Add scatter-gather-single-cell utility

This patch adds the method to query a single cell using
the scatter-gather utility. Although we do have the
target_cell function to target a specific cell, in situations
where we need to handle a down cell (like for nova show), it
is advantageous to have this wrapper which in turn only calls
the scatter-gather-cells utility.

Change-Id: I31f2ab499aed795812a99b116cec3663760297e3

nova/context.py

@@ -512,6 +512,29 @@ def scatter_gather_skip_cell0(context, fn, *args, **kwargs):
                                 fn, *args, **kwargs)
 
 
+def scatter_gather_single_cell(context, cell_mapping, fn, *args, **kwargs):
+    """Target the provided cell and return its results or sentinels in case of
+    failure.
+
+    The first parameter in the signature of the function to call for each cell
+    should be of type RequestContext.
+
+    :param context: The RequestContext for querying cells
+    :param cell_mapping: The CellMapping to target
+    :param fn: The function to call for each cell
+    :param args: The args for the function to call for each cell, not including
+                 the RequestContext
+    :param kwargs: The kwargs for the function to call for this cell
+    :returns: A dict {cell_uuid: result} containing the joined results. The
+              did_not_respond_sentinel will be returned if the cell did not
+              respond within the timeout. The raised_exception_sentinel will
+              be returned if the call to the cell raised an exception. The
+              exception will be logged.
+    """
+    return scatter_gather_cells(context, [cell_mapping], CELL_TIMEOUT, fn,
+                                *args, **kwargs)
+
+
 def scatter_gather_all_cells(context, fn, *args, **kwargs):
     """Target all cells in parallel and return their results.
 

nova/tests/unit/test_context.py

@@ -466,3 +466,19 @@ class ContextTestCase(test.NoDBTestCase):
         mock_scatter.assert_called_once_with(
             ctxt, [mapping1], 60, objects.InstanceList.get_by_filters, filters,
             sort_dir='foo')
+
+    @mock.patch('nova.context.scatter_gather_cells')
+    def test_scatter_gather_single_cell(self, mock_scatter):
+        ctxt = context.get_context()
+        mapping0 = objects.CellMapping(database_connection='fake://db0',
+                                       transport_url='none:///',
+                                       uuid=objects.CellMapping.CELL0_UUID)
+
+        filters = {'deleted': False}
+        context.scatter_gather_single_cell(ctxt, mapping0,
+            objects.InstanceList.get_by_filters, filters, sort_dir='foo')
+
+        mock_scatter.assert_called_once_with(
+            ctxt, [mapping0], context.CELL_TIMEOUT,
+            objects.InstanceList.get_by_filters, filters,
+            sort_dir='foo')