f2/nix: provide a function to serialise python objects to nix code

Change-Id: I863029263edbc701a18b18da4b58fcb35f575614

tests/functional2/testlib/fixtures/nix.py

@@ -16,8 +16,11 @@ from testlib.fixtures.command import CommandResult, Command
 from testlib.fixtures.env import ManagedEnv
 from testlib.utils import is_value_of_type
 
+from textwrap import dedent
+
 
 type _NixSettingValue = str | int | list[str] | bool | None
+type _NixValue = str | int | float | list[_NixValue] | dict[str, _NixValue] | bool | None
 
 
 def _serialise_config(value: _NixSettingValue) -> str:
@@ -32,6 +35,39 @@ def _serialise_config(value: _NixSettingValue) -> str:
     raise ValueError(msg)
 
 
+def serialise_nix(value: _NixValue) -> str:
+    """
+    Serialises the given python object into a nix represenatation.
+    NOTE: this interface does not allow accessing variables unless using interpolation, as all strings will be surrounded by quotes.
+    """
+
+    def escape(v: str) -> str:
+        if "\\r" in v:
+            raise ValueError("\\r is not supported for conversion")
+        escaped = v.replace("\\", "\\\\").replace("$", "\\$").replace('"', '\\"')
+        return f'"{escaped}"'
+
+    if is_value_of_type(value, list[_NixValue.__value__]):
+        return f"[{' '.join([serialise_nix(v) for v in value])}]"
+    if is_value_of_type(value, dict[str, _NixValue.__value__]):
+        return dedent(f"""
+            {{
+                {"\n                ".join([f"{escape(k)} = {serialise_nix(v)};" for k, v in value.items()])}
+            }}
+        """)
+    if is_value_of_type(value, bool):
+        return "true" if value else "false"
+    if is_value_of_type(value, int | float):
+        return str(value)
+    if is_value_of_type(value, str):
+        return escape(value)
+    if is_value_of_type(value, None):
+        return "null"
+
+    msg = f"Value is unsupported in nix code: {value!r}"
+    raise ValueError(msg)
+
+
 class NixSettings:
     """Settings for invoking Nix"""
 

tests/functional2/testlib/fixtures/test_nix.py

@@ -3,7 +3,9 @@ from pathlib import Path
 import pytest
 
 from testlib.fixtures.env import ManagedEnv
-from testlib.fixtures.nix import NixSettings
+from testlib.fixtures.nix import NixSettings, serialise_nix
+
+from textwrap import dedent
 
 
 def test_nix_settings_set_item():
@@ -115,3 +117,38 @@ def test_nix_settings_to_env_overlay_no_store_dir(tmp_path: Path):
 
     settings.to_env_overlay(env)
     assert "store = local?root=/some/path\n" in env._env["NIX_CONFIG"]
+
+
+class TestSerialiseNix:
+    def test_list(self):
+        assert serialise_nix(["a", 1, None]) == '["a" 1 null]'
+
+    def test_dict(self):
+        assert serialise_nix(
+            {"a": 1, "b": None, "c": "hello world", "d": 3.14159265, "e": True}
+        ) == dedent("""
+            {
+                "a" = 1;
+                "b" = null;
+                "c" = "hello world";
+                "d" = 3.14159265;
+                "e" = true;
+            }
+        """)
+
+    def test_escaping(self):
+        assert serialise_nix({'"a': 'Hello ""', "1": "${foo}", "${foo}": None}) == dedent("""
+            {
+                "\\"a" = "Hello \\"\\"";
+                "1" = "\\${foo}";
+                "\\${foo}" = null;
+            }
+        """)
+
+    def test_quotes(self):
+        assert serialise_nix({"a b": None, "\\n": '${awa}"'}) == dedent("""
+            {
+                "a b" = null;
+                "\\\\n" = "\\${awa}\\"";
+            }
+        """)