Refactor spawn to use UndoManager.

UndoManager provides a mechanism for automatically rolling back on
exceptions.  An additional benefit of this approach is that undo code is
spatially close to the roll-forward code.

This patch should be considered an intermediate step towards a more
complete Command pattern based approach down the road.

Change-Id: Ib429420d67324422a5d13cdea6872fd9c57c857e

1 files changed, 26 insertions, 0 deletions

diff --git a/nova/utils.py b/nova/utils.py
index a224b3878..0ea6ee93b 100644
--- a/nova/utils.py
+++ b/nova/utils.py
@@ -1582,3 +1582,29 @@ def strcmp_const_time(s1, s2):
     for (a, b) in zip(s1, s2):
         result |= ord(a) ^ ord(b)
     return result == 0
+
+
+class UndoManager(object):
+    """Provides a mechanism to facilitate rolling back a series of actions
+    when an exception is raised.
+    """
+    def __init__(self):
+        self.undo_stack = []
+
+    def undo_with(self, undo_func):
+        self.undo_stack.append(undo_func)
+
+    def _rollback(self):
+        for undo_func in reversed(self.undo_stack):
+            undo_func()
+
+    def rollback_and_reraise(self, msg=None):
+        """Rollback a series of actions then re-raise the exception.
+
+        NOTE(sirp): This should only be called within an exception handler.
+        """
+        with save_and_reraise_exception():
+            if msg:
+                LOG.exception(msg)
+
+            self._rollback()