Improve error message for YAQL task function

Improve the error message in case a non existing task name is passed
to the "task" YAQL function

Change-Id: I93a33365779ba2a5301431c6769697d8b4861fae
Closes-Bug: 1520930

doc/source/dsl/dsl_v2.rst

@@ -830,7 +830,7 @@ Example:
         - radix: 16
 
       output:
-        uuid: <% task(generated_uuid).result %>
+        uuid: <% task(generate_uuid_task).result %>
 
       tasks:
         generate_uuid_task:

mistral/tests/unit/engine/test_yaql_functions.py

@@ -109,3 +109,28 @@ class YAQLFunctionsEngineTest(engine_test_base.EngineTestCase):
             },
             task2.published
         )
+
+    def test_task_function_non_existing(self):
+        wf_text = """---
+            version: '2.0'
+
+            wf:
+              type: direct
+              output:
+                task_name: <% task(non_existing_task).name %>
+
+              tasks:
+                task1:
+                  action: std.noop
+            """
+
+        wf_service.create_workflows(wf_text)
+
+        wf_ex = self.engine.start_workflow('wf', {})
+
+        self._await(lambda: self.is_execution_error(wf_ex.id))
+
+        wf_ex = db_api.get_workflow_execution(wf_ex.id)
+
+        self.assertEqual(states.ERROR, wf_ex.state)
+        self.assertIn('non_existing_task', wf_ex.state_info)

mistral/utils/yaql_utils.py

@@ -75,7 +75,7 @@ def task_(context, task_name):
 
     # TODO(rakhmerov): Account for multiple executions (i.e. in case of
     # cycles).
-    task_ex = task_execs[-1]
+    task_ex = task_execs[-1] if len(task_execs) > 0 else None
 
     if not task_ex:
         raise ValueError(