Track step nesting in StreamEngine (Reland).

recipes/example/nested.py

Index: recipes/example/nested.py
diff --git a/recipes/example/nested.py b/recipes/example/nested.py
index d9e3671c38a8736024fb1154b9596d4fbba93c7e..c16844aca179c895b31e80ea776565d5b358d1f4 100644
--- a/recipes/example/nested.py
+++ b/recipes/example/nested.py
@@ -12,7 +12,7 @@ def RunSteps(api):
   # Nest all steps below this.
   with api.step.nest('complicated thing'):
     with api.step.nest('first part'):
-      api.step('wait a bit', ['sleep', '10'])
+      api.step('wait a bit', ['sleep', '1'])
 
     # Prefix the name without indenting.
     with api.step.context({'name': 'attempt number'}):
@@ -20,7 +20,21 @@ def RunSteps(api):
       assert step_result.step['name'] == 'complicated thing.attempt number.one'
       api.step('two', ['echo', 'derpy'])
 
-  api.step('simple thing', ['sleep', '10'])
+  # Outer nested step's status should not inherit from inner.
+  with api.step.nest('inherit status') as nest_step:
+    with api.step.nest('inner step') as other_nest_step:
+      other_nest_step.presentation.status = api.step.EXCEPTION
+  assert nest_step.presentation.status == api.step.SUCCESS
+
+  # Change outer status after nesting is complete.
+  with api.step.nest('versatile status') as nest_step:
+    with api.step.nest('inner step'):
+      with api.step.nest('even deeper'):
+        pass
+    nest_step.presentation.status = api.step.FAILURE
+  assert nest_step.presentation.status == api.step.FAILURE
+
+  api.step('simple thing', ['sleep', '1'])
 
 
 def GenTests(api):