stages(ostree.post-copy): add stage unit test and comment

This commit adds a small stage unit test and most importantly
a comemnt why `devices` is part of the schema (but appears unused).

The reason "devices" is explained by Alex Larsson:
"""
The mounts don't work without devices that have the filesystems.
In sample-images for example, this is typically used like so:
```
type: org.osbuild.ostree.post-copy
devices:
  root:
    type: org.osbuild.loopback
    options:
      filename: disk.img
mounts:
- name: root
  type: org.osbuild.ext4
  source: root
  target: /
```
"""

stages/test/test_kickstart.py

@@ -162,9 +162,10 @@ TEST_INPUT = [
 
 def schema_validate_kickstart_stage(test_data):
     name = "org.osbuild.kickstart"
+    version = "1"
     root = os.path.join(os.path.dirname(__file__), "../..")
     mod_info = osbuild.meta.ModuleInfo.load(root, "Stage", name)
-    schema = osbuild.meta.Schema(mod_info.get_schema(), name)
+    schema = osbuild.meta.Schema(mod_info.get_schema(version=version), name)
     test_input = {
         "name": "org.osbuild.kickstart",
         "options": {

stages/test/test_ostree_post_copy.py

@@ -0,0 +1,61 @@
+#!/usr/bin/python3
+
+import os.path
+from unittest.mock import call, patch
+
+import pytest
+
+import osbuild.meta
+from osbuild.testutil.imports import import_module_from_path
+
+
+def schema_validate_stage_ostree_post_copy(test_data):
+    name = "org.osbuild.ostree.post-copy"
+    version = "2"
+    root = os.path.join(os.path.dirname(__file__), "../..")
+    mod_info = osbuild.meta.ModuleInfo.load(root, "Stage", name)
+    schema = osbuild.meta.Schema(mod_info.get_schema(version=version), name)
+    test_input = {
+        "type": "org.osbuild.ostree.post-copy",
+        "options": {
+            "sysroot": "/some/sysroot",
+        },
+    }
+    test_input.update(test_data)
+    return schema.validate(test_input)
+
+
+@patch("osbuild.util.ostree.cli")
+def test_ostree_post_copy_smoke(mock_ostree_cli):
+    stage_path = os.path.join(os.path.dirname(__file__), "../org.osbuild.ostree.post-copy")
+    stage = import_module_from_path("stage", stage_path)
+
+    paths = {
+        "mounts": "/run/osbuild/mounts",
+    }
+    options = {
+        "sysroot": "/some/sysroot",
+    }
+    stage.main(paths, options)
+
+    assert mock_ostree_cli.call_args_list == [
+        call("admin", "post-copy", sysroot="/run/osbuild/mounts/some/sysroot")]
+
+
+@pytest.mark.parametrize(
+    "test_data,expected_err",
+    [
+        # devices is not used directly in the stage but it's required
+        # because it's used as input for "mounts", see
+        # https://github.com/osbuild/osbuild/pull/1343/files#r1402161208
+        ({"devices": "must-be-object"}, " is not of type 'object'"),
+        ({"mounts": "must-be-array"}, " is not of type 'array'"),
+    ],
+)
+def test_schema_validation_ostree_post_copy(test_data, expected_err):
+    res = schema_validate_stage_ostree_post_copy(test_data)
+
+    assert res.valid is False
+    err_msgs = [e.as_dict()["message"] for e in res.errors]
+    assert len(res.errors) == 1, err_msgs
+    assert expected_err in err_msgs[0]