Add get_context helper method

The context.get_admin_context is used in places that it's not necessary,
and that's likely because there's no helper method to retrieve a non
admin context. This adds such a helper method and adds a note to
get_admin_context warning that it's not usually the right choice.

Change-Id: I2e6a2efa4bcdf3f8688897972a6cf8a5af3f90d6

nova/context.py

@@ -286,7 +286,25 @@ class RequestContext(context.RequestContext):
         return "<Context %s>" % self.to_dict()
 
 
+def get_context():
+    """A helper method to get a blank context.
+
+    Note that overwrite is False here so this context will not update the
+    greenthread-local stored context that is used when logging.
+    """
+    return RequestContext(user_id=None,
+                          project_id=None,
+                          is_admin=False,
+                          overwrite=False)
+
+
 def get_admin_context(read_deleted="no"):
+    # NOTE(alaski): This method should only be used when an admin context is
+    # necessary for the entirety of the context lifetime. If that's not the
+    # case please use get_context(), or create the RequestContext manually, and
+    # use context.elevated() where necessary. Some periodic tasks may use
+    # get_admin_context so that their database calls are not filtered on
+    # project_id.
     return RequestContext(user_id=None,
                           project_id=None,
                           is_admin=True,

nova/tests/unit/test_context.py

@@ -146,6 +146,15 @@ class ContextTestCase(test.NoDBTestCase):
                                overwrite=False)
         self.assertIs(o_context.get_current(), ctx1)
 
+    def test_get_context_no_overwrite(self):
+        # If there is already a context in the cache creating another context
+        # should not overwrite it.
+        ctx1 = context.RequestContext('111',
+                                      '222',
+                                      overwrite=True)
+        context.get_context()
+        self.assertIs(ctx1, o_context.get_current())
+
     def test_admin_no_overwrite(self):
         # If there is already a context in the cache creating an admin
         # context will not overwrite it.
@@ -290,3 +299,9 @@ class ContextTestCase(test.NoDBTestCase):
         with context.target_cell(ctxt, mapping):
             self.assertEqual(ctxt.db_connection, mock.sentinel.cm)
         self.assertEqual(mock.sentinel.db_conn, ctxt.db_connection)
+
+    def test_get_context(self):
+        ctxt = context.get_context()
+        self.assertIsNone(ctxt.user_id)
+        self.assertIsNone(ctxt.project_id)
+        self.assertFalse(ctxt.is_admin)