mercurial: don't load C extensions from PyPy

PyPy isn't compatible with Python C extensions.

With this patch, the module load policy is automatically to "Python
only" when run under PyPy. `hg` and other Python scripts importing
mercurial.* modules will run from the source checkout or any
installation when executed with PyPy. This should enable people to
more easily experiment with PyPy and its potentially significant
performance benefits over CPython!

mercurial/__init__.py

@@ -1,106 +1,113 b''
 # __init__.py - Startup and module loading logic for Mercurial.
 #
 # Copyright 2015 Gregory Szorc <[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.
 
 from __future__ import absolute_import
 
 import imp
 import os
 import sys
 
 __all__ = []
 
 # Rules for how modules can be loaded. Values are:
 #
 #    c - require C extensions
 #    allow - allow pure Python implementation when C loading fails
 #    py - only load pure Python modules
 modulepolicy = '@MODULELOADPOLICY@'
 
 # By default, require the C extensions for performance reasons.
 if modulepolicy == '@' 'MODULELOADPOLICY' '@':
     modulepolicy = 'c'
 
+# PyPy doesn't load C extensions.
+#
+# The canonical way to do this is to test platform.python_implementation().
+# But we don't import platform and don't bloat for it here.
+if '__pypy__' in sys.builtin_module_names:
+    modulepolicy = 'py'
+
 # Environment variable can always force settings.
 modulepolicy = os.environ.get('HGMODULEPOLICY', modulepolicy)
 
 # Modules that have both Python and C implementations. See also the
 # set of .py files under mercurial/pure/.
 _dualmodules = set([
     'mercurial.base85',
     'mercurial.bdiff',
     'mercurial.diffhelpers',
     'mercurial.mpatch',
     'mercurial.osutil',
     'mercurial.parsers',
 ])
 
 class hgimporter(object):
     """Object that conforms to import hook interface defined in PEP-302."""
     def find_module(self, name, path=None):
         # We only care about modules that have both C and pure implementations.
         if name in _dualmodules:
             return self
         return None
 
     def load_module(self, name):
         mod = sys.modules.get(name, None)
         if mod:
             return mod
 
         mercurial = sys.modules['mercurial']
 
         # Unlike the default importer which searches special locations and
         # sys.path, we only look in the directory where "mercurial" was
         # imported from.
 
         # imp.find_module doesn't support submodules (modules with ".").
         # Instead you have to pass the parent package's __path__ attribute
         # as the path argument.
         stem = name.split('.')[-1]
 
         try:
             if modulepolicy == 'py':
                 raise ImportError()
 
             modinfo = imp.find_module(stem, mercurial.__path__)
 
             # The Mercurial installer used to copy files from
             # mercurial/pure/*.py to mercurial/*.py. Therefore, it's possible
             # for some installations to have .py files under mercurial/*.
             # Loading Python modules when we expected C versions could result
             # in a) poor performance b) loading a version from a previous
             # Mercurial version, potentially leading to incompatibility. Either
             # scenario is bad. So we verify that modules loaded from
             # mercurial/* are C extensions. If the current policy allows the
             # loading of .py modules, the module will be re-imported from
             # mercurial/pure/* below.
             if modinfo[2][2] != imp.C_EXTENSION:
                 raise ImportError('.py version of %s found where C '
                                   'version should exist' % name)
 
         except ImportError:
             if modulepolicy == 'c':
                 raise
 
             # Could not load the C extension and pure Python is allowed. So
             # try to load them.
             from . import pure
             modinfo = imp.find_module(stem, pure.__path__)
             if not modinfo:
                 raise ImportError('could not find mercurial module %s' %
                                   name)
 
         mod = imp.load_module(name, *modinfo)
         sys.modules[name] = mod
         return mod
 
 # We automagically register our custom importer as a side-effect of loading.
 # This is necessary to ensure that any entry points are able to import
 # mercurial.* modules without having to perform this registration themselves.
 if not any(isinstance(x, hgimporter) for x in sys.meta_path):
     # meta_path is used before any implicit finders and before sys.path.
     sys.meta_path.insert(0, hgimporter())