Ensure session_interface.open_session is called after URL matching (#3776)

tests/test_session_interface.py

@@ -0,0 +1,22 @@
+import flask
+from flask.sessions import SessionInterface
+
+
+def test_open_session_endpoint_not_none():
+    # Define a session interface that breaks if request.endpoint is None
+    class MySessionInterface(SessionInterface):
+        def save_session(self):
+            pass
+
+        def open_session(self, _, request):
+            assert request.endpoint is not None
+
+    def index():
+        return "Hello World!"
+
+    # Confirm a 200 response, indicating that request.endpoint was NOT None
+    app = flask.Flask(__name__)
+    app.route("/")(index)
+    app.session_interface = MySessionInterface()
+    response = app.test_client().open("/")
+    assert response.status_code == 200