Return NoContent when mocking HTTP 204 operations (#2000)

There's never a need to return a response for HTTP 204 - and these code
are unlikely to have any "example" response documented in the spec.

Avoids a uvicorn RuntimeError `"Response content longer than
Content-Length"` when mocking a (3.0) spec containing:

```
...
      responses:
        '204':
          description: No Content
        '401':
          description: Unauthorized
          content:
            application/json:
              schema:
                $ref: '#/components/schemas/ProblemDetails'
...
```

connexion/operations/openapi.py

@@ -4,8 +4,9 @@
 
 import logging
 import typing as t
+from http import HTTPStatus
 
-from connexion.datastructures import MediaTypeDict
+from connexion.datastructures import MediaTypeDict, NoContent
 from connexion.operations.abstract import AbstractOperation
 from connexion.uri_parsing import OpenAPIURIParser
 from connexion.utils import build_example_from_schema, deep_get
@@ -170,6 +171,10 @@ def example_response(self, status_code=None, content_type=None):
             status_code = int(status_code)
         except ValueError:
             status_code = 200
+
+        if status_code == HTTPStatus.NO_CONTENT:
+            return NoContent, status_code
+
         try:
             # TODO also use example header?
             return (

connexion/operations/swagger2.py

@@ -4,7 +4,9 @@
 
 import logging
 import typing as t
+from http import HTTPStatus
 
+from connexion.datastructures import NoContent
 from connexion.exceptions import InvalidSpecification
 from connexion.operations.abstract import AbstractOperation
 from connexion.uri_parsing import Swagger2URIParser
@@ -196,6 +198,10 @@ def example_response(self, status_code=None, *args, **kwargs):
             status_code = int(status_code)
         except ValueError:
             status_code = 200
+
+        if status_code == HTTPStatus.NO_CONTENT:
+            return NoContent, status_code
+
         try:
             return (
                 list(deep_get(self._responses, examples_path).values())[0],

tests/test_mock.py

@@ -1,3 +1,4 @@
+from connexion.datastructures import NoContent
 from connexion.mock import MockResolver
 from connexion.operations import OpenAPIOperation, Swagger2Operation
 
@@ -261,6 +262,28 @@ def test_mock_resolver_no_example_nested_in_list_openapi():
     assert all(isinstance(c, str) for c in response)
 
 
+def test_mock_resolver_no_content():
+    resolver = MockResolver(mock_all=True)
+
+    responses = {"204": {}}
+
+    operation = Swagger2Operation(
+        method="GET",
+        path="endpoint",
+        path_parameters=[],
+        operation={"responses": responses},
+        app_produces=["application/json"],
+        app_consumes=["application/json"],
+        definitions={},
+        resolver=resolver,
+    )
+    assert operation.operation_id == "mock-1"
+
+    response, status_code = resolver.mock_operation(operation)
+    assert status_code == 204
+    assert response == NoContent
+
+
 def test_mock_resolver_no_examples():
     resolver = MockResolver(mock_all=True)
 

tests/test_mock3.py

@@ -1,3 +1,4 @@
+from connexion.datastructures import NoContent
 from connexion.mock import MockResolver
 from connexion.operations import OpenAPIOperation
 
@@ -87,6 +88,25 @@ def test_mock_resolver_inline_schema_example():
     assert response == {"foo": "bar"}
 
 
+def test_mock_resolver_no_content():
+    resolver = MockResolver(mock_all=True)
+
+    responses = {"204": {}}
+
+    operation = OpenAPIOperation(
+        method="GET",
+        path="endpoint",
+        path_parameters=[],
+        operation={"responses": responses},
+        resolver=resolver,
+    )
+    assert operation.operation_id == "mock-1"
+
+    response, status_code = resolver.mock_operation(operation)
+    assert status_code == 204
+    assert response == NoContent
+
+
 def test_mock_resolver_no_examples():
     resolver = MockResolver(mock_all=True)