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Impure derivations are derivations that can produce a different result
every time they're built. Example:
stdenv.mkDerivation {
name = "impure";
__impure = true; # marks this derivation as impure
outputHashAlgo = "sha256";
outputHashMode = "recursive";
buildCommand = "date > $out";
};
Some important characteristics:
* This requires the 'impure-derivations' experimental feature.
* Impure derivations are not "cached". Thus, running "nix-build" on
the example above multiple times will cause a rebuild every time.
* They are implemented similar to CA derivations, i.e. the output is
moved to a content-addressed path in the store. The difference is
that we don't register a realisation in the Nix database.
* Pure derivations are not allowed to depend on impure derivations. In
the future fixed-output derivations will be allowed to depend on
impure derivations, thus forming an "impurity barrier" in the
dependency graph.
* When sandboxing is enabled, impure derivations can access the
network in the same way as fixed-output derivations. In relaxed
sandboxing mode, they can access the local filesystem.
46 lines
950 B
Nix
46 lines
950 B
Nix
with import ./config.nix;
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rec {
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impure = mkDerivation {
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name = "impure";
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outputs = [ "out" "stuff" ];
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buildCommand =
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''
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x=$(< $TEST_ROOT/counter)
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mkdir $out $stuff
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echo $x > $out/n
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ln -s $out/n $stuff/bla
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printf $((x + 1)) > $TEST_ROOT/counter
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'';
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__impure = true;
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outputHashAlgo = "sha256";
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outputHashMode = "recursive";
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impureEnvVars = [ "TEST_ROOT" ];
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};
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impureOnImpure = mkDerivation {
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name = "impure-on-impure";
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buildCommand =
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''
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x=$(< ${impure}/n)
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mkdir $out
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printf X$x > $out/n
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ln -s ${impure.stuff} $out/symlink
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ln -s $out $out/self
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'';
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__impure = true;
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outputHashAlgo = "sha256";
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outputHashMode = "recursive";
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};
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# This is not allowed.
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inputAddressed = mkDerivation {
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name = "input-addressed";
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buildCommand =
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''
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cat ${impure} > $out
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'';
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};
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}
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