Retain chain of missing dependencies

Change-Id: I170bedb5c667aa30764ad29634910dda40c6cd49

taskflow/engines/action_engine/engine.py

@@ -225,12 +225,34 @@ class ActionEngine(base.Engine):
                         execution_graph.number_of_edges(),
                         nx.density(execution_graph))
         missing = set()
-        fetch = self.storage.fetch_unsatisfied_args
+        # Attempt to retain a chain of what was missing (so that the final
+        # raised exception for the flow has the nodes that had missing
+        # dependencies).
+        last_cause = None
+        last_node = None
+        missing_nodes = 0
+        fetch_func = self.storage.fetch_unsatisfied_args
         for node in execution_graph.nodes_iter():
-            missing.update(fetch(node.name, node.rebind,
-                                 optional_args=node.optional))
+            node_missing = fetch_func(node.name, node.rebind,
+                                      optional_args=node.optional)
+            if node_missing:
+                cause = exc.MissingDependencies(node,
+                                                sorted(node_missing),
+                                                cause=last_cause)
+                last_cause = cause
+                last_node = node
+                missing_nodes += 1
+                missing.update(node_missing)
         if missing:
-            raise exc.MissingDependencies(self._flow, sorted(missing))
+            # For when a task is provided (instead of a flow) and that
+            # task is the only item in the graph and its missing deps, avoid
+            # re-wrapping it in yet another exception...
+            if missing_nodes == 1 and last_node is self._flow:
+                raise last_cause
+            else:
+                raise exc.MissingDependencies(self._flow,
+                                              sorted(missing),
+                                              cause=last_cause)
 
     @lock_utils.locked
     def prepare(self):

taskflow/tests/unit/test_engines.py

@@ -651,6 +651,26 @@ class EngineGraphFlowTest(utils.EngineTestBase):
         self.assertIsInstance(graph, gr.DiGraph)
 
 
+class EngineMissingDepsTest(utils.EngineTestBase):
+    def test_missing_deps_deep(self):
+        flow = gf.Flow('missing-many').add(
+            utils.TaskOneReturn(name='task1',
+                                requires=['a', 'b', 'c']),
+            utils.TaskMultiArgOneReturn(name='task2',
+                                        rebind=['e', 'f', 'g']))
+        engine = self._make_engine(flow)
+        engine.compile()
+        engine.prepare()
+        self.assertRaises(exc.MissingDependencies, engine.validate)
+        c_e = None
+        try:
+            engine.validate()
+        except exc.MissingDependencies as e:
+            c_e = e
+        self.assertIsNotNone(c_e)
+        self.assertIsNotNone(c_e.cause)
+
+
 class EngineCheckingTaskTest(utils.EngineTestBase):
     # FIXME: this test uses a inner class that workers/process engines can't
     # get to, so we need to do something better to make this test useful for
@@ -682,6 +702,7 @@ class SerialEngineTest(EngineTaskTest,
                        EngineLinearAndUnorderedExceptionsTest,
                        EngineOptionalRequirementsTest,
                        EngineGraphFlowTest,
+                       EngineMissingDepsTest,
                        EngineCheckingTaskTest,
                        test.TestCase):
     def _make_engine(self, flow,
@@ -707,6 +728,7 @@ class ParallelEngineWithThreadsTest(EngineTaskTest,
                                     EngineLinearAndUnorderedExceptionsTest,
                                     EngineOptionalRequirementsTest,
                                     EngineGraphFlowTest,
+                                    EngineMissingDepsTest,
                                     EngineCheckingTaskTest,
                                     test.TestCase):
     _EXECUTOR_WORKERS = 2
@@ -744,6 +766,7 @@ class ParallelEngineWithEventletTest(EngineTaskTest,
                                      EngineLinearAndUnorderedExceptionsTest,
                                      EngineOptionalRequirementsTest,
                                      EngineGraphFlowTest,
+                                     EngineMissingDepsTest,
                                      EngineCheckingTaskTest,
                                      test.TestCase):
 
@@ -764,6 +787,7 @@ class ParallelEngineWithProcessTest(EngineTaskTest,
                                     EngineLinearAndUnorderedExceptionsTest,
                                     EngineOptionalRequirementsTest,
                                     EngineGraphFlowTest,
+                                    EngineMissingDepsTest,
                                     test.TestCase):
     _EXECUTOR_WORKERS = 2
 
@@ -789,6 +813,7 @@ class WorkerBasedEngineTest(EngineTaskTest,
                             EngineLinearAndUnorderedExceptionsTest,
                             EngineOptionalRequirementsTest,
                             EngineGraphFlowTest,
+                            EngineMissingDepsTest,
                             test.TestCase):
     def setUp(self):
         super(WorkerBasedEngineTest, self).setUp()