Ensure non-overlapping cidrs in subnetpools without galera

_get_allocated_cidrs[1] locks only allocated subnets in a subnetpool
(with mysql/postgresql at least). It ensures we don't allocate a cidr
overlapping with existent cidrs but nothing disallows a concurrent
subnet allocation to create a subnet in the same subnetpool.

This change replaces the lock on subnetpool subnets by a lock on the
subnetpool itself. It disallows to allocate concurrently 2 subnets in
the same subnetpool and ensure non-overlapping cidrs in the same
subnetpool.

Moreover this change solves a trouble with postgresql which disallows
to lock an empty select with an outer join: it happens on first subnet
allocation in a subnetpool when no specific cidr is provided. Moving
the lock ensures the lock is done on a non-empty select.

But this change does not ensure non-overlapping cidrs in subnetpools
with galera because galera doesn't support SELECT FOR UPDATE locks. A
follow-up change will (try to?) remove locks from subnet allocation[1]
in order to ensure non-overlapping cidrs in subnetpools also with galera.

[1] in neutron.ipam.subnet_alloc.SubnetAllocator

Closes-Bug: #1451558
Partial-Bug: #1451576
Change-Id: I73854f9863f44621ae0d89c5dc4893ccc16d07e4

neutron/ipam/subnet_alloc.py

@@ -38,9 +38,20 @@ class SubnetAllocator(driver.Pool):
         super(SubnetAllocator, self).__init__(subnetpool, context)
         self._sp_helper = SubnetPoolHelper()
 
+    def _lock_subnetpool(self):
+        """Lock subnetpool associated row.
+
+        This method disallows to allocate concurrently 2 subnets in the same
+        subnetpool, it's required to ensure non-overlapping cidrs in the same
+        subnetpool.
+        """
+        # FIXME(cbrandily): not working with Galera
+        (self._context.session.query(models_v2.SubnetPool.id).
+         filter_by(id=self._subnetpool['id']).
+         with_lockmode('update').first())
+
     def _get_allocated_cidrs(self):
-        query = self._context.session.query(
-            models_v2.Subnet).with_lockmode('update')
+        query = self._context.session.query(models_v2.Subnet)
         subnets = query.filter_by(subnetpool_id=self._subnetpool['id'])
         return (x.cidr for x in subnets)
 
@@ -62,8 +73,7 @@ class SubnetAllocator(driver.Pool):
         subnetpool_id = self._subnetpool['id']
         tenant_id = self._subnetpool['tenant_id']
         with self._context.session.begin(subtransactions=True):
-            qry = self._context.session.query(
-                 models_v2.Subnet).with_lockmode('update')
+            qry = self._context.session.query(models_v2.Subnet)
             allocations = qry.filter_by(subnetpool_id=subnetpool_id,
                                         tenant_id=tenant_id)
             value = 0
@@ -88,6 +98,7 @@ class SubnetAllocator(driver.Pool):
 
     def _allocate_any_subnet(self, request):
         with self._context.session.begin(subtransactions=True):
+            self._lock_subnetpool()
             self._check_subnetpool_tenant_quota(request.tenant_id,
                                                 request.prefixlen)
             prefix_pool = self._get_available_prefix_list()
@@ -111,6 +122,7 @@ class SubnetAllocator(driver.Pool):
 
     def _allocate_specific_subnet(self, request):
         with self._context.session.begin(subtransactions=True):
+            self._lock_subnetpool()
             self._check_subnetpool_tenant_quota(request.tenant_id,
                                                 request.prefixlen)
             cidr = request.subnet_cidr