nixos-rebuild-ng: validate NixOS config path in target host

In #418243 we started to validate NixOS config path to avoid a nasty bug
in Nix, but this doesn't work in the `--build-host` and `--target-host`
case because the configuration will not be available in the local host
to check if it contains a `nixos-version`. This moves the check to
`--target-host` instead, that is probably the correct choice anyway.

Fix #418868.

pkgs/by-name/ni/nixos-rebuild-ng/src/nixos_rebuild/__init__.py

@@ -5,7 +5,6 @@ import os
 import sys
 from pathlib import Path
 from subprocess import CalledProcessError, run
-from textwrap import dedent
 from typing import Final, assert_never
 
 from . import nix, tmpdir
@@ -338,29 +337,6 @@ def validate_image_variant(image_variant: str, variants: ImageVariants) -> None:
         )
 
 
-def validate_nixos_config(path_to_config: Path) -> None:
-    if not (path_to_config / "nixos-version").exists() and not os.environ.get(
-        "NIXOS_REBUILD_I_UNDERSTAND_THE_CONSEQUENCES_PLEASE_BREAK_MY_SYSTEM"
-    ):
-        msg = dedent(
-            # the lowercase for the first letter below is proposital
-            f"""
-                your NixOS configuration path seems to be missing essential files.
-                To avoid corrupting your current NixOS installation, the activation will abort.
-
-                This could be caused by Nix bug: https://github.com/NixOS/nix/issues/13367.
-                This is the evaluated NixOS configuration path: {path_to_config}.
-                Change the directory to somewhere else (e.g., `cd $HOME`) before trying again.
-
-                If you think this is a mistake, you can set the environment variable
-                NIXOS_REBUILD_I_UNDERSTAND_THE_CONSEQUENCES_PLEASE_BREAK_MY_SYSTEM to 1
-                and re-run the command to continue.
-                Please open an issue if this is the case.
-            """
-        ).strip()
-        raise NixOSRebuildError(msg)
-
-
 def execute(argv: list[str]) -> None:
     args, args_groups = parse_args(argv)
 
@@ -514,7 +490,6 @@ def execute(argv: list[str]) -> None:
                     copy_flags=copy_flags,
                 )
                 if action in (Action.SWITCH, Action.BOOT):
-                    validate_nixos_config(path_to_config)
                     nix.set_profile(
                         profile,
                         path_to_config,

pkgs/by-name/ni/nixos-rebuild-ng/src/nixos_rebuild/nix.py

@@ -9,6 +9,7 @@ from importlib.resources import files
 from pathlib import Path
 from string import Template
 from subprocess import PIPE, CalledProcessError
+from textwrap import dedent
 from typing import Final, Literal
 
 from . import tmpdir
@@ -613,6 +614,33 @@ def set_profile(
     sudo: bool,
 ) -> None:
     "Set a path as the current active Nix profile."
+    if not os.environ.get(
+        "NIXOS_REBUILD_I_UNDERSTAND_THE_CONSEQUENCES_PLEASE_BREAK_MY_SYSTEM"
+    ):
+        r = run_wrapper(
+            ["test", "-f", path_to_config / "nixos-version"],
+            remote=target_host,
+            check=False,
+        )
+        if r.returncode:
+            msg = dedent(
+                # the lowercase for the first letter below is proposital
+                f"""
+                    your NixOS configuration path seems to be missing essential files.
+                    To avoid corrupting your current NixOS installation, the activation will abort.
+
+                    This could be caused by Nix bug: https://github.com/NixOS/nix/issues/13367.
+                    This is the evaluated NixOS configuration path: {path_to_config}.
+                    Change the directory to somewhere else (e.g., `cd $HOME`) before trying again.
+
+                    If you think this is a mistake, you can set the environment variable
+                    NIXOS_REBUILD_I_UNDERSTAND_THE_CONSEQUENCES_PLEASE_BREAK_MY_SYSTEM to 1
+                    and re-run the command to continue.
+                    Please open an issue if this is the case.
+                """
+            ).strip()
+            raise NixOSRebuildError(msg)
+
     run_wrapper(
         ["nix-env", "-p", profile.path, "--set", path_to_config],
         remote=target_host,

pkgs/by-name/ni/nixos-rebuild-ng/src/tests/test_nix.py

@@ -674,6 +674,8 @@ def test_rollback_temporary_profile(tmp_path: Path) -> None:
 def test_set_profile(mock_run: Mock) -> None:
     profile_path = Path("/path/to/profile")
     config_path = Path("/path/to/config")
+    mock_run.return_value = CompletedProcess([], 0)
+
     n.set_profile(
         m.Profile("system", profile_path),
         config_path,
@@ -687,6 +689,19 @@ def test_set_profile(mock_run: Mock) -> None:
         sudo=False,
     )
 
+    mock_run.return_value = CompletedProcess([], 1)
+
+    with pytest.raises(m.NixOSRebuildError) as e:
+        n.set_profile(
+            m.Profile("system", profile_path),
+            config_path,
+            target_host=None,
+            sudo=False,
+        )
+    assert str(e.value).startswith(
+        "error: your NixOS configuration path seems to be missing essential files."
+    )
+
 
 @patch(get_qualified_name(n.run_wrapper, n), autospec=True)
 def test_switch_to_configuration_without_systemd_run(