chore: refactor "features"

Instead of having a "loose" boolean in Application, formalize the
concept a bit by adding an AppFeatures object to be kept in the
AppMetadata (as part of the app's "static" info).

This complicates testing a bit because the features have to be enabled
at AppMetadata-creation-time and can't be set "a posteriori" anymore,
but the end result isn't too onerous and better reflects what the
applications will do anyway.

craft_application/__init__.py

@@ -15,7 +15,7 @@
 # with this program.  If not, see <http://www.gnu.org/licenses/>.
 """Framework for *craft applications."""
 
-from craft_application.application import Application, AppMetadata
+from craft_application.application import Application, AppFeatures, AppMetadata
 from craft_application import models
 from craft_application.services import (
     BaseService,
@@ -38,6 +38,7 @@
 __all__ = [
     "__version__",
     "Application",
+    "AppFeatures",
     "AppMetadata",
     "models",
     "BaseService",

craft_application/application.py

@@ -53,6 +53,14 @@ def parsed_args(self) -> argparse.Namespace:
         return self._parsed_command_args or argparse.Namespace()
 
 
+@dataclass(frozen=True)
+class AppFeatures:
+    """Specific features that can be enabled/disabled per-application."""
+
+    build_secrets: bool = False
+    """Support for build-time secrets."""
+
+
 @final
 @dataclass(frozen=True)
 class AppMetadata:
@@ -63,6 +71,7 @@ class AppMetadata:
     version: str = field(init=False)
     source_ignore_patterns: list[str] = field(default_factory=lambda: [])
     managed_instance_project_path = pathlib.PurePosixPath("/root/project")
+    features: AppFeatures = AppFeatures()
 
     ProjectClass: type[models.Project] = models.Project
 
@@ -94,10 +103,6 @@ class Application:
     :ivar services: A ServiceFactory for this application
     """
 
-    # Feature flag for build-time secrets. Applications that want to use the feature
-    # must set this to True.
-    enable_build_secrets = False
-
     def __init__(
         self,
         app: AppMetadata,
@@ -384,7 +389,7 @@ def _transform_project_yaml(self, yaml_data: dict[str, Any]) -> dict[str, Any]:
         self._expand_environment(yaml_data)
 
         # Handle build secrets.
-        if self.enable_build_secrets:
+        if self.app.features.build_secrets:
             self._render_secrets(yaml_data)
 
         # Perform extra, application-specific transformations.

tests/conftest.py

@@ -38,14 +38,33 @@ def get_build_plan(self) -> list[models.BuildInfo]:
 
 
 @pytest.fixture()
-def app_metadata() -> craft_application.AppMetadata:
+def features(request) -> dict[str, bool]:
+    """Fixture that controls the enabled features.
+
+    To use it, mark the test with the features that should be enabled. For example:
+
+    @pytest.mark.enable_features("build_secrets")
+    def test_with_build_secrets(...)
+    """
+    features = {}
+
+    for feature_marker in request.node.iter_markers("enable_features"):
+        for feature_name in feature_marker.args:
+            features[feature_name] = True
+
+    return features
+
+
+@pytest.fixture()
+def app_metadata(features) -> craft_application.AppMetadata:
     with pytest.MonkeyPatch.context() as m:
         m.setattr(metadata, "version", lambda _: "3.14159")
         return craft_application.AppMetadata(
             "testcraft",
             "A fake app for testing craft-application",
             ProjectClass=MyProject,
             source_ignore_patterns=["*.snap", "*.charm", "*.starcraft"],
+            features=craft_application.AppFeatures(**features),
         )
 
 

tests/integration/test_application.py

@@ -259,6 +259,7 @@ def test_global_environment(
     assert variables["project_version"] == "1.2.3"
 
 
+@pytest.mark.enable_features("build_secrets")
 def test_build_secrets_destructive(create_app, monkeypatch, tmp_path, capsys):
     """Test the use of build secrets in destructive mode."""
     monkeypatch.setenv("CRAFT_DEBUG", "1")
@@ -269,7 +270,6 @@ def test_build_secrets_destructive(create_app, monkeypatch, tmp_path, capsys):
     monkeypatch.setattr("sys.argv", ["testcraft", "prime", "-v", "--destructive-mode"])
 
     app = create_app()
-    app.enable_build_secrets = True
 
     # Set the environment variables that the project needs
     monkeypatch.setenv("HOST_SOURCE_FOLDER", "secret-source")

tests/unit/test_application.py

@@ -540,13 +540,13 @@ def build_secrets_project(monkeypatch, tmp_path):
 
 
 @pytest.mark.usefixtures("build_secrets_project")
+@pytest.mark.enable_features("build_secrets")
 def test_application_build_secrets(app_metadata, fake_services, monkeypatch, mocker):
     monkeypatch.setenv("SECRET_VAR_1", "source-folder")
     monkeypatch.setenv("SECRET_VAR_2", "secret-value")
     spied_set_secrets = mocker.spy(craft_cli.emit, "set_secrets")
 
     app = application.Application(app_metadata, fake_services)
-    app.enable_build_secrets = True
     project = app.project
 
     mypart = project.parts["mypart"]