Fixed #24847 -- Prevented items set on a RequestContext from being lost.

Backport of 300e8baf94 from master

django/template/context.py

@@ -219,7 +219,13 @@ class RequestContext(Context):
         self.request = request
         self._processors = () if processors is None else tuple(processors)
         self._processors_index = len(self.dicts)
-        self.update({})         # placeholder for context processors output
+
+        # placeholder for context processors output
+        self.update({})
+
+        # empty dict for any new modifications
+        # (so that context processors don't overwrite them)
+        self.update({})
 
     @contextmanager
     def bind_template(self, template):

docs/releases/1.8.3.txt

@@ -22,3 +22,6 @@ Bugfixes
 
 * Reverted an optimization to the CSRF template context processor which caused
   a regression (:ticket:`24836`).
+
+* Fixed a regression which caused template context processors to overwrite
+  variables set on a ``RequestContext`` after it's created (:ticket:`24847`).

tests/template_tests/test_context.py

@@ -134,8 +134,8 @@ class RequestContextTests(SimpleTestCase):
         request = RequestFactory().get('/')
         ctx = RequestContext(request, {})
         # The stack should now contain 3 items:
-        # [builtins, supplied context, context processor]
-        self.assertEqual(len(ctx.dicts), 3)
+        # [builtins, supplied context, context processor, empty dict]
+        self.assertEqual(len(ctx.dicts), 4)
 
     def test_context_comparable(self):
         # Create an engine without any context processors.
@@ -149,3 +149,10 @@ class RequestContextTests(SimpleTestCase):
             RequestContext(request, dict_=test_data),
             RequestContext(request, dict_=test_data),
         )
+
+    def test_modify_context_and_render(self):
+        template = Template('{{ foo }}')
+        request = RequestFactory().get('/')
+        context = RequestContext(request, {})
+        context['foo'] = 'foo'
+        self.assertEqual(template.render(context), 'foo')