Ensure error while opening session pops context

CHANGES

@@ -25,11 +25,14 @@ Major release, unreleased
   adding OPTIONS method when the ``view_func`` argument is not a class.
   (`#1489`_).
 - ``MethodView`` can inherit method handlers from base classes. (`#1936`_)
+- Errors caused while opening the session at the beginning of the request are
+  handled by the app's error handlers. (`#2254`_)
 
 .. _#1489: https://github.com/pallets/flask/pull/1489
 .. _#1936: https://github.com/pallets/flask/pull/1936
 .. _#2017: https://github.com/pallets/flask/pull/2017
 .. _#2223: https://github.com/pallets/flask/pull/2223
+.. _#2254: https://github.com/pallets/flask/pull/2254
 
 Version 0.12.1
 --------------

flask/app.py

@@ -2002,10 +2002,10 @@ def wsgi_app(self, environ, start_response):
                                exception context to start the response
         """
         ctx = self.request_context(environ)
-        ctx.push()
         error = None
         try:
             try:
+                ctx.push()
                 response = self.full_dispatch_request()
             except Exception as e:
                 error = e

tests/test_reqctx.py

@@ -12,6 +12,7 @@
 import pytest
 
 import flask
+from flask.sessions import SessionInterface
 
 try:
     from greenlet import greenlet
@@ -193,3 +194,27 @@ def g():
 
     result = greenlets[0].run()
     assert result == 42
+
+
+def test_session_error_pops_context():
+    class SessionError(Exception):
+        pass
+
+    class FailingSessionInterface(SessionInterface):
+        def open_session(self, app, request):
+            raise SessionError()
+
+    class CustomFlask(flask.Flask):
+        session_interface = FailingSessionInterface()
+
+    app = CustomFlask(__name__)
+
+    @app.route('/')
+    def index():
+        # shouldn't get here
+        assert False
+
+    response = app.test_client().get('/')
+    assert response.status_code == 500
+    assert not flask.request
+    assert not flask.current_app