hgdemandimport/demandimportpy3.py

vfs: create copy at renaming to avoid file stat ambiguity if needed

In order to fix issue5418, bff5ccbe5ead made vfs.rename(checkambig=True)
omit advancing mtime of renamed file, if renamed file is owned by
another (EPERM is raised in this case).

But this omission causes rewinding mtime at restoration in such
situation, and makes avoiding file stat ambiguity difficult, because
ExactCacheValidationPlan assumes that mtime should be advanced, if a
file is changed in same ctime.

    https://www.mercurial-scm.org/wiki/ExactCacheValidationPlan

Ambiguity of file stat also requires issue5584 to be fixed with other
than file stat, but "hash of file", "generation ID" and so on were
already rejected ideas (please see original RFC linked from "Outline
of issue" in ExactCacheValidationPlan page).

This omission occurs:

  - only for non append-only files (dirstate, bookmarks, and phaseroots), and
  - only if previous transaction is rollbacked by another user

The latter means "sharing a repository clone via group permission".
This is reasonable usecase, but not ordinary for many users, IMHO.
"hg rollback" itself has been deprecated since Mercurial 2.7, too.

Therefore, increasing the cost at rollbacking previous transaction
executed by another a little seems reasonable, for avoidance of file
stat ambiguity.

This patch does:

  - create copy of (already renamed) source file, if advancing mtime
    fails for EPERM
  - rename from copied file to destination file, and
  - advance mtime of renamed file, which is now owned by current user

This patch also factors "self.join(src)" out to reduce redundancy.

# demandimportpy3 - global demand-loading of modules for Mercurial
#
# Copyright 2017 Facebook Inc.
#
# This software may be used and distributed according to the terms of the
# GNU General Public License version 2 or any later version.

"""Lazy loading for Python 3.6 and above.

This uses the new importlib finder/loader functionality available in Python 3.5
and up. The code reuses most of the mechanics implemented inside importlib.util,
but with a few additions:

* Allow excluding certain modules from lazy imports.
* Expose an interface that's substantially the same as demandimport for
  Python 2.

This also has some limitations compared to the Python 2 implementation:

* Much of the logic is per-package, not per-module, so any packages loaded
  before demandimport is enabled will not be lazily imported in the future. In
  practice, we only expect builtins to be loaded before demandimport is
  enabled.
"""

# This line is unnecessary, but it satisfies test-check-py3-compat.t.
from __future__ import absolute_import

import contextlib
import os
import sys

import importlib.abc
import importlib.machinery
import importlib.util

_deactivated = False

class _lazyloaderex(importlib.util.LazyLoader):
    """This is a LazyLoader except it also follows the _deactivated global and
    the ignore list.
    """
    def exec_module(self, module):
        """Make the module load lazily."""
        if _deactivated or module.__name__ in ignore:
            self.loader.exec_module(module)
        else:
            super().exec_module(module)

# This is 3.6+ because with Python 3.5 it isn't possible to lazily load
# extensions. See the discussion in https://python.org/sf/26186 for more.
_extensions_loader = _lazyloaderex.factory(
    importlib.machinery.ExtensionFileLoader)
_bytecode_loader = _lazyloaderex.factory(
    importlib.machinery.SourcelessFileLoader)
_source_loader = _lazyloaderex.factory(importlib.machinery.SourceFileLoader)

def _makefinder(path):
    return importlib.machinery.FileFinder(
        path,
        # This is the order in which loaders are passed in in core Python.
        (_extensions_loader, importlib.machinery.EXTENSION_SUFFIXES),
        (_source_loader, importlib.machinery.SOURCE_SUFFIXES),
        (_bytecode_loader, importlib.machinery.BYTECODE_SUFFIXES),
    )

ignore = []

def init(ignorelist):
    global ignore
    ignore = ignorelist

def isenabled():
    return _makefinder in sys.path_hooks and not _deactivated

def disable():
    try:
        while True:
            sys.path_hooks.remove(_makefinder)
    except ValueError:
        pass

def enable():
    if os.environ.get('HGDEMANDIMPORT') != 'disable':
        sys.path_hooks.insert(0, _makefinder)

@contextlib.contextmanager
def deactivated():
    # This implementation is a bit different from Python 2's. Python 3
    # maintains a per-package finder cache in sys.path_importer_cache (see
    # PEP 302). This means that we can't just call disable + enable.
    # If we do that, in situations like:
    #
    #   demandimport.enable()
    #   ...
    #   from foo.bar import mod1
    #   with demandimport.deactivated():
    #       from foo.bar import mod2
    #
    # mod2 will be imported lazily. (The converse also holds -- whatever finder
    # first gets cached will be used.)
    #
    # Instead, have a global flag the LazyLoader can use.
    global _deactivated
    demandenabled = isenabled()
    if demandenabled:
        _deactivated = True
    try:
        yield
    finally:
        if demandenabled:
            _deactivated = False