Add add_json_log='on_failure' to json module.

README.recipes.md

 <!--- AUTOGENERATED BY `./recipes.py test train` -->
 # Package documentation for [recipe\_engine](https://github.com/luci/recipes-py)
 ## Table of Contents
 
 **[Recipe Modules](#recipe-modules)**
   * [context](#recipe_modules--context) &mdash; The context module provides APIs for manipulating a few pieces of 'ambient' data that affect how steps are run.
   * [file](#recipe_modules--file) &mdash; File manipulation (read/write/delete/glob) methods.
   * [generator_script](#recipe_modules--generator_script) &mdash; A simplistic method for running steps generated by an external script.
   * [json](#recipe_modules--json) &mdash; Methods for producing and consuming JSON.
   * [path](#recipe_modules--path) &mdash; All functions related to manipulating paths in recipes.
@@ skipping 31 matching lines @@
   * [engine_tests/step_stack_exhaustion](#recipes--engine_testsstep_stack_exhaustion) &mdash; Tests that placeholders can't wreck the world by exhausting the step stack.
   * [engine_tests/undeclared_method](#recipes--engine_testsundeclared_method)
   * [engine_tests/unicode](#recipes--engine_testsunicode)
   * [engine_tests/whitelist_steps](#recipes--engine_testswhitelist_steps) &mdash; Tests that step_data can accept multiple specs at once.
   * [file:examples/copy](#recipes--fileexamplescopy)
   * [file:examples/copytree](#recipes--fileexamplescopytree)
   * [file:examples/error](#recipes--fileexampleserror)
   * [file:examples/glob](#recipes--fileexamplesglob)
   * [generator_script:examples/full](#recipes--generator_scriptexamplesfull)
   * [json:examples/full](#recipes--jsonexamplesfull)
+  * [json:tests/add_json_log](#recipes--jsontestsadd_json_log)
   * [path:examples/full](#recipes--pathexamplesfull)
   * [platform:examples/full](#recipes--platformexamplesfull)
   * [properties:examples/full](#recipes--propertiesexamplesfull)
   * [python:examples/full](#recipes--pythonexamplesfull) &mdash; Launches the repo bundler.
   * [raw_io:examples/full](#recipes--raw_ioexamplesfull)
   * [step:examples/full](#recipes--stepexamplesfull)
   * [step:tests/active_result](#recipes--steptestsactive_result)
   * [step:tests/defer](#recipes--steptestsdefer)
   * [step:tests/inject_paths](#recipes--steptestsinject_paths)
   * [step:tests/nested](#recipes--steptestsnested)
@@ skipping 377 matching lines @@
         read them back from inside of the recipes.
   * allow_subannotations: allow this step to emit legacy buildbot
     subannotations. If you don't know what this is, you shouldn't use it. If
     you know what it is, you also shouldn't use it.
 ### *recipe_modules* / [json](/recipe_modules/json)
 
 [DEPS](/recipe_modules/json/__init__.py#L5): [properties](#recipe_modules--properties), [python](#recipe_modules--python), [raw\_io](#recipe_modules--raw_io)
 
 Methods for producing and consuming JSON.
 
-#### **class [JsonApi](/recipe_modules/json/api.py#L80)([RecipeApi](/recipe_engine/recipe_api.py#L884)):**
+#### **class [JsonApi](/recipe_modules/json/api.py#L83)([RecipeApi](/recipe_engine/recipe_api.py#L884)):**
 
-&emsp; **@[returns\_placeholder](/recipe_engine/util.py#L119)**<br>&mdash; **def [input](/recipe_modules/json/api.py#L99)(self, data):**
+&emsp; **@[returns\_placeholder](/recipe_engine/util.py#L119)**<br>&mdash; **def [input](/recipe_modules/json/api.py#L102)(self, data):**
 
 A placeholder which will expand to a file path containing <data>.
 
-&mdash; **def [is\_serializable](/recipe_modules/json/api.py#L91)(self, obj):**
+&mdash; **def [is\_serializable](/recipe_modules/json/api.py#L94)(self, obj):**
 
 Returns True if the object is JSON-serializable.
 
-&emsp; **@staticmethod**<br>&mdash; **def [loads](/recipe_modules/json/api.py#L85)(data, \*\*kwargs):**
+&emsp; **@staticmethod**<br>&mdash; **def [loads](/recipe_modules/json/api.py#L88)(data, \*\*kwargs):**
 
 Works like `json.loads`, but strips out unicode objects (replacing them
 with utf8-encoded str objects).
 
-&emsp; **@[returns\_placeholder](/recipe_engine/util.py#L119)**<br>&mdash; **def [output](/recipe_modules/json/api.py#L104)(self, add_json_log=True, name=None, leak_to=None):**
+&emsp; **@[returns\_placeholder](/recipe_engine/util.py#L119)**<br>&mdash; **def [output](/recipe_modules/json/api.py#L107)(self, add_json_log=True, name=None, leak_to=None):**
 
 A placeholder which will expand to '/tmp/file'.
 
 If leak_to is provided, it must be a Path object. This path will be used in
 place of a random temporary file, and the file will not be deleted at the
 end of the step.
 
-&mdash; **def [read](/recipe_modules/json/api.py#L115)(self, name, path, add_json_log=True, output_name=None, \*\*kwargs):**
+Args:
+  * add_json_log (True|False|'on_failure') - Log a copy of the output json
+    to a step link named `name`. If this is 'on_failure', only create this
+    log when the step has a non-SUCCESS status.
+
+&mdash; **def [read](/recipe_modules/json/api.py#L123)(self, name, path, add_json_log=True, output_name=None, \*\*kwargs):**
 
 Returns a step that reads a JSON file.
 ### *recipe_modules* / [path](/recipe_modules/path)
 
 [DEPS](/recipe_modules/path/__init__.py#L5): [platform](#recipe_modules--platform), [properties](#recipe_modules--properties)
 
 All functions related to manipulating paths in recipes.
 
 Recipes handle paths a bit differently than python does. All path manipulation
 in recipes revolves around Path objects. These objects store a base path (always
@@ skipping 746 matching lines @@
 ### *recipes* / [generator\_script:examples/full](/recipe_modules/generator_script/examples/full.py)
 
 [DEPS](/recipe_modules/generator_script/examples/full.py#L7): [generator\_script](#recipe_modules--generator_script), [json](#recipe_modules--json), [path](#recipe_modules--path), [properties](#recipe_modules--properties), [step](#recipe_modules--step)
 
 &mdash; **def [RunSteps](/recipe_modules/generator_script/examples/full.py#L19)(api, script_name):**
 ### *recipes* / [json:examples/full](/recipe_modules/json/examples/full.py)
 
 [DEPS](/recipe_modules/json/examples/full.py#L5): [json](#recipe_modules--json), [path](#recipe_modules--path), [properties](#recipe_modules--properties), [python](#recipe_modules--python), [raw\_io](#recipe_modules--raw_io), [step](#recipe_modules--step)
 
 &mdash; **def [RunSteps](/recipe_modules/json/examples/full.py#L16)(api):**
+### *recipes* / [json:tests/add\_json\_log](/recipe_modules/json/tests/add_json_log.py)
+
+[DEPS](/recipe_modules/json/tests/add_json_log.py#L5): [json](#recipe_modules--json), [step](#recipe_modules--step)
+
+&mdash; **def [RunSteps](/recipe_modules/json/tests/add_json_log.py#L10)(api):**
 ### *recipes* / [path:examples/full](/recipe_modules/path/examples/full.py)
 
 [DEPS](/recipe_modules/path/examples/full.py#L5): [path](#recipe_modules--path), [platform](#recipe_modules--platform), [properties](#recipe_modules--properties), [step](#recipe_modules--step)
 
 &mdash; **def [RunSteps](/recipe_modules/path/examples/full.py#L14)(api):**
 ### *recipes* / [platform:examples/full](/recipe_modules/platform/examples/full.py)
 
 [DEPS](/recipe_modules/platform/examples/full.py#L5): [platform](#recipe_modules--platform), [step](#recipe_modules--step)
 
 &mdash; **def [RunSteps](/recipe_modules/platform/examples/full.py#L10)(api):**
@@ skipping 78 matching lines @@
 
 [DEPS](/recipe_modules/url/tests/join.py#L5): [step](#recipe_modules--step), [url](#recipe_modules--url)
 
 &mdash; **def [RunSteps](/recipe_modules/url/tests/join.py#L11)(api):**
 ### *recipes* / [url:tests/validate\_url](/recipe_modules/url/tests/validate_url.py)
 
 [DEPS](/recipe_modules/url/tests/validate_url.py#L5): [properties](#recipe_modules--properties), [step](#recipe_modules--step), [url](#recipe_modules--url)
 
 &mdash; **def [RunSteps](/recipe_modules/url/tests/validate_url.py#L12)(api):**