From 5cd72598feaff3c4bbcc7304a4844768f64a1ee0 Mon Sep 17 00:00:00 2001
From: Eelco Dolstra <edolstra@gmail.com>
Date: Wed, 30 Mar 2022 16:31:01 +0200
Subject: Add support for impure derivations

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.
---
 src/libutil/experimental-features.cc | 1 +
 src/libutil/experimental-features.hh | 1 +
 2 files changed, 2 insertions(+)

(limited to 'src/libutil')

diff --git a/src/libutil/experimental-features.cc b/src/libutil/experimental-features.cc
index 01f318fa3..e033a4116 100644
--- a/src/libutil/experimental-features.cc
+++ b/src/libutil/experimental-features.cc
@@ -7,6 +7,7 @@ namespace nix {
 
 std::map<ExperimentalFeature, std::string> stringifiedXpFeatures = {
     { Xp::CaDerivations, "ca-derivations" },
+    { Xp::ImpureDerivations, "impure-derivations" },
     { Xp::Flakes, "flakes" },
     { Xp::NixCommand, "nix-command" },
     { Xp::RecursiveNix, "recursive-nix" },
diff --git a/src/libutil/experimental-features.hh b/src/libutil/experimental-features.hh
index b5140dcfe..3a254b423 100644
--- a/src/libutil/experimental-features.hh
+++ b/src/libutil/experimental-features.hh
@@ -16,6 +16,7 @@ namespace nix {
 enum struct ExperimentalFeature
 {
     CaDerivations,
+    ImpureDerivations,
     Flakes,
     NixCommand,
     RecursiveNix,
-- 
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