[DHCP] Break reference chain to any Exception object when resync

In the DHCP agent, if an exception is raised during the driver call,
"DhcpAgent.schedule_resync" is called. Before this patch, the
exception instance was passed instead of a string. This instance
reference was stored in the dictionary "needs_resync_reasons" and
used in "_periodic_resync_helper" to resync the DHCP agent
information.

The call to "sync_state" passed the dictionary ".keys()" method. In
python2.7 when that was implemented, this method was creating a list
with the dictionary keys. In python3, this method is a generator
that holds the dictionary content.

This patch breaks this reference chain in two points (actually only
one is needed):
- "sync_state" now passes a list created from the mentioned generator.
- The dictionary "needs_resync_reasons" now stores the exception
  strings only, instead of the exception instance.

Closes-Bug: #1969270
Change-Id: I07e9818021283d321fc32066be7e0f8e2b81e639
(cherry picked from commit e3b3ec930967305e5fce314c0a4cf74151ad711c)

2 files changed, 4 insertions, 4 deletions

diff --git a/neutron/agent/dhcp/agent.py b/neutron/agent/dhcp/agent.py
index e3becaa1ed..028845465a 100644
--- a/neutron/agent/dhcp/agent.py
+++ b/neutron/agent/dhcp/agent.py
@@ -208,7 +208,7 @@ class DhcpAgent(manager.Manager):
                 # allocation failure. When the subnet is updated with a new
                 # allocation pool or a port is  deleted to free up an IP, this
                 # will automatically be retried on the notification
-                self.schedule_resync(e, network.id)
+                self.schedule_resync(str(e), network.id)
             if (isinstance(e, oslo_messaging.RemoteError) and
                     e.exc_type == 'NetworkNotFound' or
                     isinstance(e, exceptions.NetworkNotFound)):
@@ -333,7 +333,7 @@ class DhcpAgent(manager.Manager):
                         net = "*"
                     LOG.debug("resync (%(network)s): %(reason)s",
                               {"reason": r, "network": net})
-                self.sync_state(reasons.keys())
+                self.sync_state(list(reasons.keys()))
 
     def periodic_resync(self):
         """Spawn a thread to periodically resync the dhcp state."""
diff --git a/neutron/tests/unit/agent/dhcp/test_agent.py b/neutron/tests/unit/agent/dhcp/test_agent.py
index d109ba25ed..86e9a131d1 100644
--- a/neutron/tests/unit/agent/dhcp/test_agent.py
+++ b/neutron/tests/unit/agent/dhcp/test_agent.py
@@ -605,7 +605,7 @@ class TestDhcpAgent(base.BaseTestCase):
             sync_state.side_effect = RuntimeError
             with testtools.ExpectedException(RuntimeError):
                 dhcp._periodic_resync_helper()
-            sync_state.assert_called_once_with(resync_reasons.keys())
+            sync_state.assert_called_once_with(list(resync_reasons.keys()))
             self.assertEqual(0, len(dhcp.needs_resync_reasons))
 
     def test_periodic_resync_helper_with_event(self):
@@ -613,7 +613,7 @@ class TestDhcpAgent(base.BaseTestCase):
             dhcp = dhcp_agent.DhcpAgent(HOSTNAME)
             dhcp.schedule_resync('reason1', 'a')
             dhcp.schedule_resync('reason1', 'b')
-            reasons = dhcp.needs_resync_reasons.keys()
+            reasons = list(dhcp.needs_resync_reasons.keys())
             with mock.patch.object(dhcp, 'sync_state') as sync_state:
                 sync_state.side_effect = RuntimeError
                 with testtools.ExpectedException(RuntimeError):