Add interface to add WSGI middleware (#1814)

As discussed in #1807.

Allowing the injection of WSGI middleware can enable easier migration
from Connexion 2 to Connexion 3. The use cases are limited though, as
this will only work for middleware that can work at the end of the
middleware stack.

connexion/apps/flask.py

@@ -23,7 +23,7 @@
 from connexion.operations import AbstractOperation
 from connexion.options import SwaggerUIOptions
 from connexion.resolver import Resolver
-from connexion.types import MaybeAwaitable
+from connexion.types import MaybeAwaitable, WSGIApp
 from connexion.uri_parsing import AbstractURIParser
 
 
@@ -259,3 +259,19 @@ def add_error_handler(
         ],
     ) -> None:
         self.middleware.add_error_handler(code_or_exception, function)
+
+    def add_wsgi_middleware(
+        self, middleware: t.Type[WSGIApp], **options: t.Any
+    ) -> None:
+        """Wrap the underlying Flask application with a WSGI middleware. Note that it will only be
+        called at the end of the middleware stack. Middleware that needs to act sooner, needs to
+        be added as ASGI middleware instead.
+
+        Adding multiple middleware using this method wraps each middleware around the previous one.
+
+        :param middleware: Middleware class to add
+        :param options: Options to pass to the middleware_class on initialization
+        """
+        self._middleware_app.asgi_app.app = middleware(
+            self._middleware_app.asgi_app.app, **options  # type: ignore
+        )

connexion/types.py

@@ -1,4 +1,32 @@
+import types
 import typing as t
 
-ReturnType = t.TypeVar("ReturnType")
-MaybeAwaitable = t.Union[t.Awaitable[ReturnType], ReturnType]
+# Maybe Awaitable
+_ReturnType = t.TypeVar("_ReturnType")
+MaybeAwaitable = t.Union[t.Awaitable[_ReturnType], _ReturnType]
+
+# WSGIApp
+Environ = t.Mapping[str, object]
+
+_WriteCallable = t.Callable[[bytes], t.Any]
+_ExcInfo = t.Tuple[type, BaseException, types.TracebackType]
+
+_StartResponseCallable = t.Callable[
+    [
+        str,  # status
+        t.Sequence[t.Tuple[str, str]],  # response headers
+    ],
+    _WriteCallable,  # write() callable
+]
+_StartResponseCallableWithExcInfo = t.Callable[
+    [
+        str,  # status
+        t.Sequence[t.Tuple[str, str]],  # response headers
+        t.Optional[_ExcInfo],  # exc_info
+    ],
+    _WriteCallable,  # write() callable
+]
+StartResponse = t.Union[_StartResponseCallable, _StartResponseCallableWithExcInfo]
+ResponseStream = t.Iterable[bytes]
+
+WSGIApp = t.Callable[[Environ, StartResponse], ResponseStream]

docs/middleware.rst

@@ -64,6 +64,20 @@ You can easily add additional ASGI middleware to the middleware stack with the
             .. automethod:: connexion.FlaskApp.add_middleware
                 :noindex:
 
+        You can also add WSGI middleware to a ``FlaskApp``. Note that it will only be called at the
+        end of the middleware stack. If you need your middleware to act sooner, you will have to
+        use an ASGI middleware instead.
+
+        .. code-block:: python
+
+            app.add_wsgi_middleware(MiddlewareClass, **options)
+
+        .. dropdown:: View a detailed reference of the :code:`add_middleware` method
+            :icon: eye
+
+            .. automethod:: connexion.FlaskApp.add_wsgi_middleware
+                :noindex:
+
     .. tab-item:: ConnexionMiddleware
         :sync: ConnexionMiddleware
 
@@ -77,7 +91,7 @@ You can easily add additional ASGI middleware to the middleware stack with the
 
             app.add_middleware(MiddlewareClass, **options)
 
-        .. dropdown:: View a detailed reference of the :code:`add_middleware` method
+        .. dropdown:: View a detailed reference of the :code:`add_wsgi_middleware` method
             :icon: eye
 
             .. automethod:: connexion.ConnexionMiddleware.add_middleware

tests/test_middleware.py

@@ -1,6 +1,11 @@
+import typing as t
+from unittest.mock import Mock
+
 import pytest
+from connexion import FlaskApp
 from connexion.middleware import ConnexionMiddleware, MiddlewarePosition
 from connexion.middleware.swagger_ui import SwaggerUIMiddleware
+from connexion.types import Environ, ResponseStream, StartResponse, WSGIApp
 from starlette.datastructures import MutableHeaders
 
 from conftest import build_app_from_fixture
@@ -81,3 +86,26 @@ def test_position(spec, app_class):
         == f"Could not insert middleware at position BEFORE_SWAGGER. "
         f"Please make sure you have a {SwaggerUIMiddleware} in your stack."
     )
+
+
+def test_add_wsgi_middleware(spec):
+    app: FlaskApp = build_app_from_fixture("simple", app_class=FlaskApp, spec_file=spec)
+
+    class WSGIMiddleware:
+        def __init__(self, app_: WSGIApp, mock_counter):
+            self.next_app = app_
+            self.mock_counter = mock_counter
+
+        def __call__(
+            self, environ: Environ, start_response: StartResponse
+        ) -> ResponseStream:
+            self.mock_counter()
+            return self.next_app(environ, start_response)
+
+    mock = Mock()
+    app.add_wsgi_middleware(WSGIMiddleware, mock_counter=mock)
+
+    app_client = app.test_client()
+    app_client.post("/v1.0/greeting/robbe")
+
+    mock.assert_called_once()