Over to @jgraham -- looks like this needs some code changes inside wptrunner. WDYT?

I strongly suspect it actually depends on getting an updated version of some other mozbase package; we are running against the master rev in gecko infra so this kind of thing shouldn't require changes in our code.

So if I'm reading right, the main error is:

ERROR Traceback (most recent call last):
  File "/home/test/web-platform-tests/tools/wptrunner/wptrunner/testrunner.py", line 222, in init
    self.browser.start(group_metadata=group_metadata, **self.browser_settings)
  File "/home/test/web-platform-tests/tools/wptrunner/wptrunner/browsers/chrome.py", line 123, in start
    self.server.start(block=False)
  File "/home/test/web-platform-tests/tools/wptrunner/wptrunner/webdriver_server.py", line 51, in start
    self._run(block)
  File "/home/test/web-platform-tests/tools/wptrunner/wptrunner/webdriver_server.py", line 61, in _run
    storeOutput=False)
  File "/home/test/web-platform-tests/_venv3/lib/python3.6/site-packages/mozprocess/processhandler.py", line 1238, in __init__
    stdout = sys.stdout.buffer
AttributeError: 'LoggingWrapper' object has no attribute 'buffer'

sys.stdout is overwritten (!!) in mozlog/capture.py inside any CaptureIO scope, to be a LoggingWrapper, which is an io.BytesIO subclass with little extra stuff as far as I can tell. Now, as far as I know, io.BytesIO doesn't have a buffer attribute, so that's a bit confusing.

It's a Python 3 issue; Python 3 gives sys.stdout a buffer attribute which makes wrapping it excitingly error-prone since it doesn't behave like a normal file. I've fixed the bug in mozilla-central but a new mozlog release is needed. Maybe tomorrow, or if you're reading this and it's later than tomorrow please ping me to get it done.

I've fixed the bug in mozilla-central but a new mozlog release is needed.

Awesome, thanks jgraham! Be assured I will chase it up ;)

mozlog has been updated to 6.1, retrying this.

Community-TC now passes (woo!). Azure Pipeline failures are various existing issues I believe (I'm currently working on investigating those elsewhere), so merging.