
     env["NIX_BUILD_CORES"] = fmt("%d", settings.buildCores);
 
+    /* The maximum average load to target for buildsystems supporting it. */
+    env["NIX_LOAD_LIMIT"] = fmt("%d", settings.loadLimit);
+
     initTmpDir();
 
     /* Compatibility hack with Nix <= 0.7: if this is a fixed-output

src/libstore/daemon.cc

@@ -189,6 +189,7 @@ struct ClientSettings
     time_t maxSilentTime;
     bool verboseBuild;
     unsigned int buildCores;
+    unsigned int loadLimit;
     bool useSubstitutes;
     StringMap overrides;
 
@@ -202,6 +203,7 @@ struct ClientSettings
         settings.maxSilentTime = maxSilentTime;
         settings.verboseBuild = verboseBuild;
         settings.buildCores = buildCores;
+        settings.loadLimit = loadLimit;
         settings.useSubstitutes = useSubstitutes;
 
         for (auto & i : overrides) {

src/libstore/globals.hh

@@ -150,6 +150,22 @@ public:
         )",
         {"build-cores"}, false};
 
+    Setting<unsigned int> loadLimit{
+        this,
+        getDefaultCores(),
+        "load-limit",
+        R"(
+          Sets the value of the `NIX_LOAD_LIMIT` environment variable in the
+          invocation of builders. Builders can use this variable at their
+          discretion to schedule jobs based on the average load of the system.
+          For instance, in Nixpkgs, if the derivation attribute
+          `enableParallelBuilding` is set to `true`, the builder passes the
+          `-lN` flag to GNU Make. It can be overriden using the `--max-build-load`
+          command line switch and defaults to `1`. The value `0`means that
+          the builder should ignore the average system load.
+        )",
+        {}, false};
+
     /* Read-only mode.  Don't copy stuff to the store, don't change
        the database. */
     bool readOnlyMode = false;

src/nix-build/nix-build.cc

@@ -465,6 +465,7 @@ static void main_nix_build(int argc, char * * argv)
         env["NIX_BUILD_TOP"] = env["TMPDIR"] = env["TEMPDIR"] = env["TMP"] = env["TEMP"] = *tmp;
         env["NIX_STORE"] = store->storeDir;
         env["NIX_BUILD_CORES"] = std::to_string(settings.buildCores);
+        env["NIX_LOAD_LIMIT"] = std::to_string(settings.loadLimit);
 
         auto passAsFile = tokenizeString<StringSet>(getOr(drv.env, "passAsFile", ""));