Fix racey snapshots.

Fixes bug 949475

Atomically tests and sets the instance task_state before allowing a
snapshot or backup to be initiated.

Change-Id: I40671a80f5e75337e176a715837f62d400cc21b6

2 files changed, 43 insertions, 0 deletions

diff --git a/nova/db/api.py b/nova/db/api.py
index bf94747df..9d4fae11e 100644
--- a/nova/db/api.py
+++ b/nova/db/api.py
@@ -647,6 +647,15 @@ def instance_get_all_hung_in_rebooting(context, reboot_window):
     return IMPL.instance_get_all_hung_in_rebooting(context, reboot_window)
 
 
+def instance_test_and_set(context, instance_id, attr, ok_states,
+                          new_state):
+    """Atomically check if an instance is in a valid state, and if it is, set
+    the instance into a new state.
+    """
+    return IMPL.instance_test_and_set(
+            context, instance_id, attr, ok_states, new_state)
+
+
 def instance_update(context, instance_id, values):
     """Set the given properties on an instance and update it.
 
diff --git a/nova/db/sqlalchemy/api.py b/nova/db/sqlalchemy/api.py
index 07d48a229..ff9ffd0bc 100644
--- a/nova/db/sqlalchemy/api.py
+++ b/nova/db/sqlalchemy/api.py
@@ -1691,6 +1691,40 @@ def instance_get_all_hung_in_rebooting(context, reboot_window, session=None):
 
 
 @require_context
+def instance_test_and_set(context, instance_id, attr, ok_states,
+                          new_state, session=None):
+    """Atomically check if an instance is in a valid state, and if it is, set
+    the instance into a new state.
+    """
+    if not session:
+        session = get_session()
+
+    with session.begin():
+        query = model_query(context, models.Instance, session=session,
+                            project_only=True)
+
+        if utils.is_uuid_like(instance_id):
+            query = query.filter_by(uuid=instance_id)
+        else:
+            query = query.filter_by(id=instance_id)
+
+        # NOTE(vish): if with_lockmode isn't supported, as in sqlite,
+        #             then this has concurrency issues
+        instance = query.with_lockmode('update').first()
+
+        state = instance[attr]
+        if state not in ok_states:
+            raise exception.InstanceInvalidState(
+                attr=attr,
+                instance_uuid=instance['uuid'],
+                state=state,
+                method='instance_test_and_set')
+
+        instance[attr] = new_state
+        instance.save(session=session)
+
+
+@require_context
 def instance_update(context, instance_id, values):
     session = get_session()