Fix "join" when the last indirect inbound task failed

* See bug description for the example that didn't work. It was
  caused by a simple mistake in a python expression of type
  "my_set = my_set or set()" that didn't work as expected, i.e.
  it created a new set even if my_set is already an empty set.
  So, the proper expression that's needed is
  "my_set = set() if my_set is None else my_set"

Change-Id: I2a787921449fecf3301013a770ffe712e9606baf
Closes-Bug: #1803677

mistral/tests/unit/engine/test_join.py

@@ -1293,3 +1293,58 @@ class JoinEngineTest(base.EngineTestCase):
         self.assertEqual(states.SUCCESS, task1.state)
         self.assertEqual(states.SUCCESS, task2.state)
         self.assertEqual(states.SUCCESS, task3.state)
+
+    def test_join_last_inbound_indirect_error(self):
+        wf_text = """---
+        version: '2.0'
+
+        wf:
+          tasks:
+            task1:
+              action: std.noop
+              on-success:
+                - join_task
+
+            task2:
+              action: std.fail
+              wait-before: 2
+              on-success:
+                - task3
+
+            task3:
+              action: std.noop
+              on-success:
+                - join_task
+
+            join_task:
+              join: all
+        """
+
+        wf_service.create_workflows(wf_text)
+
+        wf_ex = self.engine.start_workflow('wf')
+
+        self.await_workflow_error(wf_ex.id)
+
+        with db_api.transaction():
+            wf_ex = db_api.get_workflow_execution(wf_ex.id)
+
+            task_execs = wf_ex.task_executions
+
+            self.assertEqual(3, len(task_execs))
+
+            self._assert_single_item(
+                task_execs,
+                name='task1',
+                state=states.SUCCESS
+            )
+            self._assert_single_item(
+                task_execs,
+                name='task2',
+                state=states.ERROR
+            )
+            self._assert_single_item(
+                task_execs,
+                name='join_task',
+                state=states.ERROR
+            )

mistral/workflow/direct_workflow.py

@@ -321,14 +321,16 @@ class DirectWorkflowController(base.WorkflowController):
     @profiler.trace('direct-wf-controller-find-downstream-joins')
     def _find_indirectly_affected_created_joins(self, task_name, result=None,
                                                 visited_task_names=None):
-        visited_task_names = visited_task_names or set()
+        visited_task_names = (
+            set() if visited_task_names is None else visited_task_names
+        )
 
         if task_name in visited_task_names:
             return
 
         visited_task_names.add(task_name)
 
-        result = result or set()
+        result = set() if result is None else result
 
         def _process_clause(clause):
             for t_name, condition, params in clause:

releasenotes/notes/fix_join_when_last_finished_indirect_error-b0e5adf99cde9a58.yaml

@@ -0,0 +1,5 @@
+---
+fixes:
+  - |
+    Fixed the issue when "join" task remained in WAITING state forever if
+    the last inbound task failed and it was not a direct predecessor.