ensure error while opening session pops context

errors will be handled by the app error handlers
closes #1538, closes #1528

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 test_greenlet_context_copying_api():
 
     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