fix(instances): a failed migration says what libvirt said

A live migration whose source host could not resolve the destination's
name for the migration stream failed with "The host of this virtual
machine cannot be reached right now": libvirt reports that failure as a
connection error, and a job's error hides those behind that sentence so
as not to name hosts. Only superusers migrate, so the migration job now
fails with libvirt's own words, here "Unable to resolve address ...",
which points at the destination's Migration address.

instances/jobs.py

@@ -898,11 +898,13 @@ class MigrateInstance(Kind):
             utils.migrate_instance(Compute.objects.get(pk=p["target"]), instance, op.user,
                                    disk_pools=p["disk_pools"],
                                    **{k: p[k] for k in MIGRATE_OPTIONS})
-        except libvirtError:
+        except libvirtError as err:
             op.refresh_from_db(fields=["cancel_requested"])
             if op.cancel_requested:
                 raise Cancelled from None
-            raise
+            # a superuser's job: libvirt's words, which a host the source cannot
+            # reach for the migration would hide behind "cannot be reached"
+            raise util.OperationError(str(err)) from err
         return {}
 
     def progress(self, op):

instances/tests/test_migrate_job.py

@@ -11,9 +11,10 @@ from appsettings.settings import get_settings
 from computes.models import Compute
 from instances.jobs import MigrateInstance
 from instances.models import Instance
-from libvirt import VIR_DOMAIN_XML_INACTIVE, libvirtError
+from libvirt import VIR_DOMAIN_XML_INACTIVE, VIR_ERR_SYSTEM_ERROR, VIR_FROM_RPC, libvirtError
 from operations.jobs import Cancelled, Context, get_kind
 from operations.models import Claim, Operation, acquire
+from operations.utils import public_error_text
 
 UUID = "31313131-4242-5353-6464-757575757575"
 
@@ -156,6 +157,18 @@ class MigrateJobTestCase(TransactionTestCase):
         self.vm.refresh_from_db()
         self.assertEqual(self.vm.compute, self.source)
 
+    def test_a_failed_migration_says_what_libvirt_said(self):
+        # libvirt reports the source host's failure to reach the destination's migration
+        # port as a connection error; only superusers migrate, so its text is shown
+        op = self.queue(live_migrate="true")
+        ctx = self.start(op)
+        err = libvirtError("Unable to resolve address 'kvm2.example.com' service '49152': Name or service not known")
+        err.err = (VIR_ERR_SYSTEM_ERROR, VIR_FROM_RPC, str(err), 2, None, None, None, 0, 0)
+        self.dest_conn.moveto.side_effect = err
+        with self.assertRaises(Exception) as raised:
+            get_kind(op.kind).run(op, ctx)
+        self.assertIn("Unable to resolve address 'kvm2.example.com'", public_error_text(raised.exception))
+
     def test_cancel_and_progress_ask_the_source_vm(self):
         op = self.queue(live_migrate="true")
         with patch("instances.jobs.wvmInstance") as proxy: