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build-cores: detect cores automatically if set to 0

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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 commit is contained in:
DavHau 2025-06-27 15:35:46 +07:00
parent df2d5f276a
commit fb5e22e318
8 changed files with 58 additions and 10 deletions
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2
src/nix-build/nix-build.cc
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@ -542,7 +542,7 @@ static void main_nix_build(int argc, char * * argv)
env["NIX_BUILD_TOP"] = env["TMPDIR"] = env["TEMPDIR"] = env["TMP"] = env["TEMP"] = tmpDir.path().string();
env["NIX_STORE"] = store->storeDir;
env["NIX_BUILD_CORES"] = std::to_string(settings.buildCores);
env["NIX_BUILD_CORES"] = fmt("%d", settings.buildCores ? settings.buildCores : settings.getDefaultCores());
auto parsedDrv = StructuredAttrs::tryParse(drv.env);
DerivationOptions drvOptions;