At most there should only be 1 entry in this DB table.

Enforce this by using the same value for the primary key column.

Change-Id: Iedbd2e9b90200f703d05dc3932bfbace398e81e9

gbpservice/neutron/db/migration/alembic_migrations/versions/4f70135dc089_vm_name.py

@@ -35,8 +35,9 @@ def upgrade():
     )
     op.create_table(
         'apic_aim_vm_name_updates',
+        sa.Column('purpose', sa.String(36), nullable=False),
+        sa.PrimaryKeyConstraint('purpose'),
         sa.Column('host_id', sa.String(36), nullable=False),
-        sa.PrimaryKeyConstraint('host_id'),
         sa.Column('last_incremental_update_time', sa.DateTime()),
         sa.Column('last_full_update_time', sa.DateTime()),
     )

gbpservice/neutron/plugins/ml2plus/drivers/apic_aim/db.py

@@ -25,6 +25,7 @@ import sqlalchemy as sa
 from sqlalchemy.ext import baked
 from sqlalchemy import orm
 
+VM_UPDATE_PURPOSE = 'VmUpdate'
 
 LOG = log.getLogger(__name__)
 
@@ -109,12 +110,14 @@ class VMName(model_base.BASEV2):
 
 
 # At any point of time, there should only be one entry in this table.
-# That entry is used to make sure only one controller is actively updating
-# the VMName table.
+# We will enforce that by using the same value for the purpose column which
+# is the primary key. That entry is used to make sure only one controller is
+# actively updating the VMName table.
 class VMNameUpdate(model_base.BASEV2):
     __tablename__ = 'apic_aim_vm_name_updates'
 
-    host_id = sa.Column(sa.String(36), primary_key=True)
+    purpose = sa.Column(sa.String(36), primary_key=True)
+    host_id = sa.Column(sa.String(36))
     last_incremental_update_time = sa.Column(sa.DateTime)
     last_full_update_time = sa.Column(sa.DateTime)
 
@@ -522,7 +525,8 @@ class DbMixin(object):
                 if last_full_update_time:
                     db_obj.last_full_update_time = last_full_update_time
             else:
-                db_obj = VMNameUpdate(host_id=host_id,
+                db_obj = VMNameUpdate(
+                    purpose=VM_UPDATE_PURPOSE, host_id=host_id,
                     last_incremental_update_time=last_incremental_update_time,
                     last_full_update_time=last_full_update_time)
             session.add(db_obj)

gbpservice/neutron/plugins/ml2plus/drivers/apic_aim/mechanism_driver.py

@@ -292,9 +292,16 @@ class ApicMechanismDriver(api_plus.MechanismDriver,
         if nova_vms is None:
             return
 
-        self._set_vm_name_update(session, vm_name_update, self.host_id,
-                                 current_time,
-                                 current_time if is_full_update else None)
+        try:
+            self._set_vm_name_update(session, vm_name_update, self.host_id,
+                                     current_time,
+                                     current_time if is_full_update else None)
+        # This means another controller is also adding an entry at the same
+        # time and he has beat us. This is fine so we will just return.
+        except Exception as e:
+            LOG.warning(e)
+            return
+
         vm_list = []
         for vm in nova_vms:
             vm_list.append((vm.id, vm.name))

gbpservice/neutron/tests/unit/plugins/ml2plus/test_apic_aim.py

@@ -19,6 +19,7 @@ import fixtures
 import mock
 import netaddr
 import six
+from sqlalchemy.orm import exc as sql_exc
 import testtools
 
 from aim.aim_lib import nat_strategy
@@ -1952,6 +1953,13 @@ class TestAimMapping(ApicAimTestCase):
         self.assertEqual(self.driver._get_vm_names(self.db_session),
                          [])
 
+        # This should throw an exception as there will be only one
+        # entry in this DB table at any given time.
+        current_time = datetime.datetime.now()
+        self.assertRaises(sql_exc.FlushError,
+                          self.driver._set_vm_name_update,
+                          self.db_session, None, 'host_id1', current_time)
+
     def test_multi_scope_routing_with_unscoped_pools(self):
         self._test_multi_scope_routing(True)