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