fix(instances): recording a migration holds the ownership lock

record_migration moved or merged a VM's owners without lock_ownership(),
which every other step that moves owners takes; on PostgreSQL a sync of
a third compute with the same UUID could move them at the same time, and
owners moved onto the source record went with its delete.

instances/tests/test_migrate_job.py

@@ -228,3 +228,13 @@ class MigrateJobTestCase(TransactionTestCase):
                            resource_label="mig-src/mig-vm")
         self.assertEqual(MigrateInstance().log_entry(queued, Operation.CANCELLED),
                          ("mig-src", "mig-vm", UUID, "VM mig-vm: migrate to mig-dst (cancelled)"))
+
+    def test_recording_a_migration_holds_the_ownership_lock(self):
+        # a sync of a third compute must not move the owners at the same time
+        from instances import utils
+
+        with patch.object(utils, "lock_ownership") as lock:
+            utils.record_migration(self.vm, self.dest)
+        lock.assert_called_once_with()
+        self.vm.refresh_from_db()
+        self.assertEqual(self.vm.compute, self.dest)

instances/utils.py

@@ -7,6 +7,7 @@ from collections import namedtuple
 from accounts.models import UserInstance, UserAttributes
 from appsettings.settings import app_settings
 from computes.models import Compute
+from computes.utils import lock_ownership
 from django.conf import settings
 from django.db import transaction
 from django.utils.translation import gettext_lazy as _
@@ -321,6 +322,8 @@ def record_migration(instance, new_compute):
     """Moves instance to new_compute in the database. A record the list of
     new_compute made for it meanwhile takes its owners and template mark."""
     with transaction.atomic():
+        # owners are moved here: not while a sync of a third compute moves them too
+        lock_ownership()
         target_inst = Instance.objects.filter(compute=new_compute, uuid=instance.uuid).first()
         if target_inst and target_inst.id != instance.id:
             for ui in UserInstance.objects.filter(instance=instance):