fix(instances): a snapshot job checks its name again in the worker

The name was checked only when the job was queued; recovery counts any
snapshot of that name as the job's, so one taken outside the panel
meanwhile made a failed job succeed. precheck now refuses the name
before any effect.

instances/jobs.py

@@ -384,7 +384,10 @@ class CreateSnapshot(Kind):
         return _vm_plan(params)
 
     def precheck(self, op, ctx):
-        _checked_proxy(op)
+        # a snapshot taken outside the panel since the job was queued may have
+        # the name, and recovery would count it as this job's
+        if op.params["name"] in _checked_proxy(op).instance.snapshotListNames(0):
+            raise util.OperationError(_("A snapshot named %(name)s already exists"), name=op.params["name"])
 
     def run(self, op, ctx):
         p = op.params

instances/tests/test_snapshot_jobs.py

@@ -70,6 +70,15 @@ class SnapshotJobsTestCase(TransactionTestCase):
             self.assertContains(response, "Snapshot names are 1-64 letters")
         self.assertFalse(Operation.objects.exists())
 
+    def test_a_name_taken_outside_the_panel_meanwhile_is_refused_before_effects(self):
+        self.client.post(reverse("instances:snapshot", args=[self.instance.pk]),
+                         {"name": "b", "submission_key": "key-snap0001"})
+        op = Operation.objects.get()
+        self.proxy.instance.snapshotListNames.return_value = ["b"]  # virsh snapshot-create-as
+        with self.assertRaisesMessage(OperationError, "A snapshot named b already exists"):
+            CreateSnapshot().precheck(op, None)
+        self.proxy.create_snapshot.assert_not_called()
+
     def test_create_recovery(self):
         op = Operation(params={"instance": self.instance.pk, "name": "a", "external": True},
                        intent={"compute": self.instance.compute_id})