fix: reorganise charmcraft.yaml preprocessing (#1622)

This makes a few related changes:

1. Extensions are applied on the rawest charmcraft.yaml data we have
2. Charms with a "bases" key (not just bundles) get a default part.
3. Always expect a dictionary for parts.

charmcraft/application/main.py

@@ -65,15 +65,14 @@ def _project_vars(self, yaml_data: dict[str, Any]) -> dict[str, str]:
     def _extra_yaml_transform(
         self, yaml_data: dict[str, Any], *, build_on: str, build_for: str | None
     ) -> dict[str, Any]:
-        yaml_data = yaml_data.copy()
+        # Extensions get applied on as close as possible to what the user provided.
+        yaml_data = extensions.apply_extensions(self.project_dir, yaml_data.copy())
 
+        # Preprocessing "magic" to create a fully-formed charm.
         preprocess.add_default_parts(yaml_data)
         preprocess.add_bundle_snippet(self.project_dir, yaml_data)
         preprocess.add_config(self.project_dir, yaml_data)
         preprocess.add_actions(self.project_dir, yaml_data)
-
-        yaml_data = extensions.apply_extensions(self.project_dir, yaml_data)
-
         preprocess.add_metadata(self.project_dir, yaml_data)
 
         return yaml_data

charmcraft/preprocess.py

@@ -32,13 +32,16 @@ def add_default_parts(yaml_data: dict[str, Any]) -> None:
     :param yaml_data: The raw YAML dictionary of the project.
     :returns: The same dictionary passed in, with necessary mutations.
     """
-    if (yaml_data.get("type")) != "bundle":
-        return
-    parts = yaml_data.setdefault("parts", {})
-    if parts:  # Only operate if there aren't any parts declared.
+    if "parts" in yaml_data:  # Only operate if there isn't a parts key
         return
 
-    parts["bundle"] = {"plugin": "bundle", "source": "."}
+    if yaml_data.get("type") == "bundle":
+        yaml_data["parts"] = {"bundle": {"plugin": "bundle", "source": "."}}
+    elif yaml_data.get("type") == "charm":
+        # Only for backwards compatibility for bases charms.
+        # Platforms charms expect parts to be explicit.
+        if "bases" in yaml_data:
+            yaml_data["parts"] = {"charm": {"plugin": "charm", "source": "."}}
 
 
 def add_metadata(project_dir: pathlib.Path, yaml_data: dict[str, Any]) -> None:

tests/unit/models/test_project.py

@@ -55,6 +55,7 @@
     "build-on": [{"channel": "22.04", "name": "ubuntu"}],
     "run-on": [{"channel": "22.04", "name": "ubuntu"}],
 }
+BASIC_CHARM_PARTS = {"charm": {"plugin": "charm", "source": "."}}
 
 MINIMAL_CHARMCRAFT_YAML = """\
 type: charm
@@ -477,35 +478,39 @@ def test_unmarshal_invalid_type(type_):
             None,
             None,
             None,
-            {},
+            {"parts": BASIC_CHARM_PARTS},
         ),
         (
             MINIMAL_CHARMCRAFT_YAML,
             SIMPLE_METADATA_YAML,
             None,
             None,
-            {},
+            {"parts": BASIC_CHARM_PARTS},
         ),
         (
             SIMPLE_CHARMCRAFT_YAML,
             None,
             SIMPLE_CONFIG_YAML,
             None,
-            {"config": SIMPLE_CONFIG_DICT},
+            {"config": SIMPLE_CONFIG_DICT, "parts": BASIC_CHARM_PARTS},
         ),
         (
             SIMPLE_CHARMCRAFT_YAML,
             None,
             None,
             SIMPLE_ACTIONS_YAML,
-            {"actions": SIMPLE_ACTIONS_DICT},
+            {"actions": SIMPLE_ACTIONS_DICT, "parts": BASIC_CHARM_PARTS},
         ),
         (
             MINIMAL_CHARMCRAFT_YAML,
             SIMPLE_METADATA_YAML,
             SIMPLE_CONFIG_YAML,
             SIMPLE_ACTIONS_YAML,
-            {"actions": SIMPLE_ACTIONS_DICT, "config": SIMPLE_CONFIG_DICT},
+            {
+                "actions": SIMPLE_ACTIONS_DICT,
+                "config": SIMPLE_CONFIG_DICT,
+                "parts": BASIC_CHARM_PARTS,
+            },
         ),
     ],
 )

tests/unit/models/valid_charms_yaml/basic-bases.yaml

@@ -11,3 +11,5 @@ bases:
   run-on:
   - name: ubuntu
     channel: '22.04'
+
+parts: {}

tests/unit/test_preprocess.py

@@ -22,6 +22,8 @@
 from charmcraft import const, errors, preprocess
 
 BASIC_BUNDLE = {"type": "bundle", "parts": {"bundle": {"plugin": "bundle", "source": "."}}}
+BASIC_CHARM = {"type": "charm", "parts": {"charm": {"plugin": "charm", "source": "."}}}
+BASIC_BASES_CHARM = {**BASIC_CHARM, "bases": [{"name": "ubuntu", "channel": "22.04"}]}
 
 
 @pytest.mark.parametrize(
@@ -30,6 +32,8 @@
         pytest.param({}, {}, id="no-type"),
         pytest.param({"type": "bundle"}, BASIC_BUNDLE, id="empty-bundle"),
         pytest.param(BASIC_BUNDLE.copy(), BASIC_BUNDLE, id="prefilled-bundle"),
+        pytest.param({"type": "charm"}, {"type": "charm"}, id="empty-charm"),
+        pytest.param(BASIC_CHARM.copy(), BASIC_CHARM, id="empty-charm"),
     ],
 )
 def test_add_default_parts_correct(yaml_data, expected):