view mercurial/policy.py @ 48671:f1ed5c304f45

encoding: fix trim() to be O(n) instead of O(n^2)

`encoding.trim()` iterated over the possible lengths smaller than the
input and created a slice for each. It then calculated the column
width of the result, which is of course O(n), so the overall algorithm
was O(n). This patch rewrites it to iterate over the unicode
characters, keeping track of the length so far. Also, the old
algorithm started from the end of the string, which made it much worse
when the input is large and the limit is small (such as the typical 72
we pass to it).

You can time it by running something like this:

```
time python3 -c 'from mercurial.utils import stringutil; print(stringutil.ellipsis(b"0123456789" * 1000, 5))'
```

That drops from 4.05 s to 83 ms with this patch (and most of that is
of course startup time).

Differential Revision: https://phab.mercurial-scm.org/D12089

# policy.py - module policy 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 os
import sys

from .pycompat import getattr

# Rules for how modules can be loaded. Values are:
#
#    c - require C extensions
#    rust+c - require Rust and C extensions
#    rust+c-allow - allow Rust and C extensions with fallback to pure Python
#                   for each
#    allow - allow pure Python implementation when C loading fails
#    cffi - required cffi versions (implemented within pure module)
#    cffi-allow - allow pure Python implementation if cffi version is missing
#    py - only load pure Python modules
#
# By default, fall back to the pure modules so the in-place build can
# run without recompiling the C extensions. This will be overridden by
# __modulepolicy__ generated by setup.py.
policy = b'allow'
_packageprefs = {
    # policy: (versioned package, pure package)
    b'c': ('cext', None),
    b'allow': ('cext', 'pure'),
    b'cffi': ('cffi', None),
    b'cffi-allow': ('cffi', 'pure'),
    b'py': (None, 'pure'),
    # For now, rust policies impact importrust only
    b'rust+c': ('cext', None),
    b'rust+c-allow': ('cext', 'pure'),
}

try:
    from . import __modulepolicy__

    policy = __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:
    policy = b'cffi'

# Environment variable can always force settings.
if sys.version_info[0] >= 3:
    if 'HGMODULEPOLICY' in os.environ:
        policy = os.environ['HGMODULEPOLICY'].encode('utf-8')
else:
    policy = os.environ.get('HGMODULEPOLICY', policy)


def _importfrom(pkgname, modname):
    # from .<pkgname> import <modname> (where . is looked through this module)
    fakelocals = {}
    pkg = __import__(pkgname, globals(), fakelocals, [modname], level=1)
    try:
        fakelocals[modname] = mod = getattr(pkg, modname)
    except AttributeError:
        raise ImportError('cannot import name %s' % modname)
    # force import; fakelocals[modname] may be replaced with the real module
    getattr(mod, '__doc__', None)
    return fakelocals[modname]


# keep in sync with "version" in C modules
_cextversions = {
    ('cext', 'base85'): 1,
    ('cext', 'bdiff'): 3,
    ('cext', 'mpatch'): 1,
    ('cext', 'osutil'): 4,
    ('cext', 'parsers'): 20,
}

# map import request to other package or module
_modredirects = {
    ('cext', 'charencode'): ('cext', 'parsers'),
    ('cffi', 'base85'): ('pure', 'base85'),
    ('cffi', 'charencode'): ('pure', 'charencode'),
    ('cffi', 'parsers'): ('pure', 'parsers'),
}


def _checkmod(pkgname, modname, mod):
    expected = _cextversions.get((pkgname, modname))
    actual = getattr(mod, 'version', None)
    if actual != expected:
        raise ImportError(
            'cannot import module %s.%s '
            '(expected version: %d, actual: %r)'
            % (pkgname, modname, expected, actual)
        )


def importmod(modname):
    """Import module according to policy and check API version"""
    try:
        verpkg, purepkg = _packageprefs[policy]
    except KeyError:
        raise ImportError('invalid HGMODULEPOLICY %r' % policy)
    assert verpkg or purepkg
    if verpkg:
        pn, mn = _modredirects.get((verpkg, modname), (verpkg, modname))
        try:
            mod = _importfrom(pn, mn)
            if pn == verpkg:
                _checkmod(pn, mn, mod)
            return mod
        except ImportError:
            if not purepkg:
                raise
    pn, mn = _modredirects.get((purepkg, modname), (purepkg, modname))
    return _importfrom(pn, mn)


def _isrustpermissive():
    """Assuming the policy is a Rust one, tell if it's permissive."""
    return policy.endswith(b'-allow')


def importrust(modname, member=None, default=None):
    """Import Rust module according to policy and availability.

    If policy isn't a Rust one, this returns `default`.

    If either the module or its member is not available, this returns `default`
    if policy is permissive and raises `ImportError` if not.
    """
    if not policy.startswith(b'rust'):
        return default

    try:
        mod = _importfrom('rustext', modname)
    except ImportError:
        if _isrustpermissive():
            return default
        raise
    if member is None:
        return mod

    try:
        return getattr(mod, member)
    except AttributeError:
        if _isrustpermissive():
            return default
        raise ImportError("Cannot import name %s" % member)