Filed by: nbeleuzu [at] mozilla.com
Parsed log: https://treeherder.mozilla.org/logviewer.html#?job_id=313428154&repo=mozilla-central
Full log: https://firefox-ci-tc.services.mozilla.com/api/queue/v1/task/Q08raE9dToOJSMZ6FMVA4A/runs/0/artifacts/public/logs/live_backing.log

[task 2020-08-19T11:17:39.210Z] 11:17:39 INFO - STDOUT: self = <Session (disconnected)>, method = 'POST', url = 'session'
[task 2020-08-19T11:17:39.211Z] 11:17:39 INFO - STDOUT: body = {'capabilities': {'alwaysMatch': {'moz:firefoxOptions': {'binary': '/builds/worker/workspace/build/application/firefox...test,xn--lve-6lad.www2.not-web-platform.test', 'network.process.enabled': False, 'toolkit.asyncshutdown.log': True}}}}}
[task 2020-08-19T11:17:39.212Z] 11:17:39 INFO - STDOUT: timeout = None
[task 2020-08-19T11:17:39.212Z] 11:17:39 INFO - STDOUT: def send_command(self, method, url, body=None, timeout=None):
[task 2020-08-19T11:17:39.212Z] 11:17:39 INFO - STDOUT: """
[task 2020-08-19T11:17:39.212Z] 11:17:39 INFO - STDOUT: Send a command to the remote end and validate its success.
[task 2020-08-19T11:17:39.214Z] 11:17:39 INFO - STDOUT:
[task 2020-08-19T11:17:39.214Z] 11:17:39 INFO - STDOUT: :param method: HTTP method to use in request.
[task 2020-08-19T11:17:39.214Z] 11:17:39 INFO - STDOUT: :param uri: "Command part" of the HTTP request URL,
[task 2020-08-19T11:17:39.214Z] 11:17:39 INFO - STDOUT: e.g. window/rect.
[task 2020-08-19T11:17:39.214Z] 11:17:39 INFO - STDOUT: :param body: Optional body of the HTTP request.
[task 2020-08-19T11:17:39.215Z] 11:17:39 INFO - STDOUT:
[task 2020-08-19T11:17:39.216Z] 11:17:39 INFO - STDOUT: :return: None if the HTTP response body was empty, otherwise
[task 2020-08-19T11:17:39.216Z] 11:17:39 INFO - STDOUT: the value field returned after parsing the response
[task 2020-08-19T11:17:39.217Z] 11:17:39 INFO - STDOUT: body as JSON.
[task 2020-08-19T11:17:39.217Z] 11:17:39 INFO - STDOUT:
[task 2020-08-19T11:17:39.217Z] 11:17:39 INFO - STDOUT: :raises error.WebDriverException: If the remote end returns
[task 2020-08-19T11:17:39.218Z] 11:17:39 INFO - STDOUT: an error.
[task 2020-08-19T11:17:39.219Z] 11:17:39 INFO - STDOUT: :raises ValueError: If the response body does not contain a
[task 2020-08-19T11:17:39.219Z] 11:17:39 INFO - STDOUT: value key.
[task 2020-08-19T11:17:39.219Z] 11:17:39 INFO - STDOUT: """
[task 2020-08-19T11:17:39.219Z] 11:17:39 INFO - STDOUT:
[task 2020-08-19T11:17:39.220Z] 11:17:39 INFO - STDOUT: response = self.transport.send(
[task 2020-08-19T11:17:39.220Z] 11:17:39 INFO - STDOUT: method, url, body,
[task 2020-08-19T11:17:39.221Z] 11:17:39 INFO - STDOUT: encoder=protocol.Encoder, decoder=protocol.Decoder,
[task 2020-08-19T11:17:39.221Z] 11:17:39 INFO - STDOUT: session=self, timeout=timeout)
[task 2020-08-19T11:17:39.222Z] 11:17:39 INFO - STDOUT:
[task 2020-08-19T11:17:39.222Z] 11:17:39 INFO - STDOUT: if response.status != 200:
[task 2020-08-19T11:17:39.222Z] 11:17:39 INFO - STDOUT: err = error.from_response(response)
[task 2020-08-19T11:17:39.223Z] 11:17:39 INFO - STDOUT:
[task 2020-08-19T11:17:39.224Z] 11:17:39 INFO - STDOUT: if isinstance(err, error.InvalidSessionIdException):
[task 2020-08-19T11:17:39.224Z] 11:17:39 INFO - STDOUT: # The driver could have already been deleted the session.
[task 2020-08-19T11:17:39.224Z] 11:17:39 INFO - STDOUT: self.session_id = None
[task 2020-08-19T11:17:39.224Z] 11:17:39 INFO - STDOUT:
[task 2020-08-19T11:17:39.225Z] 11:17:39 INFO - STDOUT: > raise err
[task 2020-08-19T11:17:39.226Z] 11:17:39 INFO - STDOUT: E UnknownErrorException: unknown error (500): invalid argument: can't kill an exited process
[task 2020-08-19T11:17:39.226Z] 11:17:39 INFO - STDOUT: body = {'capabilities': {'alwaysMatch': {'moz:firefoxOptions': {'binary': '/builds/worker/workspace/build/application/firefox...test,xn--lve-6lad.www2.not-web-platform.test', 'network.process.enabled': False, 'toolkit.asyncshutdown.log': True}}}}}
[task 2020-08-19T11:17:39.227Z] 11:17:39 INFO - STDOUT: err = <UnknownErrorException http_status=500>
[task 2020-08-19T11:17:39.227Z] 11:17:39 INFO - STDOUT: method = 'POST'
[task 2020-08-19T11:17:39.227Z] 11:17:39 INFO - STDOUT: response = <Response status=500 error=<UnknownErrorException http_status=500>>
[task 2020-08-19T11:17:39.228Z] 11:17:39 INFO - STDOUT: self = <Session (disconnected)>
[task 2020-08-19T11:17:39.228Z] 11:17:39 INFO - STDOUT: timeout = None
[task 2020-08-19T11:17:39.229Z] 11:17:39 INFO - STDOUT: url = 'session'
[task 2020-08-19T11:17:39.230Z] 11:17:39 INFO - STDOUT: tests/web-platform/tests/tools/webdriver/webdriver/client.py
[task 2020-08-19T11:17:39.230Z] 11:17:39 INFO - STDOUT: :487: UnknownErrorException
[task 2020-08-19T11:17:39.230Z] 11:17:39 INFO - STDOUT: =============================== warnings summary ===============================
[task 2020-08-19T11:17:39.232Z] 11:17:39 INFO - STDOUT: <undetermined location>
[task 2020-08-19T11:17:39.232Z] 11:17:39 INFO - STDOUT: Module already imported so cannot be rewritten: mozlog
[task 2020-08-19T11:17:39.232Z] 11:17:39 INFO - STDOUT: -- Docs: http://doc.pytest.org/en/latest/warnings.html
[task 2020-08-19T11:17:39.233Z] 11:17:39 INFO - STDOUT: ===================== 1 warnings, 2 error in 0.31 seconds ======================
[task 2020-08-19T11:17:39.236Z] 11:17:39 INFO -
[task 2020-08-19T11:17:39.237Z] 11:17:39 INFO - TEST-UNEXPECTED-ERROR | /webdriver/tests/element_send_keys/content_editable.py | test_sets_insertion_point_to_end - setup error

The connection attempt gets aborted within a second. See bug 1575021.

This should have been bug 1659965 which is fixed now.