[step_runner] Add a Placeholder Exception step when rendering placeholders raises a StepFailure.

recipe_engine/step_runner.py

 # Copyright 2016 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.
 
 import calendar
 import collections
 import contextlib
 import datetime
 import itertools
 import json
@@ skipping 154 matching lines @@
 
         def finalize(inner):
           step_stream.close()
 
         @property
         def stream(inner):
           return step_stream
 
       return EmptyOpenStep()
 
-    rendered_step = render_step(
-        step_config, recipe_test_api.DisabledTestData()
-    )
-    step_config = None # Make sure we use rendered step config.
+    try:
+      rendered_step = render_step(
+          step_config, recipe_test_api.DisabledTestData()
+      )
+      step_config = None  # Make sure we use rendered step config.
+    except:
+      with self.stream_engine.make_step_stream('Placeholder Exception') as s:
+        with s.new_log_stream('exception') as l:
+          l.write_split(traceback.format_exc())
+      raise
 
     step_env = _merge_envs(os.environ, (rendered_step.config.env or {}))
     # Now that the step's environment is all sorted, evaluate PATH on windows
     # to find the actual intended executable.
     rendered_step = _hunt_path(rendered_step, step_env)
     self._print_step(step_stream, rendered_step, step_env)
 
     class ReturnOpenStep(OpenStep):
       def run(inner):
         step_config = rendered_step.config
@@ skipping 214 matching lines @@
     self._test_data = test_data
     self._stream_engine = stream_engine
     self._annotator = annotator
     self._step_history = collections.OrderedDict()
 
   @property
   def stream_engine(self):
     return self._stream_engine
 
   def open_step(self, step_config):
-    test_data_fn = step_config.step_test_data or recipe_test_api.StepTestData
-    step_test = self._test_data.pop_step_test_data(step_config.name,
-                                                   test_data_fn)
-    rendered_step = render_step(step_config, step_test)
-    step_config = None # Make sure we use rendered step config.
+    try:
+      test_data_fn = step_config.step_test_data or recipe_test_api.StepTestData
+      step_test = self._test_data.pop_step_test_data(step_config.name,
+                                                     test_data_fn)
+      rendered_step = render_step(step_config, step_test)
+      step_config = None  # Make sure we use rendered step config.
+    except:
+      with self.stream_engine.make_step_stream('Placeholder Exception') as s:
+        with s.new_log_stream('exception') as l:
+          l.write_split(traceback.format_exc())
+      raise
 
     # Layer the simulation step on top of the given stream engine.
     step_stream = self._stream_engine.new_step_stream(rendered_step.config)
 
     class ReturnOpenStep(OpenStep):
       def run(inner):
         timeout = rendered_step.config.timeout
         if (timeout and step_test.times_out_after and
             step_test.times_out_after > timeout):
           raise recipe_api.StepTimeout(rendered_step.config.name, timeout)
@@ skipping 298 matching lines @@
   supplied command, and only uses the |env| kwarg for modifying the environment
   of the child process.
   """
   saved_path = os.environ['PATH']
   try:
     if path is not None:
       os.environ['PATH'] = path
     yield
   finally:
     os.environ['PATH'] = saved_path