nixpkgs/pkgs/build-support/setup-hooks/reproducible-builds.sh

# Use the last part of the out path as hash input for the build.
# This should ensure that it is deterministic across rebuilds of the same
# derivation and not easily collide with other builds.
# We also truncate the hash so that it cannot cause reference cycles.
export NIX_CFLAGS_COMPILE+=" -frandom-seed=$(
    outbase="${out##*/}"
    randomseed="${outbase:0:10}"
    echo $randomseed
)"
stdenv: add -frandom-seed to NIX_CFLAGS_COMPILE for reproducibility This adds -frandom-seed to each compiler invocation in stdenv. The object here is to make the compierl invocations produce the same output every time they are called (for the same derivation). When the -frandom-seed option is not set the compiler will use a combination of random numbers (in GCC's case from /dev/urandom) and the durrent time to produce a "random" input per file. This can (among other things) lead to different ordering of symbols in the produced object files. For reason of reproducibility we prefer having the same derivation produce the exact same outputs. This is not a silver bullet but one way to tame the compiler. 2020-10-31 16:29:54 +00:00			`# Use the last part of the out path as hash input for the build.`
			`# This should ensure that it is deterministic across rebuilds of the same`
			`# derivation and not easily collide with other builds.`
stdenv: trim random seed to avoid reference cycles Using the full store hash as the random seed occasionally caused reference cycles when the invocation was stored in output artifacts. For example, cross-compiled gcc was failing due to this: https://hydra.nixos.org/eval/1631713#tabs-now-fail Simply truncating the hash is sufficient to avoid this. 2020-12-15 08:05:57 +00:00			`# We also truncate the hash so that it cannot cause reference cycles.`
			`export NIX_CFLAGS_COMPILE+=" -frandom-seed=$(`
			`outbase="${out##*/}"`
			`randomseed="${outbase:0:10}"`
			`echo $randomseed`
			`)"`