Actually, this looks like a failure of the Mash Login test in vm_sanity, not the Tast version. I can see a failure on TOT (which is hopefully the same one).

Thanks, that matches what I saw when I looked. Here's my diagnosis (which conflicted with your comment :-P):

---

Where's the actual failure? In http://stainless/browse/chromeos-autotest-results/236437598-chromeos-test/, the client.0.DEBUG file appears to show local_test_runner (the thing that runs Tast tests, including the one that was enabled by https://crrev.com/c/1211644) completing successfully:

09/10 16:53:58.083 DEBUG|             utils:0219| Running 'local_test_runner '(!informational && !disabled && ("dep:chrome" || "dep:chrome_login") && !"dep:android")''
09/10 16:53:58.083 DEBUG|      global_hooks:0056| 'local_test_runner \'(!informational && !disabled && ("dep:chrome" || "dep:chrome_login") && !"dep:android")\''
09/10 16:53:58.198 DEBUG|             utils:0287| [stdout] 2018/09/10 16:53:58 Running ui.ChromeCrashLoggedIn
...
09/10 16:54:29.435 DEBUG|             utils:0287| [stdout] 2018/09/10 16:54:29 Finished ui.ChromeRespawn
09/10 16:54:29.436 DEBUG|             utils:0287| [stdout] 2018/09/10 16:54:29 --------------------------------------------------------------------------------
09/10 16:54:29.436 DEBUG|             utils:0287| [stdout] 2018/09/10 16:54:29 Running ui.MashLogin
09/10 16:54:29.436 DEBUG|             utils:0287| [stdout] 2018/09/10 16:54:29 Restarting ui job
...
09/10 16:54:38.460 DEBUG|             utils:0287| [stdout] 2018/09/10 16:54:38 Finished ui.MashLogin
...
09/10 16:54:49.950 DEBUG|             utils:0287| [stdout] 2018/09/10 16:54:49 Finished ui.SessionManagerRespawn
09/10 16:54:49.950 DEBUG|             utils:0287| [stdout] 2018/09/10 16:54:49 --------------------------------------------------------------------------------
09/10 16:54:49.953 DEBUG|             utils:0287| [stdout] 2018/09/10 16:54:49 Ran 6 test(s) in 51.751s

The stack traces that I see near the end of the log are in Python code:

Traceback (most recent call last):
  File "/usr/local/autotest/common_lib/test.py", line 800, in _call_test_function
    return func(*args, **dargs)
  File "/usr/local/autotest/common_lib/test.py", line 464, in execute
    postprocess_profiled_run, args, dargs)
  File "/usr/local/autotest/common_lib/test.py", line 371, in _call_run_once
    self.run_once(*args, **dargs)
  File "/usr/local/autotest/tests/login_VMSanity/login_VMSanity.py", line 13, in run_once
    vm_sanity.Sanity()
  File "/usr/local/autotest/bin/vm_sanity.py", line 84, in Sanity
    with chrome.Chrome(extra_browser_args=mash_browser_args):
  File "/usr/local/autotest/common_lib/cros/chrome.py", line 179, in __init__
    self._browser = self._browser_to_create.Create()
  File "/usr/local/telemetry/src/third_party/catapult/telemetry/telemetry/internal/backends/chrome/cros_browser_finder.py", line 103, in Create
    browser_backend, self._platform_backend, startup_args)
  File "/usr/local/telemetry/src/third_party/catapult/telemetry/telemetry/internal/backends/chrome/cros_browser_with_oobe.py", line 14, in __init__
    backend, platform_backend, startup_args)
  File "/usr/local/telemetry/src/third_party/catapult/telemetry/telemetry/internal/browser/browser.py", line 49, in __init__
    self._LogBrowserInfo()
  File "/usr/local/telemetry/src/third_party/catapult/telemetry/telemetry/internal/browser/browser.py", line 107, in _LogBrowserInfo
    system_info = self.GetSystemInfo()
  File "/usr/local/telemetry/src/third_party/catapult/telemetry/telemetry/internal/browser/browser.py", line 276, in GetSystemInfo
    return self._browser_backend.GetSystemInfo()
  File "/usr/local/telemetry/src/third_party/catapult/common/py_trace_event/py_trace_event/trace_event_impl/decorators.py", line 52, in traced_function
    return func(*args, **kwargs)
  File "/usr/local/telemetry/src/third_party/catapult/telemetry/telemetry/internal/backends/chrome/chrome_browser_backend.py", line 239, in GetSystemInfo
    raise exceptions.BrowserGoneException(self.browser, e)
BrowserGoneException: WebsocketException of type <class 'websocket._exceptions.WebSocketConnectionClosedException'>. Error message: Connection is already closed.

Same stack as 882073 I believe:
https://storage.cloud.google.com/chromeos-autotest-results/236437598-chromeos-test/chromeos4-row2-rack4-host14/login_VMSanity/sysinfo/iteration.1/var/spool/crash/chrome.20180910.165458.21357.dmp.txt