From e899eb6564a27d8e1a8ae94f3c601afff9cdd7b6 Mon Sep 17 00:00:00 2001
From: Antoine Viallon <antoine@lesviallon.fr>
Date: Fri, 24 Mar 2023 23:05:57 +0100
Subject: [PATCH] libstore+nix-build: add load-limit setting and use its value
 for NIX_LOAD_LIMIT env var Aimed at solving issue
 https://github.com/NixOS/nix/issues/7091

Signed-off-by: Antoine Viallon <antoine@lesviallon.fr>
---
 src/libstore/build/local-derivation-goal.cc |  3 +++
 src/libstore/daemon.cc                      |  2 ++
 src/libstore/globals.hh                     | 16 ++++++++++++++++
 src/nix-build/nix-build.cc                  |  1 +
 4 files changed, 22 insertions(+)

diff --git a/src/libstore/build/local-derivation-goal.cc b/src/libstore/build/local-derivation-goal.cc
index 3484d204441..847cb173f17 100644
--- a/src/libstore/build/local-derivation-goal.cc
+++ b/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
diff --git a/src/libstore/daemon.cc b/src/libstore/daemon.cc
index 7f8b0f90533..9196c13fddd 100644
--- a/src/libstore/daemon.cc
+++ b/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) {
diff --git a/src/libstore/globals.hh b/src/libstore/globals.hh
index 93086eaf8bd..07f97fae911 100644
--- a/src/libstore/globals.hh
+++ b/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;
diff --git a/src/nix-build/nix-build.cc b/src/nix-build/nix-build.cc
index a4b3b1f968c..7cecc1d3a35 100644
--- a/src/nix-build/nix-build.cc
+++ b/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", ""));