Allow CLI to pass environment variables to FOD builder (#8830)

Add a new experimental `impure-env` setting that is a key-value list of
environment variables to inject into FOD derivations that specify the
corresponding `impureEnvVars`.

This allows clients to make use of this feature (without having to change the
environment of the daemon itself) and might eventually deprecate the current
behaviour (pick whatever is in the environment of the daemon) as it's more
principled and might prevent information leakage.

doc/manual/src/language/advanced-attributes.md

@@ -112,6 +112,13 @@ Derivations can declare some infrequently used optional attributes.
     > environmental variables come from the environment of the
     > `nix-build`.
 
+    If the [`configurable-impure-env` experimental
+    feature](@docroot@/contributing/experimental-features.md#xp-feature-configurable-impure-env)
+    is enabled, these environment variables can also be controlled
+    through the
+    [`impure-env`](@docroot@/command-ref/conf-file.md#conf-impure-env)
+    configuration setting.
+
   - [`outputHash`]{#adv-attr-outputHash}; [`outputHashAlgo`]{#adv-attr-outputHashAlgo}; [`outputHashMode`]{#adv-attr-outputHashMode}\
     These attributes declare that the derivation is a so-called
     *fixed-output derivation*, which means that a cryptographic hash of

src/libstore/build/local-derivation-goal.cc

@@ -1135,8 +1135,18 @@ void LocalDerivationGoal::initEnv()
        fixed-output derivations is by definition pure (since we
        already know the cryptographic hash of the output). */
     if (!derivationType->isSandboxed()) {
-        for (auto & i : parsedDrv->getStringsAttr("impureEnvVars").value_or(Strings()))
-            env[i] = getEnv(i).value_or("");
+        auto & impureEnv = settings.impureEnv.get();
+        if (!impureEnv.empty())
+            experimentalFeatureSettings.require(Xp::ConfigurableImpureEnv);
+
+        for (auto & i : parsedDrv->getStringsAttr("impureEnvVars").value_or(Strings())) {
+            auto envVar = impureEnv.find(i);
+            if (envVar != impureEnv.end()) {
+                env[i] = envVar->second;
+            } else {
+                env[i] = getEnv(i).value_or("");
+            }
+        }
     }
 
     /* Currently structured log messages piggyback on stderr, but we

src/libstore/globals.hh

@@ -4,6 +4,7 @@
 #include "types.hh"
 #include "config.hh"
 #include "util.hh"
+#include "experimental-features.hh"
 
 #include <map>
 #include <limits>
@@ -1052,6 +1053,25 @@ public:
           ```
         )"
     };
+
+    Setting<StringMap> impureEnv {this, {}, "impure-env",
+        R"(
+          A list of items, each in the format of:
+
+          - `name=value`: Set environment variable `name` to `value`.
+
+          If the user is trusted (see `trusted-users` option), when building
+          a fixed-output derivation, environment variables set in this option
+          will be passed to the builder if they are listed in [`impureEnvVars`](@docroot@/language/advanced-attributes.md##adv-attr-impureEnvVars).
+
+          This option is useful for, e.g., setting `https_proxy` for
+          fixed-output derivations and in a multi-user Nix installation, or
+          setting private access tokens when fetching a private repository.
+        )",
+        {}, // aliases
+        true, // document default
+        Xp::ConfigurableImpureEnv
+    };
 };
 
 

src/libutil/experimental-features.cc

@@ -12,7 +12,7 @@ struct ExperimentalFeatureDetails
     std::string_view description;
 };
 
-constexpr std::array<ExperimentalFeatureDetails, 14> xpFeatureDetails = {{
+constexpr std::array<ExperimentalFeatureDetails, 15> xpFeatureDetails = {{
     {
         .tag = Xp::CaDerivations,
         .name = "ca-derivations",
@@ -221,6 +221,13 @@ constexpr std::array<ExperimentalFeatureDetails, 14> xpFeatureDetails = {{
             Allow the use of the `read-only` parameter in [local store](@docroot@/command-ref/new-cli/nix3-help-stores.md#local-store) URIs.
         )",
     },
+    {
+        .tag = Xp::ConfigurableImpureEnv,
+        .name = "configurable-impure-env",
+        .description = R"(
+            Allow the use of the [impure-env](@docroot@/command-ref/conf-file.md#conf-impure-env) setting.
+        )",
+    }
 }};
 
 static_assert(

src/libutil/experimental-features.hh

@@ -31,6 +31,7 @@ enum struct ExperimentalFeature
     DynamicDerivations,
     ParseTomlTimestamps,
     ReadOnlyLocalStore,
+    ConfigurableImpureEnv,
 };
 
 /**

tests/functional/impure-env.nix

@@ -0,0 +1,16 @@
+{ var, value }:
+
+with import ./config.nix;
+
+mkDerivation {
+  name = "test";
+  buildCommand = ''
+    echo ${var} = "''$${var}"
+    echo -n "''$${var}" > "$out"
+  '';
+
+  impureEnvVars = [ var ];
+
+  outputHashAlgo = "sha256";
+  outputHash = builtins.hashString "sha256" value;
+}

tests/functional/impure-env.sh

@@ -0,0 +1,33 @@
+source common.sh
+
+# Needs the config option 'impure-env' to work
+requireDaemonNewerThan "2.18.0pre20230816"
+
+enableFeatures "configurable-impure-env"
+restartDaemon
+
+varTest() {
+    local var="$1"; shift
+    local value="$1"; shift
+    nix build --no-link -vL --argstr var "$var" --argstr value "$value" --impure "$@" --file impure-env.nix
+    clearStore
+}
+
+clearStore
+startDaemon
+
+varTest env_name value --impure-env env_name=value
+
+echo 'impure-env = set_in_config=config_value' >> "$NIX_CONF_DIR/nix.conf"
+set_in_config=daemon_value restartDaemon
+
+varTest set_in_config config_value
+varTest set_in_config client_value --impure-env set_in_config=client_value
+
+sed -i -e '/^trusted-users =/d' "$NIX_CONF_DIR/nix.conf"
+
+env_name=daemon_value restartDaemon
+
+varTest env_name daemon_value --impure-env env_name=client_value
+
+killDaemon

tests/functional/local.mk

@@ -121,7 +121,8 @@ nix_tests = \
   path-from-hash-part.sh \
   toString-path.sh \
   read-only-store.sh \
-  nested-sandboxing.sh
+  nested-sandboxing.sh \
+  impure-env.sh
 
 ifeq ($(HAVE_LIBCPUID), 1)
 	nix_tests += compute-levels.sh