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Add support for impure derivations

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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.
This commit is contained in:
Eelco Dolstra 2022-03-30 16:31:01 +02:00
parent 28309352d9
commit 5cd72598fe
17 changed files with 486 additions and 154 deletions
Download patch file Download diff file Expand all files Collapse all files

32
src/libexpr/primops.cc
View file

@ -989,6 +989,7 @@ static void prim_derivationStrict(EvalState & state, const Pos & pos, Value * *
PathSet context;
bool contentAddressed = false;
bool isImpure = false;
std::optional<std::string> outputHash;
std::string outputHashAlgo;
auto ingestionMethod = FileIngestionMethod::Flat;
@ -1051,6 +1052,12 @@ static void prim_derivationStrict(EvalState & state, const Pos & pos, Value * *
settings.requireExperimentalFeature(Xp::CaDerivations);
}
else if (i->name == state.sImpure) {
isImpure = state.forceBool(*i->value, pos);
if (isImpure)
settings.requireExperimentalFeature(Xp::ImpureDerivations);
}
/* The `args' attribute is special: it supplies the
command-line arguments to the builder. */
else if (i->name == state.sArgs) {
@ -1197,15 +1204,28 @@ static void prim_derivationStrict(EvalState & state, const Pos & pos, Value * *
});
}
else if (contentAddressed) {
else if (contentAddressed || isImpure) {
if (contentAddressed && isImpure)
throw EvalError({
.msg = hintfmt("derivation cannot be both content-addressed and impure"),
.errPos = posDrvName
});
HashType ht = parseHashType(outputHashAlgo);
for (auto & i : outputs) {
drv.env[i] = hashPlaceholder(i);
drv.outputs.insert_or_assign(i,
DerivationOutput::CAFloating {
.method = ingestionMethod,
.hashType = ht,
});
if (isImpure)
drv.outputs.insert_or_assign(i,
DerivationOutput::Impure {
.method = ingestionMethod,
.hashType = ht,
});
else
drv.outputs.insert_or_assign(i,
DerivationOutput::CAFloating {
.method = ingestionMethod,
.hashType = ht,
});
}
}