Make servers API use cell-targeted context

This makes the servers API look up the CellMapping for the instance
in question and target the context to the appropriate cell for the
subsequent call into compute/api.

Note that just as with get() and get_all(), we need to avoid going
direct to the cell database and thus short-circuiting the replication
in the case of cellsv1.

Related to blueprint cells-aware-api
Change-Id: If63a4fa7349890a6e7ac0fa3fe12917dc53334d0

1 files changed, 21 insertions, 5 deletions

diff --git a/nova/context.py b/nova/context.py
index 02549f31b1..bc5a21a92e 100644
--- a/nova/context.py
+++ b/nova/context.py
@@ -358,19 +358,35 @@ def authorize_quota_class_context(context, class_name):
             raise exception.Forbidden()
 
 
-@contextmanager
-def target_cell(context, cell_mapping):
+def set_target_cell(context, cell_mapping):
     """Adds database connection information to the context
-    for communicating with the given target cell.
+    for communicating with the given target_cell.
+
+    This is used for permanently targeting a cell in a context.
+    Use this when you want all subsequent code to target a cell.
 
     :param context: The RequestContext to add connection information
-    :param cell_mapping: A objects.CellMapping object
+    :param cell_mapping: An objects.CellMapping object
     """
-    original_db_connection = context.db_connection
     # avoid circular import
     from nova import db
     db_connection_string = cell_mapping.database_connection
     context.db_connection = db.create_context_manager(db_connection_string)
+
+
+@contextmanager
+def target_cell(context, cell_mapping):
+    """Temporarily adds database connection information to the context
+    for communicating with the given target cell.
+
+    This context manager makes a temporary change to the context
+    and restores it when complete.
+
+    :param context: The RequestContext to add connection information
+    :param cell_mapping: A objects.CellMapping object
+    """
+    original_db_connection = context.db_connection
+    set_target_cell(context, cell_mapping)
     try:
         yield context
     finally: