demandimport: add _imp to ignore list

Mozilla is seeing an issue with demand importing of _imp
failing in pkg_resources/__init__.py:fixup_namespace_packages.
It strangely only reproduces when using a modern version of
setuptools/pip in certain scenarios. Adding _imp to the demand import
ignore list seems to make the problem go away.

mercurial/demandimport.py

@@ -1,280 +1,282 b''
 # demandimport.py - global demand-loading of modules for Mercurial
 #
 # Copyright 2006, 2007 Matt Mackall <[e-mail]>
 #
 # 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 demandloading of modules
 
 To enable this module, do:
 
   import demandimport; demandimport.enable()
 
 Imports of the following forms will be demand-loaded:
 
   import a, b.c
   import a.b as c
   from a import b,c # a will be loaded immediately
 
 These imports will not be delayed:
 
   from a import *
   b = __import__(a)
 '''
 
 from __future__ import absolute_import
 
 import contextlib
 import os
 import sys
 
 # __builtin__ in Python 2, builtins in Python 3.
 try:
     import __builtin__ as builtins
 except ImportError:
     import builtins
 
 contextmanager = contextlib.contextmanager
 
 _origimport = __import__
 
 nothing = object()
 
 # Python 3 doesn't have relative imports nor level -1.
 level = -1
 if sys.version_info[0] >= 3:
     level = 0
 _import = _origimport
 
 def _hgextimport(importfunc, name, globals, *args, **kwargs):
     try:
         return importfunc(name, globals, *args, **kwargs)
     except ImportError:
         if not globals:
             raise
         # extensions are loaded with "hgext_" prefix
         hgextname = 'hgext_%s' % name
         nameroot = hgextname.split('.', 1)[0]
         contextroot = globals.get('__name__', '').split('.', 1)[0]
         if nameroot != contextroot:
             raise
         # retry to import with "hgext_" prefix
         return importfunc(hgextname, globals, *args, **kwargs)
 
 class _demandmod(object):
     """module demand-loader and proxy"""
     def __init__(self, name, globals, locals, level=level):
         if '.' in name:
             head, rest = name.split('.', 1)
             after = [rest]
         else:
             head = name
             after = []
         object.__setattr__(self, "_data",
                            (head, globals, locals, after, level, set()))
         object.__setattr__(self, "_module", None)
     def _extend(self, name):
         """add to the list of submodules to load"""
         self._data[3].append(name)
 
     def _addref(self, name):
         """Record that the named module ``name`` imports this module.
 
         References to this proxy class having the name of this module will be
         replaced at module load time. We assume the symbol inside the importing
         module is identical to the "head" name of this module. We don't
         actually know if "as X" syntax is being used to change the symbol name
         because this information isn't exposed to __import__.
         """
         self._data[5].add(name)
 
     def _load(self):
         if not self._module:
             head, globals, locals, after, level, modrefs = self._data
             mod = _hgextimport(_import, head, globals, locals, None, level)
             # load submodules
             def subload(mod, p):
                 h, t = p, None
                 if '.' in p:
                     h, t = p.split('.', 1)
                 if getattr(mod, h, nothing) is nothing:
                     setattr(mod, h, _demandmod(p, mod.__dict__, mod.__dict__))
                 elif t:
                     subload(getattr(mod, h), t)
 
             for x in after:
                 subload(mod, x)
 
             # Replace references to this proxy instance with the actual module.
             if locals and locals.get(head) == self:
                 locals[head] = mod
 
             for modname in modrefs:
                 modref = sys.modules.get(modname, None)
                 if modref and getattr(modref, head, None) == self:
                     setattr(modref, head, mod)
 
             object.__setattr__(self, "_module", mod)
 
     def __repr__(self):
         if self._module:
             return "<proxied module '%s'>" % self._data[0]
         return "<unloaded module '%s'>" % self._data[0]
     def __call__(self, *args, **kwargs):
         raise TypeError("%s object is not callable" % repr(self))
     def __getattribute__(self, attr):
         if attr in ('_data', '_extend', '_load', '_module', '_addref'):
             return object.__getattribute__(self, attr)
         self._load()
         return getattr(self._module, attr)
     def __setattr__(self, attr, val):
         self._load()
         setattr(self._module, attr, val)
 
 _pypy = '__pypy__' in sys.builtin_module_names
 
 def _demandimport(name, globals=None, locals=None, fromlist=None, level=level):
     if not locals or name in ignore or fromlist == ('*',):
         # these cases we can't really delay
         return _hgextimport(_import, name, globals, locals, fromlist, level)
     elif not fromlist:
         # import a [as b]
         if '.' in name: # a.b
             base, rest = name.split('.', 1)
             # email.__init__ loading email.mime
             if globals and globals.get('__name__', None) == base:
                 return _import(name, globals, locals, fromlist, level)
             # if a is already demand-loaded, add b to its submodule list
             if base in locals:
                 if isinstance(locals[base], _demandmod):
                     locals[base]._extend(rest)
                 return locals[base]
         return _demandmod(name, globals, locals, level)
     else:
         # There is a fromlist.
         # from a import b,c,d
         # from . import b,c,d
         # from .a import b,c,d
 
         # level == -1: relative and absolute attempted (Python 2 only).
         # level >= 0: absolute only (Python 2 w/ absolute_import and Python 3).
         # The modern Mercurial convention is to use absolute_import everywhere,
         # so modern Mercurial code will have level >= 0.
 
         # The name of the module the import statement is located in.
         globalname = globals.get('__name__')
 
         def processfromitem(mod, attr):
             """Process an imported symbol in the import statement.
 
             If the symbol doesn't exist in the parent module, it must be a
             module. We set missing modules up as _demandmod instances.
             """
             symbol = getattr(mod, attr, nothing)
             if symbol is nothing:
                 symbol = _demandmod(attr, mod.__dict__, locals, level=1)
                 setattr(mod, attr, symbol)
 
             # Record the importing module references this symbol so we can
             # replace the symbol with the actual module instance at load
             # time.
             if globalname and isinstance(symbol, _demandmod):
                 symbol._addref(globalname)
 
         if level >= 0:
             # The "from a import b,c,d" or "from .a import b,c,d"
             # syntax gives errors with some modules for unknown
             # reasons. Work around the problem.
             if name:
                 return _hgextimport(_origimport, name, globals, locals,
                                     fromlist, level)
 
             if _pypy:
                 # PyPy's __import__ throws an exception if invoked
                 # with an empty name and no fromlist.  Recreate the
                 # desired behaviour by hand.
                 mn = globalname
                 mod = sys.modules[mn]
                 if getattr(mod, '__path__', nothing) is nothing:
                     mn = mn.rsplit('.', 1)[0]
                     mod = sys.modules[mn]
                 if level > 1:
                     mn = mn.rsplit('.', level - 1)[0]
                     mod = sys.modules[mn]
             else:
                 mod = _hgextimport(_origimport, name, globals, locals,
                                    level=level)
 
             for x in fromlist:
                 processfromitem(mod, x)
 
             return mod
 
         # But, we still need to support lazy loading of standard library and 3rd
         # party modules. So handle level == -1.
         mod = _hgextimport(_origimport, name, globals, locals)
         # recurse down the module chain
         for comp in name.split('.')[1:]:
             if getattr(mod, comp, nothing) is nothing:
                 setattr(mod, comp,
                         _demandmod(comp, mod.__dict__, mod.__dict__))
             mod = getattr(mod, comp)
 
         for x in fromlist:
             processfromitem(mod, x)
 
         return mod
 
 ignore = [
     '__future__',
     '_hashlib',
+    # ImportError during pkg_resources/__init__.py:fixup_namespace_package
+    '_imp',
     '_xmlplus',
     'fcntl',
     'win32com.gen_py',
     '_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__',
     '_ssl', # conditional imports in the stdlib, issue1964
     '_sre', # issue4920
     'rfc822',
     'mimetools',
     # setuptools 8 expects this module to explode early when not on windows
     'distutils.msvc9compiler'
     ]
 
 def isenabled():
     return builtins.__import__ == _demandimport
 
 def enable():
     "enable global demand-loading of modules"
     if os.environ.get('HGDEMANDIMPORT') != 'disable':
         builtins.__import__ = _demandimport
 
 def disable():
     "disable global demand-loading of modules"
     builtins.__import__ = _origimport
 
 @contextmanager
 def deactivated():
     "context manager for disabling demandimport in 'with' blocks"
     demandenabled = isenabled()
     if demandenabled:
         disable()
 
     try:
         yield
     finally:
         if demandenabled:
             enable()