feat(config): limit use of CRAFT_* env vars

CRAFT_* environment variables are now only used if the config item
is known to craft-application (not for app-specific config).

craft_application/services/config.py

@@ -29,7 +29,7 @@
 from craft_cli import emit
 from typing_extensions import override
 
-from craft_application import application, util
+from craft_application import application, util, _config
 from craft_application.services import base
 
 if TYPE_CHECKING:
@@ -72,8 +72,18 @@ def get_raw(self, item: str) -> str:
 class CraftEnvironmentHandler(ConfigHandler):
     """Configuration handler to get values from CRAFT environment variables."""
 
+    def __init__(self, app: application.AppMetadata) -> None:
+        super().__init__(app)
+        self._fields = _config.ConfigModel.model_fields
+
+
     @override
     def get_raw(self, item: str) -> str:
+        # Ensure that CRAFT_* env vars can only be used for configuration items
+        # known to craft-application.
+        if item not in self._fields:
+            raise KeyError(f"{item!r} not a general craft-application config item.")
+
         return os.environ[f"CRAFT_{item.upper()}"]
 
 

tests/unit/services/test_config.py

@@ -24,6 +24,7 @@
 import craft_cli
 import pytest
 import pytest_subprocess
+import craft_application
 from craft_application.services import config
 from hypothesis import given, strategies
 
@@ -100,7 +101,7 @@ def test_app_environment_handler(app_environment_handler, item: str, content: st
 
 
 @given(
-    item=strategies.text(alphabet=string.ascii_letters + "_", min_size=1),
+    item=strategies.sampled_from(list(craft_application.ConfigModel.model_fields)),
     content=strategies.text(
         alphabet=strategies.characters(categories=["L", "M", "N", "P", "S", "Z"])
     ),
@@ -112,6 +113,25 @@ def test_craft_environment_handler(craft_environment_handler, item: str, content
         assert craft_environment_handler.get_raw(item) == content
 
 
+@pytest.mark.parametrize(
+    ("item", "content", "_"), CRAFT_APPLICATION_TEST_ENTRY_VALUES
+)
+def test_craft_environment_handler_success(monkeypatch, craft_environment_handler, item: str, content: str, _):
+    monkeypatch.setenv(f"CRAFT_{item.upper()}", content)
+
+    assert craft_environment_handler.get_raw(item) == content
+
+
+@pytest.mark.parametrize(
+    ("item", "content", "_"), APP_SPECIFIC_TEST_ENTRY_VALUES
+)
+def test_craft_environment_handler_error(monkeypatch, craft_environment_handler, item: str, content: str, _):
+    monkeypatch.setenv(f"CRAFT_{item.upper()}", content)
+
+    with pytest.raises(KeyError):
+        assert craft_environment_handler.get_raw(item) == content
+
+
 @given(
     item=strategies.text(alphabet=string.ascii_letters + "_", min_size=1),
     content=strategies.text(