Create _mech_context before delete to avoid race

When a network is deleted, precommit handlers are notified prior to the
deletion of the network from the database. One handler exists in the ML2
plugin - _network_delete_precommit_handler. This handler queries the
database for the current state of the network and uses it to create a
NetworkContext which it saves under context._mech_context. When the
postcommit handler _network_delete_after_delete_handler is triggered
later, it passess the saved context._mech_context to mechanism drivers.

A problem can occur with provider networks since the segments service
also registers a precommit handler - _delete_segments_for_network. Both
precommit handlers use the default priority, so the order in which they
are called is random, and determined by dict ordering. If the segment
precommit handler executes first, it will delete the segments associated
with the network. When the ML2 plugin precommit handler runs it then
sees no segments for the network and sets the provider attributes of the
network in the NetworkContext to None.

A mechanism driver that is passed a NetworkContext without provider
attributes in its delete_network_postcommit method will not have the
information to perform the necessary actions.  In the case of the
networking-generic-switch mechanism driver where this was observed, this
resulted in the driver ignoring the event, because the network did not
look like a VLAN.

This change uses a priority of zero for ML2 network delete precommit
handler, to ensure they query the network and store the NetworkContext
before the segments service has a chance to delete segments.

A similar change has been made for subnets, both to keep the pattern
consistent and avoid any similar issues.

Change-Id: I6482223ed2a479de4f5ef4cef056c311c0281408
Closes-Bug: #1841967
Depends-On: https://review.opendev.org/680001

neutron/plugins/ml2/plugin.py

@@ -1200,7 +1200,11 @@ class Ml2Plugin(db_base_plugin_v2.NeutronDbPluginV2,
         # called inside of a transaction.
         return super(Ml2Plugin, self).delete_network(context, id)
 
-    @registry.receives(resources.NETWORK, [events.PRECOMMIT_DELETE])
+    # NOTE(mgoddard): Use a priority of zero to ensure this handler runs before
+    # other precommit handlers. This is necessary to ensure we avoid another
+    # handler deleting a subresource of the network, e.g. segments.
+    @registry.receives(resources.NETWORK, [events.PRECOMMIT_DELETE],
+                       priority=0)
     def _network_delete_precommit_handler(self, rtype, event, trigger,
                                           context, network_id, **kwargs):
         network = self.get_network(context, network_id)
@@ -1311,7 +1315,10 @@ class Ml2Plugin(db_base_plugin_v2.NeutronDbPluginV2,
         # called inside of a transaction.
         return super(Ml2Plugin, self).delete_subnet(context, id)
 
-    @registry.receives(resources.SUBNET, [events.PRECOMMIT_DELETE])
+    # NOTE(mgoddard): Use a priority of zero to ensure this handler runs before
+    # other precommit handlers. This is necessary to ensure we avoid another
+    # handler deleting a subresource of the subnet.
+    @registry.receives(resources.SUBNET, [events.PRECOMMIT_DELETE], priority=0)
     def _subnet_delete_precommit_handler(self, rtype, event, trigger,
                                          context, subnet_id, **kwargs):
         subnet_obj = self._get_subnet_object(context, subnet_id)

releasenotes/notes/fix-net-delete-race-f2fa5bac3ab35a5b.yaml

@@ -0,0 +1,9 @@
+---
+fixes:
+  - |
+    Fixes an issue where deletion of a provider network could result in ML2
+    mechanism drivers not being passed information about the network's provider
+    fields. The consequences of this depend on the mechanism driver in use, but
+    could result in the event being ignored, leading to an incorrectly
+    configured network. See `bug 1841967
+    <https://bugs.launchpad.net/neutron/+bug/1841967>`__ for details.