mercurial/__init__.py

changelog: add class to represent parsed changelog revisions

Currently, changelog entries are parsed into their respective
components at read time. Many operations are only interested
in a subset of fields of a changelog entry. The parsing and
storing of all the fields adds avoidable overhead.

This patch introduces the "changelogrevision" class. It takes
changelog raw text and exposes the parsed results as attributes.
The code for parsing changelog entries has been moved into its
construction function. changelog.read() has been modified to use
the new class internally while maintaining its existing API.
Future patches will make revision parsing lazy.

We implement the construction function of the new class with
__new__ instead of __init__ so we can use a named tuple to
represent the empty revision. This saves overhead and complexity
of coercing later versions of this class to represent an empty
instance.

While we are here, we add a method on changelog to obtain an
instance of the new type.

The overhead of constructing the new class regresses performance
of revsets accessing this data:

author(mpm)
0.896565
0.929984

desc(bug)
0.887169
0.935642 105%

date(2015)
0.878797
0.908094

extra(rebase_source)
0.865446
0.922624 106%

author(mpm) or author(greg)
1.801832
1.902112 105%

author(mpm) or desc(bug)
1.812438
1.860977

date(2015) or branch(default)
0.968276
1.005824

author(mpm) or desc(bug) or date(2015) or extra(rebase_source)
3.656193
3.743381

Once lazy parsing is implemented, these revsets will all be faster
than before. There is no performance change on revsets that do not
access this data. There /could/ be a performance regression on
operations that perform several changelog reads. However, I can't
think of anything outside of revsets and `hg log` (basically the
same as a revset) that would be impacted.

# __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
#
# By default, require the C extensions for performance reasons.
modulepolicy = 'c'
try:
    from . import __modulepolicy__
    modulepolicy = __modulepolicy__.modulepolicy
except ImportError:
    pass

# 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())