Track step nesting in StreamEngine (Reland).

recipes/example/nested.py

 # Copyright 2015 The LUCI Authors. All rights reserved.
 # Use of this source code is governed under the Apache License, Version 2.0
 # that can be found in the LICENSE file.
 
 
 DEPS = [
   'step',
 ]
 
 
 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'}):
       step_result = api.step('one', ['echo', 'herpy'])
       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):
   yield api.test('basic')