f2/nix: provide a high-level wrapper for calling builtins

Change-Id: I357bd7630b96e55d000fbff1032efcf8c98dfb7b

tests/functional2/testlib/fixtures/nix.py

@@ -238,6 +238,20 @@ class Nix:
         self._settings = orig
         return cmd.run()
 
+    def eval_builtin(self, name: str, *args: _NixValue) -> CommandResult:
+        """
+        This is a high-level wrapper, to easily evaluate and obtain the result of a builtin.
+
+        In the background, it calls `nix eval --json --expr builtins.{name} {args}`
+        It is recommended to call `.json()` on the return value.
+
+        :param name: name of the builtin
+        :param args: list of arguments to provide to the builtin
+        :return: result of the evaluation
+        """
+        args = [serialise_nix(arg) for arg in args]
+        return self.eval(f"builtins.{name} {' '.join(args)}")
+
     @property
     def store_dir(self) -> Path:
         """

tests/functional2/testlib/fixtures/test_nix.py

@@ -3,7 +3,7 @@ from pathlib import Path
 import pytest
 
 from testlib.fixtures.env import ManagedEnv
-from testlib.fixtures.nix import NixSettings, serialise_nix
+from testlib.fixtures.nix import NixSettings, Nix, serialise_nix
 
 from textwrap import dedent
 
@@ -119,6 +119,17 @@ def test_nix_settings_to_env_overlay_no_store_dir(tmp_path: Path):
     assert "store = local?root=/some/path\n" in env._env["NIX_CONFIG"]
 
 
+class TestNixEvalBuiltins:
+    def test_add(self, nix: Nix):
+        assert nix.eval_builtin("add", 1, 2).ok().stdout_plain == "3"
+
+    def test_attrnames(self, nix: Nix):
+        assert nix.eval_builtin("attrNames", {"a": "b", "c": 1}).json() == ["a", "c"]
+
+    def test_attrvalues(self, nix: Nix):
+        assert nix.eval_builtin("attrValues", {"a": "b", "c": 1}).json() == ["b", 1]
+
+
 class TestSerialiseNix:
     def test_list(self):
         assert serialise_nix(["a", 1, None]) == '["a" 1 null]'