Add eval-system option

`eval-system` option overrides just the value of `builtins.currentSystem`. This is more useful than overriding `system` since you can build these derivations on remote builders which can work on the given system. Co-authored-by: John Ericson <John.Ericson@Obsidian.Systems> Co-authored-by: Valentin Gagarin <valentin.gagarin@tweag.io>
matthewbauer · Dec 14, 2023 · f40267c · f40267c
1 parent fc0accf
commit f40267c
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Showing 4 changed files with 34 additions and 4 deletions.
diff --git a/src/libexpr/eval-settings.hh b/src/libexpr/eval-settings.hh
@@ -27,6 +27,20 @@ struct EvalSettings : Config
           [`builtins.nixPath`](@docroot@/language/builtin-constants.md#builtins-nixPath).
         )"};
 
+    Setting<std::string> thisSystem{
+        this, "", "eval-system",
+        R"(
+          This option defines
+          [`builtins.currentSystem`](@docroot@/language/builtin-constants.md#builtins-currentSystem)
+          in the Nix language if it is set as a non-empty string.
+          Otherwise, if it is defined as the empty string (the default), the value of the
+          [`system` ](#conf-system)
+          configuration setting is used instead.
+
+          Unlike `system`, this setting does not change what kind of derivations can be built locally.
+          This is useful for evaluating Nix code on one system to produce derivations to be built on another type of system.
+        )"};
+
     Setting<bool> restrictEval{
         this, false, "restrict-eval",
         R"(

diff --git a/src/libexpr/primops.cc b/src/libexpr/primops.cc
@@ -4384,12 +4384,19 @@ void EvalState::createBaseEnv()
     });
 
     if (!evalSettings.pureEval) {
-        v.mkString(settings.thisSystem.get());
+        if (evalSettings.thisSystem.get() != "")
+            v.mkString(evalSettings.thisSystem.get());
+        else
+            v.mkString(settings.thisSystem.get());
     }
     addConstant("__currentSystem", v, {
         .type = nString,
         .doc = R"(
-          The value of the [`system` configuration option](@docroot@/command-ref/conf-file.md#conf-system).
+          The value of the
+          [`eval-system`](@docroot@/command-ref/conf-file.md#conf-eval-system)
+          or else
+          [`system`](@docroot@/command-ref/conf-file.md#conf-system)
+          configuration option.
 
           It can be used to set the `system` attribute for [`builtins.derivation`](@docroot@/language/derivations.md) such that the resulting derivation can be built on the same system that evaluates the Nix expression:
 

diff --git a/src/libstore/globals.hh b/src/libstore/globals.hh
@@ -214,7 +214,11 @@ public:
           In general, you do not have to modify this setting.
           While you can force Nix to run a Darwin-specific `builder` executable on a Linux machine, the result would obviously be wrong.
 
-          This value is available in the Nix language as [`builtins.currentSystem`](@docroot@/language/builtin-constants.md#builtins-currentSystem).
+          This value is available in the Nix language as
+          [`builtins.currentSystem`](@docroot@/language/builtin-constants.md#builtins-currentSystem)
+          if the
+          [`eval-system`](#conf-eval-system)
+          configuration option is set as the empty string.
         )"};
 
     Setting<time_t> maxSilentTime{

diff --git a/tests/unit/libexpr/primops.cc b/tests/unit/libexpr/primops.cc
@@ -1,6 +1,7 @@
 #include <gmock/gmock.h>
 #include <gtest/gtest.h>
 
+#include "eval-settings.hh"
 #include "memory-input-accessor.hh"
 
 #include "tests/libexpr.hh"
@@ -631,7 +632,11 @@ namespace nix {
 
     TEST_F(PrimOpTest, currentSystem) {
         auto v = eval("builtins.currentSystem");
-        ASSERT_THAT(v, IsStringEq(settings.thisSystem.get()));
+        if (evalSettings.thisSystem.get() != "") {
+            ASSERT_THAT(v, IsStringEq(evalSettings.thisSystem.get()));
+        } else {
+            ASSERT_THAT(v, IsStringEq(settings.thisSystem.get()));
+        }
     }
 
     TEST_F(PrimOpTest, derivation) {