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

CHANGES.rst

@@ -60,6 +60,9 @@ Unreleased
     conditional requests instead of using a timed cache. :pr:`3828`
 -   ``helpers.safe_join`` is deprecated. Use
     ``werkzeug.utils.safe_join`` instead. :pr:`3828`
+-   The request context does route matching before opening the session.
+    This could allow a session interface to change behavior based on
+    ``request.endpoint``. :issue:`3776`
 
 
 Version 1.1.2

src/flask/ctx.py

@@ -361,6 +361,9 @@ class RequestContext:
 
         _request_ctx_stack.push(self)
 
+        if self.url_adapter is not None:
+            self.match_request()
+
         # Open the session at the moment that the request context is available.
         # This allows a custom open_session method to use the request context.
         # Only open a new session if this is the first time the request was
@@ -372,9 +375,6 @@ class RequestContext:
             if self.session is None:
                 self.session = session_interface.make_null_session(self.app)
 
-        if self.url_adapter is not None:
-            self.match_request()
-
     def pop(self, exc=_sentinel):
         """Pops the request context and unbinds it by doing that.  This will
         also trigger the execution of functions registered by the

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