fix(flask): handle SyntaxErrors when parsing app.py

rockcraft/extensions/gunicorn.py

@@ -259,9 +259,14 @@ def _wsgi_path_error_messages(self):
             return [
                 "flask application can not be imported from app:app, no app.py file found in the project root."
             ]
-        if not self.has_global_variable(app_file, "app"):
+        try:
+            has_app = self.has_global_variable(app_file, "app")
+        except SyntaxError as err:
+            return [f"error parsing app.py: {err.msg}"]
+
+        if not has_app:
             return [
-                "flask application can not be imported from app:app in file app.py file in the project root."
+                "flask application can not be imported from app:app in app.py in the project root."
             ]
 
         return []

tests/unit/extensions/test_gunicorn.py

@@ -13,6 +13,7 @@
 #
 # You should have received a copy of the GNU General Public License
 # along with this program.  If not, see <http://www.gnu.org/licenses/>.
+import textwrap
 
 import pytest
 from rockcraft import extensions
@@ -333,6 +334,34 @@ def test_flask_extension_requirements_txt_no_flask_error(tmp_path):
     )
 
 
+@pytest.mark.usefixtures("flask_extension")
+def test_flask_extension_bad_app_py(tmp_path):
+    bad_code = textwrap.dedent(
+        """
+    import flask
+
+    app = flask.Flask()
+
+    bad = "my value
+    """
+    ).strip()
+    (tmp_path / "app.py").write_text(bad_code)
+    (tmp_path / "requirements.txt").write_text("flask")
+    flask_input_yaml = {
+        "extensions": ["flask-framework"],
+        "base": "bare",
+        "build-base": "[email protected]",
+        "platforms": {"amd64": {}},
+    }
+    with pytest.raises(ExtensionError) as exc:
+        extensions.apply_extensions(tmp_path, flask_input_yaml)
+
+    expected = (
+        "- error parsing app.py: unterminated string literal (detected at line 5)"
+    )
+    assert str(exc.value) == expected
+
+
 @pytest.mark.usefixtures("flask_extension")
 def test_flask_extension_no_requirements_txt_no_app_py_error(tmp_path):
     flask_input_yaml = {