Since Tuesday Oct 27 9AM UTC merge of autoland to m-c, it is no longer possible to run tests locally. Building works fine.

Steps I took:
1.

$ ./mach test --headless accessible/tests/mochitest/text/test_wordboundary.html > mochitest.log
  File "<string>", line 11
    return (f, path, (", r, imp.PY_SOURCE))
                                          ^
SyntaxError: EOL while scanning string literal
Sentry is attempting to send 0 pending error messages
Waiting up to 2 seconds
Press Ctrl-Break to quit
Error in atexit._run_exitfuncs:

After the "Press CTRL+C..." prompt, the system hangs forever.

After pressing CTRL+C, mochitest.log contains:

Test configuration changed. Regenerating backend.
Error running mach:

    ['test', '--headless', 'accessible/tests/mochitest/text/test_wordboundary.html']

The error occurred in code that was called by the mach command. This is either
a bug in the called code itself or in the way that mach is calling it.
You can invoke |./mach busted| to check if this issue is already on file. If it
isn't, please use |./mach busted file test| to report it. If |./mach busted| is
misbehaving, you can also inspect the dependencies of bug 1543241.

If filing a bug, please include the full output of mach, including this error
message.

The details of the failure are as follows:

BuildReaderError: 
==============================
FATAL ERROR PROCESSING MOZBUILD FILE
==============================

The error occurred while processing the following file:

    c:/devel/mozilla-central/security/moz.build

The error appears to be part of the mozbuild.frontend.reader Python module itself! It is possible you have stumbled across a legitimate bug.

Traceback (most recent call last):
  File "c:\devel\mozilla-central\python/mozbuild\mozbuild\frontend\reader.py", line 1090, in read_mozbuild
    path, config, descend=descend, metadata=metadata
  File "c:\devel\mozilla-central\python/mozbuild\mozbuild\frontend\reader.py", line 1208, in _read_mozbuild
    non_unified_sources,
  File "c:\devel\mozilla-central\python/mozbuild\mozbuild\frontend\gyp_reader.py", line 470, in __init__
    load_gyp, [path], "mozbuild", str_vars, includes, depth, params
  File "c:\devel\mozilla-central\third_party/python/futures\concurrent\futures\process.py", line 339, in submit
    self._adjust_process_count()
  File "c:\devel\mozilla-central\third_party/python/futures\concurrent\futures\process.py", line 321, in _adjust_process_count
    p.start()
  File "c:\mozilla-build\python\lib\multiprocessing\process.py", line 130, in start
    self._popen = Popen(self)
  File "c:\mozilla-build\python\lib\multiprocessing\forking.py", line 280, in __init__
    to_child.close()
IOError: [Errno 22] Invalid argument


  File "c:\devel\mozilla-central\testing/mach_commands.py", line 378, in test
    run_suites, run_tests = resolver.resolve_metadata(what)
  File "c:\devel\mozilla-central\testing/mozbase/moztest\moztest\resolve.py", line 984, in resolve_metadata
    tests = list(self.resolve_tests(paths=[relpath]))
  File "c:\devel\mozilla-central\testing/mozbase/moztest\moztest\resolve.py", line 949, in resolve_tests
    for test in self._resolve(**kwargs):
  File "c:\devel\mozilla-central\testing/mozbase/moztest\moztest\resolve.py", line 713, in _resolve
    if path in self.test_dirs or any(
  File "c:\devel\mozilla-central\testing/mozbase/moztest\moztest\resolve.py", line 625, in test_dirs
    for test in self.tests:
  File "c:\devel\mozilla-central\testing/mozbase/moztest\moztest\resolve.py", line 589, in tests
    for test in self.load_tests():
  File "c:\devel\mozilla-central\testing/mozbase/moztest\moztest\resolve.py", line 461, in __call__
    gen_test_backend()
  File "c:\devel\mozilla-central\python/mozbuild\mozbuild\gen_test_backend.py", line 50, in gen_test_backend
    backend.consume(data)
  File "c:\devel\mozilla-central\python/mozbuild\mozbuild\backend\base.py", line 130, in consume
    for obj in objs:
  File "c:\devel\mozilla-central\python/mozbuild\mozbuild\frontend\emitter.py", line 168, in emit
    for out in output:
  File "c:\devel\mozilla-central\python/mozbuild\mozbuild\frontend\reader.py", line 924, in read_topsrcdir
    for r in self.read_mozbuild(path, self.config):
  File "c:\devel\mozilla-central\python/mozbuild\mozbuild\frontend\reader.py", line 1095, in read_mozbuild
    raise bre

MozillaBuild is 3.3, I've bootstrapped again, I've clobbered again, nothing helps. Even the latest m-c checkout as of 5AM UTC this morning doesn't help.

Not even nuking the whole object directory and re-bootstrapping, making sure pip is up-to-date in the virtual env, etc., fixes this.