build-cores: detect cores automatically if set to 0

This changes makes nix detect a machines available cores automatically whenever build-cores is set to 0.

So far, nix simply passed NIX_BUILD_CORES=0 whenever build-cores is set to 0. (only when build-cores is unset it was detecting cores automatically)

The behavior of passing NIX_BUILD_CORES=0 leads to a performance penalty when sourcing nixpkgs' generic builder's `setup.sh`, as setup.sh has to execute `nproc`. This significantly slows down sourcing of setup.sh

This affects all nixos machines since the introduction of the nix settings module a couple releases ago which sets build-cores to 0 be default.

Author: DavHau

src/libstore-tests/globals.cc

@@ -0,0 +1,55 @@
+#include <gtest/gtest.h>
+
+#include "nix/store/globals.hh"
+
+namespace nix {
+
+/**
+ * Test the BuildCoresSetting to ensure build-cores=0 gets resolved to
+ * getDefaultCores() at configuration time.
+ */
+class BuildCoresSettingTest : public ::testing::Test {
+protected:
+    void SetUp() override {
+        // Save the original buildCores setting to restore after each test
+        originalBuildCores = settings.buildCores;
+    }
+
+    void TearDown() override {
+        // Restore the original buildCores setting
+        settings.buildCores = originalBuildCores;
+    }
+
+    unsigned int originalBuildCores;
+};
+
+TEST_F(BuildCoresSettingTest, buildCores_withNonZeroValue) {
+    // When buildCores is set to a non-zero value, it should store that value
+    settings.buildCores.set("4");
+    EXPECT_EQ(settings.buildCores, 4U);
+
+    settings.buildCores.set("1");
+    EXPECT_EQ(settings.buildCores, 1U);
+}
+
+TEST_F(BuildCoresSettingTest, buildCores_withZeroString) {
+    // When buildCores is set to "0", it should resolve to getDefaultCores() at config time
+    unsigned int defaultCores = settings.getDefaultCores();
+
+    settings.buildCores.set("0");
+
+    // After setting to "0", buildCores should contain the default cores value, not 0
+    EXPECT_EQ(settings.buildCores, defaultCores);
+    EXPECT_NE(settings.buildCores, 0U);
+    // The resolution of buildCores=0 should always be positive
+    EXPECT_GT(settings.buildCores, 0U);
+}
+
+TEST_F(BuildCoresSettingTest, buildCores_invalidStringThrows) {
+    // Invalid strings should throw an error
+    EXPECT_THROW(settings.buildCores.set("invalid"), UsageError);
+    EXPECT_THROW(settings.buildCores.set(""), UsageError);
+    EXPECT_THROW(settings.buildCores.set("auto"), UsageError); // auto is not supported for buildCores
+}
+
+} // namespace nix

src/libstore-tests/meson.build

@@ -59,6 +59,7 @@ sources = files(
   'derivation.cc',
   'derived-path.cc',
   'downstream-placeholder.cc',
+  'globals.cc',
   'http-binary-cache-store.cc',
   'legacy-ssh-store.cc',
   'local-binary-cache-store.cc',

src/libstore/globals.cc

@@ -140,7 +140,7 @@ std::vector<Path> getUserConfigFiles()
     return files;
 }
 
-unsigned int Settings::getDefaultCores()
+unsigned int Settings::getDefaultCores() const
 {
     const unsigned int concurrency = std::max(1U, std::thread::hardware_concurrency());
     const unsigned int maxCPU = getMaxCPU();
@@ -307,6 +307,30 @@ unsigned int MaxBuildJobsSetting::parse(const std::string & str) const
     }
 }
 
+unsigned int BuildCoresSetting::parse(const std::string & str) const
+{
+    if (auto n = string2Int<decltype(value)>(str)) {
+        if (*n == 0) {
+            // Resolve 0 to getDefaultCores() at configuration time
+            return settings.getDefaultCores();
+        } else {
+            return *n;
+        }
+    } else {
+        throw UsageError("configuration setting '%s' should be an integer", name);
+    }
+}
+
+void BuildCoresSetting::assign(const unsigned int & v)
+{
+    if (v == 0) {
+        // Resolve 0 to getDefaultCores() at assignment time
+        value = settings.getDefaultCores();
+    } else {
+        value = v;
+    }
+}
+
 
 static void preloadNSS()
 {

src/libstore/include/nix/store/globals.hh

@@ -33,6 +33,27 @@ struct MaxBuildJobsSetting : public BaseSetting<unsigned int>
     unsigned int parse(const std::string & str) const override;
 };
 
+struct BuildCoresSetting : public BaseSetting<unsigned int>
+{
+    BuildCoresSetting(Config * options,
+        unsigned int def,
+        const std::string & name,
+        const std::string & description,
+        const StringSet & aliases = {},
+        const bool documentDefault = true)
+        : BaseSetting<unsigned int>(def, documentDefault, name, description, aliases)
+    {
+        options->addSetting(this);
+    }
+
+    unsigned int parse(const std::string & str) const override;
+
+    void assign(const unsigned int & v) override;
+
+    // Inherit assignment operators from BaseSetting
+    using BaseSetting<unsigned int>::operator=;
+};
+
 const uint32_t maxIdsPerBuild =
     #ifdef __linux__
     1 << 16
@@ -43,8 +64,6 @@ const uint32_t maxIdsPerBuild =
 
 class Settings : public Config {
 
-    unsigned int getDefaultCores();
-
     StringSet getDefaultSystemFeatures();
 
     StringSet getDefaultExtraPlatforms();
@@ -57,6 +76,8 @@ public:
 
     Settings();
 
+    unsigned int getDefaultCores() const;
+
     Path nixPrefix;
 
     /**
@@ -151,9 +172,9 @@ public:
         )",
         {"substitution-max-jobs"}};
 
-    Setting<unsigned int> buildCores{
+    BuildCoresSetting buildCores{
         this,
-        getDefaultCores(),
+        0,
         "cores",
         R"(
           Sets the value of the `NIX_BUILD_CORES` environment variable in the [invocation of the `builder` executable](@docroot@/language/derivations.md#builder-execution) of a derivation.
@@ -172,9 +193,7 @@ public:
           >
           > The number of parallel local Nix build jobs is independently controlled with the [`max-jobs`](#conf-max-jobs) setting.
         )",
-        {"build-cores"},
-        // Don't document the machine-specific default value
-        false};
+        {"build-cores"}};
 
     /**
      * Read-only mode.  Don't copy stuff to the store, don't change

tests/functional/build-cores.nix

@@ -0,0 +1,11 @@
+with import ./config.nix;
+
+{
+  # Test derivation that checks the NIX_BUILD_CORES environment variable
+  testCores = mkDerivation {
+    name = "test-build-cores";
+    buildCommand = ''
+      echo "$NIX_BUILD_CORES" > $out
+    '';
+  };
+}

tests/functional/build-cores.sh

@@ -0,0 +1,32 @@
+#!/usr/bin/env bash
+
+source common.sh
+
+clearStoreIfPossible
+
+echo "Testing build-cores configuration behavior..."
+
+# Test 1: When build-cores is set to a non-zero value, NIX_BUILD_CORES should have that value
+echo "Testing build-cores=4..."
+rm -f "$TEST_ROOT"/build-cores-output
+nix-build --cores 4 build-cores.nix -A testCores -o "$TEST_ROOT"/build-cores-output
+result=$(cat $(readlink "$TEST_ROOT"/build-cores-output))
+if [[ "$result" != "4" ]]; then
+    echo "FAIL: Expected NIX_BUILD_CORES=4, got $result"
+    exit 1
+fi
+echo "PASS: build-cores=4 correctly sets NIX_BUILD_CORES=4"
+rm -f "$TEST_ROOT"/build-cores-output
+
+# Test 2: When build-cores is set to 0, NIX_BUILD_CORES should be resolved to getDefaultCores()
+echo "Testing build-cores=0..."
+nix-build --cores 0 build-cores.nix -A testCores -o "$TEST_ROOT"/build-cores-output
+result=$(cat $(readlink "$TEST_ROOT"/build-cores-output))
+if [[ "$result" == "0" ]]; then
+    echo "FAIL: NIX_BUILD_CORES should not be 0 when build-cores=0"
+    exit 1
+fi
+echo "PASS: build-cores=0 resolves to NIX_BUILD_CORES=$result (should be > 0)"
+rm -f "$TEST_ROOT"/build-cores-output
+
+echo "All build-cores tests passed!"

tests/functional/meson.build

@@ -145,6 +145,7 @@ suites = [
       'placeholders.sh',
       'ssh-relay.sh',
       'build.sh',
+      'build-cores.sh',
       'build-delete.sh',
       'output-normalization.sh',
       'selfref-gc.sh',