Only open session if request does not have open session

CHANGES

@@ -67,6 +67,9 @@ Major release, unreleased
   of the generic bad request message. (`#2348`_)
 - Allow registering new tags with ``TaggedJSONSerializer`` to support
   storing other types in the session cookie. (`#2352`_)
+- Only open the session if the request has not been pushed onto the context
+  stack yet. This allows ``stream_with_context`` generators to access the same
+  session that the containing view uses. (`#2354`_)
 
 .. _#1489: https://github.com/pallets/flask/pull/1489
 .. _#1621: https://github.com/pallets/flask/pull/1621
@@ -87,6 +90,7 @@ Major release, unreleased
 .. _#2326: https://github.com/pallets/flask/pull/2326
 .. _#2348: https://github.com/pallets/flask/pull/2348
 .. _#2352: https://github.com/pallets/flask/pull/2352
+.. _#2354: https://github.com/pallets/flask/pull/2354
 
 Version 0.12.2
 --------------

flask/ctx.py

@@ -325,15 +325,18 @@ def push(self):
 
         _request_ctx_stack.push(self)
 
-        # Open the session at the moment that the request context is
-        # available. This allows a custom open_session method to use the
-        # request context (e.g. code that access database information
-        # stored on `g` instead of the appcontext).
-        session_interface = self.app.session_interface
-        self.session = session_interface.open_session(self.app, self.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
+        # pushed, otherwise stream_with_context loses the session.
         if self.session is None:
-            self.session = session_interface.make_null_session(self.app)
+            session_interface = self.app.session_interface
+            self.session = session_interface.open_session(
+                self.app, self.request
+            )
+
+            if self.session is None:
+                self.session = session_interface.make_null_session(self.app)
 
     def pop(self, exc=_sentinel):
         """Pops the request context and unbinds it by doing that.  This will

tests/test_helpers.py

@@ -861,6 +861,20 @@ def generate():
         assert rv.data == b'Hello World!'
         assert called == [42]
 
+    def test_stream_keeps_session(self, app, client):
+        @app.route('/')
+        def index():
+            flask.session['test'] = 'flask'
+
+            @flask.stream_with_context
+            def gen():
+                yield flask.session['test']
+
+            return flask.Response(gen())
+
+        rv = client.get('/')
+        assert rv.data == b'flask'
+
 
 class TestSafeJoin(object):
     def test_safe_join(self):