chore: custom cos validation

charmcraft/extensions/app.py

@@ -17,6 +17,7 @@
 """Gunicorn based extensions."""
 
 import copy
+from pathlib import Path
 from typing import Any
 
 from overrides import override
@@ -69,6 +70,8 @@
     },
 }
 
+COS_SUBDIRS = {"grafana_dashboards", "loki_alert_rules", "prometheus_alert_rules"}
+
 
 class _AppBase(Extension):
     """A base class for 12-factor applications."""
@@ -129,6 +132,7 @@ def _check_input(self) -> None:
                 f"the '{self.framework}-framework' extension is incompatible with "
                 f"type {charm_type!r}"
             )
+        self._validate_cos_custom_dir()
         parts = self.yaml_data.get("parts")
         if parts and "charm" in parts:
             raise ExtensionError(
@@ -191,6 +195,31 @@ def _check_input(self) -> None:
                 f"Please either remove the default value or set optional field to true or remove it for the {', '.join(invalid_non_optionals)} configuration option(s)."
             )
 
+    def _validate_cos_custom_dir(self) -> None:
+        """Validate the custom COS directory if present."""
+        custom_dir = Path(self.project_root) / "cos_custom"
+        if not custom_dir.is_dir():
+            return
+        root_files: list[str] = []
+        invalid_dirs: list[str] = []
+
+        for entry in custom_dir.iterdir():
+            if entry.is_file():
+                root_files.append(entry.name)
+            elif entry.is_dir() and entry.name not in COS_SUBDIRS:
+                invalid_dirs.append(entry.name)
+
+        if root_files or invalid_dirs:
+            details: list[str] = []
+            if root_files:
+                details.append("root files: " + ", ".join(root_files))
+            if invalid_dirs:
+                details.append("invalid subdirectories: " + ", ".join(invalid_dirs))
+            raise ExtensionError(
+                "custom COS directory must only contain the following subdirectories: "
+                f"{COS_SUBDIRS}. Found {'; '.join(details)}"
+            )
+
     def _get_root_snippet(self) -> dict[str, Any]:
         """Return the root snippet to be merged into the user charmcraft.yaml.
 

tests/extensions/test_app.py

@@ -14,6 +14,7 @@
 #
 # For further info, check https://github.com/canonical/charmcraft
 import copy
+import pathlib
 
 import pytest
 
@@ -627,6 +628,45 @@ def test_flask_incompatible_fields(modification, flask_input_yaml, tmp_path):
         extensions.apply_extensions(tmp_path, copy.deepcopy(charm))
 
 
+def test_cos_custom_rejects_root_file(flask_input_yaml, tmp_path: pathlib.Path):
+    cos_dir = tmp_path / "cos_custom"
+    cos_dir.mkdir()
+    (cos_dir / "bad.txt").write_text("oops")
+
+    with pytest.raises(ExtensionError) as exc:
+        extensions.apply_extensions(tmp_path, flask_input_yaml)
+
+    error = str(exc.value)
+    assert "custom COS directory must only contain" in error
+    assert "root files: bad.txt" in error
+
+
+def test_cos_custom_rejects_unknown_subdir(flask_input_yaml, tmp_path: pathlib.Path):
+    cos_dir = tmp_path / "cos_custom"
+    (cos_dir / "unknown").mkdir(parents=True)
+
+    with pytest.raises(ExtensionError) as exc:
+        extensions.apply_extensions(tmp_path, flask_input_yaml)
+
+    error = str(exc.value)
+    assert "custom COS directory must only contain" in error
+    assert "invalid subdirectories: unknown" in error
+
+
+def test_cos_custom_allows_known_subdir(flask_input_yaml, tmp_path: pathlib.Path):
+    grafana_dir = tmp_path / "cos_custom" / "grafana_dashboards"
+    loki_dir = tmp_path / "cos_custom" / "loki_alert_rules"
+    prometheus_dir = tmp_path / "cos_custom" / "prometheus_alert_rules"
+    grafana_dir.mkdir(parents=True)
+    loki_dir.mkdir()
+    prometheus_dir.mkdir()
+    (grafana_dir / "dash.json").write_text("{}")
+    (loki_dir / "rules.yaml").write_text("groups: []")
+    (prometheus_dir / "rules.yaml").write_text("groups: []")
+
+    extensions.apply_extensions(tmp_path, flask_input_yaml)
+
+
 def test_handle_charm_part_requires_no_parts(flask_input_yaml, tmp_path):
     # Currently, in the flask-framework extension, we will reject any project that
     # includes a charm part. This is to prevent issues where a non-default charm part is