hgdemandimport: bypass demandimport for _ast module (issue6407)

This is broken on Python 3.9rc1, and while it sounds like there may be
a fix in Python, we probably also should have this workaround in place
in hg. See the bug for more details (including on bugs at redhat and
b.p.o).

Differential Revision: https://phab.mercurial-scm.org/D9004

hgdemandimport/__init__.py

@@ -1,82 +1,83
 # hgdemandimport - global demand-loading of modules for Mercurial
 #
 # Copyright 2017 Facebook Inc.
 #
 # This software may be used and distributed according to the terms of the
 # GNU General Public License version 2 or any later version.
 
 '''demandimport - automatic demand-loading of modules'''
 
 # This is in a separate package from mercurial because in Python 3,
 # demand loading is per-package. Keeping demandimport in the mercurial package
 # would disable demand loading for any modules in mercurial.
 
 from __future__ import absolute_import
 
 import os
 import sys
 
 if sys.version_info[0] >= 3:
     from . import demandimportpy3 as demandimport
 else:
     from . import demandimportpy2 as demandimport
 
 # Full module names which can't be lazy imported.
 # Extensions can add to this set.
 IGNORES = {
     '__future__',
     '_hashlib',
     # ImportError during pkg_resources/__init__.py:fixup_namespace_package
     '_imp',
     '_xmlplus',
     'fcntl',
     'nt',  # pathlib2 tests the existence of built-in 'nt' module
     'win32com.gen_py',
     'win32com.shell',  # 'appdirs' tries to import win32com.shell
     '_winreg',  # 2.7 mimetypes needs immediate ImportError
     'pythoncom',
     # imported by tarfile, not available under Windows
     'pwd',
     'grp',
     # imported by profile, itself imported by hotshot.stats,
     # not available under Windows
     'resource',
     # this trips up many extension authors
     'gtk',
     # setuptools' pkg_resources.py expects "from __main__ import x" to
     # raise ImportError if x not defined
     '__main__',
+    '_ast',  # https://bugs.python.org/issue41631
     '_ssl',  # conditional imports in the stdlib, issue1964
     '_sre',  # issue4920
     'rfc822',
     'mimetools',
     'sqlalchemy.events',  # has import-time side effects (issue5085)
     # setuptools 8 expects this module to explode early when not on windows
     'distutils.msvc9compiler',
     '__builtin__',
     'builtins',
     'urwid.command_map',  # for pudb
     'lzma',
 }
 
 _pypy = '__pypy__' in sys.builtin_module_names
 
 if _pypy:
     # _ctypes.pointer is shadowed by "from ... import pointer" (PyPy 5)
     IGNORES.add('_ctypes.pointer')
 
 demandimport.init(IGNORES)
 
 # Re-export.
 isenabled = demandimport.isenabled
 disable = demandimport.disable
 deactivated = demandimport.deactivated
 
 
 def enable():
     # chg pre-imports modules so do not enable demandimport for it
     if (
         'CHGINTERNALMARK' not in os.environ
         and os.environ.get('HGDEMANDIMPORT') != 'disable'
     ):
         demandimport.enable()