Add add_json_log='on_failure' to json module.

recipe_modules/json/tests/add_json_log.py

+# Copyright 2017 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 = [
+  'json',
+  'step',
+]
+
+def RunSteps(api):
+  example_dict = {'x': 1, 'y': 2}
+
+  # not add a log for success
+  step_result = api.step('no log on success',
+    ['cat', '{"x":1,"y":2}'],
+    stdout=api.json.output(add_json_log='on_failure', name='log1'),
+  )
+  assert step_result.stdout == example_dict
+  assert 'json.output[log1]' not in step_result.presentation.logs
+
+  # add a log for failure
+  try:
+    api.step('add log on failure',
+      ['cat', '{"x":1,"y":2}'],
+      stdout=api.json.output(add_json_log='on_failure', name='log2'),
+    )
+  except api.step.StepFailure:
+    pass # This step is expected to fail.
+  finally:
+    step_result = api.step.active_result
+    assert step_result.stdout == example_dict
+    assert 'json.output[log2]' in step_result.presentation.logs
+    actual_log_dict = api.json.loads(
+        '\n'.join(step_result.presentation.logs['json.output[log2]']))
+    assert actual_log_dict == example_dict
+
+
+def GenTests(api):
+  yield (
+    api.test('add_json_log')
+    + api.step_data(
+      'no log on success',
+      stdout=api.json.output({'x': 1, 'y': 2}, name='log1'),
+    )
+    + api.step_data(
+      'add log on failure',
+      stdout=api.json.output({'x': 1, 'y': 2}, name='log2'),
+      retcode=1,
+    )
+  )