Do not post allocations that are zero

When posting allocations for VMs with a non-local disk, the code was
sending an allocation of DISK_GB = 0, which violated the min_unit
resource constraint. Since it is pointless to enter an allocation of
zero units, this patch removes any items in the generated allocations
dict if the value is zero.

Closes-Bug: #1647316

Change-Id: I59eff4310e67ceb74086f5ade1a637f8cccec7ed

nova/scheduler/client/report.py

@@ -114,11 +114,13 @@ def _instance_to_allocations_dict(instance):
     disk = ((0 if is_bfv else instance.flavor.root_gb) +
             instance.flavor.swap +
             instance.flavor.ephemeral_gb)
-    return {
+    alloc_dict = {
         MEMORY_MB: instance.flavor.memory_mb,
         VCPU: instance.flavor.vcpus,
         DISK_GB: disk,
     }
+    # Remove any zero allocations.
+    return {key: val for key, val in alloc_dict.items() if val}
 
 
 def _extract_inventory_in_use(body):

nova/tests/unit/scheduler/client/test_report.py

@@ -928,6 +928,23 @@ class TestAllocations(SchedulerReportClientTestCase):
         }
         self.assertEqual(expected, result)
 
+    @mock.patch('nova.compute.utils.is_volume_backed_instance')
+    def test_instance_to_allocations_dict_zero_disk(self, mock_vbi):
+        mock_vbi.return_value = True
+        inst = objects.Instance(
+            uuid=uuids.inst,
+            flavor=objects.Flavor(root_gb=10,
+                                  swap=0,
+                                  ephemeral_gb=0,
+                                  memory_mb=1024,
+                                  vcpus=2))
+        result = report._instance_to_allocations_dict(inst)
+        expected = {
+            'MEMORY_MB': 1024,
+            'VCPU': 2,
+        }
+        self.assertEqual(expected, result)
+
     @mock.patch('nova.scheduler.client.report.SchedulerReportClient.'
                 'put')
     @mock.patch('nova.scheduler.client.report.SchedulerReportClient.'