This is a complicated bug :(

It looks like the try block here (https://chromium.googlesource.com/chromium/tools/build.git/+/master/scripts/slave/recipe_modules/chromium_tests/steps.py#1133) is swallowing an exception. The exception is this:

  File "/s/build/scripts/slave/recipe_modules/chromium_tests/steps.py", line 1180, in _output_chartjson_results_if_present
    args)
  File "/s/build/scripts/slave/.recipe_deps/recipe_engine/recipe_engine/recipe_api.py", line 507, in _inner
    return func(*a, **kw)
  File "/s/build/scripts/slave/.recipe_deps/recipe_engine/recipe_modules/python/api.py", line 23, in __call__
    return self.m.step(name, cmd + list(args or []), **kwargs)
  File "/s/build/scripts/slave/.recipe_deps/recipe_engine/recipe_modules/step/api.py", line 170, in __call__
    schema.set_val(kwargs)
  File "/s/build/scripts/slave/.recipe_deps/recipe_engine/recipe_engine/config.py", line 401, in set_val
    raise type(e)('While assigning key %r: %s' % (name, e))
TypeError: While assigning key 'cmd': Expected None to be of type (<type 'int'>, <type 'long'>, <type 'basestring'>, <class 'recipe_engine.config_types.Path'>, <class 'recipe_engine.util.Placeholder'>)


This is caused because you're passing in a None to the recipe engine, which it doesn't know how to interpret. The arguments to the python call are as follows:

['--chartjson-results-file', <RECIPE_MODULES.recipe_engine.raw_io.api.InputDataPlaceholder object at 0x7f5a4ca68a90>, '--perf-id', None, '--results-url', None, '--name', 'telemetry_gpu_unittests', '--buildername', 'Linux Tests', '--buildnumber', '1234']

(The InputDataPlaceholder is different in the actual code, but is effectively the same)

The perf id and results url are not set correctly. This is because the bot is "Linux Tests" on chromium.linux, which doesn't have a perf id.

I tried changing the bot id to be correct, and I got this error.

  File "/s/build/scripts/slave/recipe_modules/chromium_tests/api.py", line 831, in main_waterfall_steps
    bot_config)
  File "/s/build/scripts/slave/.recipe_deps/recipe_engine/recipe_engine/recipe_api.py", line 507, in _inner
    return func(*a, **kw)
  File "/s/build/scripts/slave/recipe_modules/chromium_tests/api.py", line 208, in prepare_checkout
    bot_config, root_solution_revision)
  File "/s/build/scripts/slave/.recipe_deps/recipe_engine/recipe_engine/recipe_api.py", line 507, in _inner
    return func(*a, **kw)
  File "/s/build/scripts/slave/recipe_modules/chromium_checkout/api.py", line 73, in ensure_checkout
    **kwargs)
  File "/s/build/scripts/slave/.recipe_deps/recipe_engine/recipe_engine/recipe_api.py", line 507, in _inner
    return func(*a, **kw)
  File "/s/build/scripts/slave/.recipe_deps/depot_tools/recipe_modules/bot_update/api.py", line 103, in ensure_checkout
    self.m.properties.get('patch_project'), cfg)
  File "/s/build/scripts/slave/.recipe_deps/recipe_engine/recipe_engine/recipe_api.py", line 507, in _inner
    return func(*a, **kw)
  File "/s/build/scripts/slave/.recipe_deps/depot_tools/recipe_modules/gclient/api.py", line 331, in calculate_patch_root
    return cfg.solutions[0].name
  File "/s/build/scripts/slave/.recipe_deps/recipe_engine/recipe_engine/config.py", line 483, in __getitem__
    return self.data.__getitem__(index)
IndexError: list index out of range
Uncaught exception (IndexError): list index out of range


Which I don't really understand, and looks like a bad misconfiguration or default somewhere.

This makes me sad :(