Safari "stable" consistently crashing on new pointerevents test

[jugglinmike]

As of this writing, this project designates Safari 12.0.1 running on macOS High Sierra as the "stable" release of that browser. Since 2019-06-20, it has been consistently failing on the same segment of WPT--segment number 5 of 20. When executing the test /pointerevents/pointerlock/pointerevent_movementxy.html (which was introduced on the date of the first
failure), the browser produces the following error:

WebDriverException: not implemented (501): Input sources with a pointerType of 'touch' are not currently supported.

...and the remaining tests time out.

Although this system automatically retries collection attempts which omit results, both such attempts fail immediately:

  File "/Users/kazooie/worker/macOS_Chunked_Runner/build/tools/wptrunner/wptrunner/executors/protocol.py", line 48, in setup
    self.connect()
  File "/Users/kazooie/worker/macOS_Chunked_Runner/build/tools/wptrunner/wptrunner/executors/executorwebdriver.py", line 225, in connect
    self.webdriver.start()
  File "/Users/kazooie/worker/macOS_Chunked_Runner/build/tools/webdriver/webdriver/client.py", line 421, in start
    value = self.send_command("POST", "session", body=body)
  File "/Users/kazooie/worker/macOS_Chunked_Runner/build/tools/webdriver/webdriver/client.py", line 463, in send_command
    session=self)
  File "/Users/kazooie/worker/macOS_Chunked_Runner/build/tools/webdriver/webdriver/transport.py", line 175, in send
    response = self._request(method, uri, payload, headers)
  File "/Users/kazooie/worker/macOS_Chunked_Runner/build/tools/webdriver/webdriver/transport.py", line 191, in _request
    return self.connection.getresponse()
  File "/System/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/httplib.py", line 1132, in getresponse
    response.begin()
  File "/System/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/httplib.py", line 453, in begin
    version, status, reason = self._read_status()
  File "/System/Library/Frameworks/Python.framework/Versions/2.7/lib/python2.7/httplib.py", line 417, in _read_status
    raise BadStatusLine(line)
BadStatusLine: ''

Normal operation is only restored when the system moves on to another segment of WPT.

This is apparently a bug in Safaridriver, but we haven't yet determined how to work around it. Given the regularity of the failure, we do not expect to upload further results from Safari "stable" until the problem is resolved.

Safari Technology Preview (version 82, also running on High Sierra) has not been affected by this bug.

[jugglinmike]

Reported in WPT since it also appears to affect Safari as running in that project's Azure Pipelines environment: web-platform-tests/wpt#17567