Merge pull request #13402 from DavHau/build-cores

build-cores: detect cores automatically if set to 0

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();

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

@@ -43,8 +43,6 @@ const uint32_t maxIdsPerBuild =
 
 class Settings : public Config {
 
-    unsigned int getDefaultCores();
-
     StringSet getDefaultSystemFeatures();
 
     StringSet getDefaultExtraPlatforms();
@@ -57,6 +55,8 @@ public:
 
     Settings();
 
+    unsigned int getDefaultCores() const;
+
     Path nixPrefix;
 
     /**
@@ -153,7 +153,7 @@ public:
 
     Setting<unsigned int> 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.
@@ -166,15 +166,13 @@ public:
           -->
           For instance, in Nixpkgs, if the attribute `enableParallelBuilding` for the `mkDerivation` build helper is set to `true`, it passes the `-j${NIX_BUILD_CORES}` flag to GNU Make.
 
-          The value `0` means that the `builder` should use all available CPU cores in the system.
+          If set to `0`, nix will detect the number of CPU cores and pass this number via NIX_BUILD_CORES.
 
           > **Note**
           >
           > 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

src/libstore/unix/build/derivation-builder.cc

@@ -1083,7 +1083,7 @@ void DerivationBuilderImpl::initEnv()
     env["NIX_STORE"] = store.storeDir;
 
     /* The maximum number of cores to utilize for parallel building. */
-    env["NIX_BUILD_CORES"] = fmt("%d", settings.buildCores);
+    env["NIX_BUILD_CORES"] = fmt("%d", settings.buildCores ? settings.buildCores : settings.getDefaultCores());
 
     /* In non-structured mode, set all bindings either directory in the
        environment or via a file, as specified by