coverage run fails with a re.error with Python 3.9a6+

[pquentin]

coverage run fails with a re.error in Python nightly

To Reproduce

1. Python version: 3.9.0a6+ (heads/master:85bdec1, May 10 2020, 05:50:51)
2. coverage 5.1
3. This failed in the CI build of urllib3 (https://github.com/urllib3/urllib3/blob/master/.travis.yml) which runs nox -s test-3.9
4. coverage run --parallel-mode -m pytest -r a --tb=native --no-success-flaky-report test/

Sorry, I did not try this outside of a failing CI job: https://travis-ci.org/github/urllib3/urllib3/jobs/685596004

By comparing the last known good build, I can tell that the problem has been introduced in one of those commits: python/cpython@f01d1be...85bdec1

Expected behavior

I would expect the tests to run.

Additional context

Instead, I get the following traceback:

Traceback (most recent call last):
  File "/home/travis/build/urllib3/urllib3/.nox/test-3-9/bin/coverage", line 8, in <module>
    sys.exit(main())
  File "/home/travis/build/urllib3/urllib3/.nox/test-3-9/lib/python3.9/site-packages/coverage/cmdline.py", line 827, in main
    status = CoverageScript().command_line(argv)
  File "/home/travis/build/urllib3/urllib3/.nox/test-3-9/lib/python3.9/site-packages/coverage/cmdline.py", line 555, in command_line
    return self.do_run(options, args)
  File "/home/travis/build/urllib3/urllib3/.nox/test-3-9/lib/python3.9/site-packages/coverage/cmdline.py", line 700, in do_run
    self.coverage.start()
  File "/home/travis/build/urllib3/urllib3/.nox/test-3-9/lib/python3.9/site-packages/coverage/control.py", line 525, in start
    self._init_for_start()
  File "/home/travis/build/urllib3/urllib3/.nox/test-3-9/lib/python3.9/site-packages/coverage/control.py", line 487, in _init_for_start
    self._inorout.configure(self.config)
  File "/home/travis/build/urllib3/urllib3/.nox/test-3-9/lib/python3.9/site-packages/coverage/inorout.py", line 192, in configure
    self.omit_match = FnmatchMatcher(self.omit)
  File "/home/travis/build/urllib3/urllib3/.nox/test-3-9/lib/python3.9/site-packages/coverage/files.py", line 273, in __init__
    self.re = fnmatches_to_regex(self.pats, case_insensitive=env.WINDOWS)
  File "/home/travis/build/urllib3/urllib3/.nox/test-3-9/li/python3.9/site-packages/coverage/files.py", line 325, in fnmatches_to_regex
    compiled = re.compile(join_regex(regexes), flags=flags)
  File "/opt/python/3.9-dev/lib/python3.9/re.py", line 252, in compile
    return _compile(pattern, flags)
  File "/opt/python/3.9-dev/lib/python3.9/re.py", line 304, in _compile
    p = sre_compile.compile(pattern, flags)
  File "/opt/python/3.9-dev/lib/python3.9/sre_compile.py", line 764, in compile
    p = sre_parse.parse(p, flags)
  File "/opt/python/3.9-dev/lib/python3.9/sre_parse.py", line 948, in parse
    p = _parse_sub(source, state, flags & SRE_FLAG_VERBOSE, 0)
  File "/opt/python/3.9-dev/lib/python3.9/sre_parse.py", line 443, in _parse_sub
    itemsappend(_parse(source, state, verbose, nested + 1,
  File "/opt/python/3.9-dev/lib/python3.9/sre_parse.py", line 834, in _parse
    p = _parse_sub(source, state, sub_verbose, nested + 1)
  File "/opt/python/3.9-dev/lib/python3.9/sre_parse.py", line 443, in _parse_sub
    itemsappend(_parse(source, state, verbose, nested + 1,
  File "/opt/python/3.9-dev/lib/python3.9/sre_parse.py", line 834, in _parse
    p = _parse_sub(source, state, sub_verbose, nested + 1)
  File "/opt/python/3.9-dev/lib/python3.9/sre_parse.py", line 443, in _parse_sub
    itemsappend(_parse(source, state, verbose, nested + 1,
  File "/opt/python/3.9-dev/lib/python3.9/sre_parse.py", line 754, in _parse
    p = _parse_sub(source, state, verbose, nested + 1)
  File "/opt/python/3.9-dev/lib/python3.9/sre_parse.py", line 443, in _parse_sub
    itemsappend(_parse(source, state, verbose, nested + 1,
  File "/opt/python/3.9-dev/lib/python3.9/sre_parse.py", line 831, in _parse
    raise source.error(err.msg, len(name) + 1) from None
re.error: redefinition of group name 'g1' as group 2; was group 1 at position 284

Since the traceback is fully in coverage.py, I expect that the details about urllib3 are irrelevant.

[nedbat]

Thanks, this was exotic.

1. https://bugs.python.org/issue40480 describes pathological behavior in fnmatch.
2. python/cpython@b9c46a2 fixes it, but now if the file pattern has more than one star, the regex has numbered group references in it.
   In 3.8:

>>> fnmatch.translate("*foo*")
'(?s:.*foo.*)\\Z'

In 3.9a6+:

>>> fnmatch.translate("*foo*")
'(?s:(?=(?P<g1>.*?foo))(?P=g1).*)\\Z'

3. Coverage.py combines multiple fnmatch-produced regexes together to make a larger regex. Now there are two groups named g1 in a single regex. Error.

This only showed up because urllib3's omit setting has two patterns with two asterisks: https://github.com/urllib3/urllib3/blob/master/.coveragerc#L6-L11

Now to figure out what to do about it.

[tim-one]

Ned, I believe this will fix it for you:

python/cpython#20049

Thanks for tracking it down! I'll push that change tonight provided the tests all pass.

[nedbat]

I can confirm that Tim's change does fix the problem.

[pquentin]

Thanks everyone!