mercurial/__init__.py

pathauditor: change parts verification order to be root first

Previously, when we verified the parts of a path in the auditor, we would
validate the deepest directory first, then it's parent, and so on up to the
root. If there happened to be a symlink in the chain, that meant our first check
would likely traverse that symlink.  In some cases that symlink might point to
a network filesystem that is expensive, and therefore this simple check could be
very slow.

The fix is to check the path parts starting at the root and working our way
down.

This has a minor performance difference in that we used to be able to short
circuit from the audit if we reached a directory that had already been checked.
Now we can't, but the cost is N dictionary look ups, where N is the number of
parts in the path, which should be fairly minor.

# __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
import zipimport

__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']

        # The zip importer behaves sufficiently differently from the default
        # importer to warrant its own code path.
        loader = getattr(mercurial, '__loader__', None)
        if isinstance(loader, zipimport.zipimporter):
            def ziploader(*paths):
                """Obtain a zipimporter for a directory under the main zip."""
                path = os.path.join(loader.archive, *paths)
                zl = sys.path_importer_cache.get(path)
                if not zl:
                    zl = zipimport.zipimporter(path)
                return zl

            try:
                if modulepolicy == 'py':
                    raise ImportError()

                zl = ziploader('mercurial')
                mod = zl.load_module(name)
                # Unlike imp, ziploader doesn't expose module metadata that
                # indicates the type of module. So just assume what we found
                # is OK (even though it could be a pure Python module).
            except ImportError:
                if modulepolicy == 'c':
                    raise
                zl = ziploader('mercurial', 'pure')
                mod = zl.load_module(name)

            sys.modules[name] = mod
            return mod

        # 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())