hgdemandimport/demandimportpy3.py

bookflow: new extension for bookmark-based branching

This extension should be helpful for feature branches - based workflows.
At my company we first considered branches, but weren't sure about creating a lot of permanent objects.
We tried bookmarks, but found some scenarios to be difficult to control.

The main problem, was the active bookmark being moved on update.
Disabling that made everything a lot more predictable.

Bookmarks move on commit, and updating means switching between them.

The extension also implements a few minor features to better guide the workflow:
  - hg bookmark NAME can be ambiguous (create or move), unlike hg branch. The extension requires -rev to move.
  - require an active bookmark on commit.
  - some bookmarks can be protected (like @), useful for teams, that require code reviews.
  - block creation of new branches.

The initial implementation requires no changes in the core, but it does rely on some implementation details.
I thought it may be useful to discuss the functionality first, and then focus making the code more robust.

Differential Revision: https://phab.mercurial-scm.org/D4312

# 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 importlib.abc
import importlib.machinery
import importlib.util
import sys

_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 ignores:
            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://bugs.python.org/issue26186 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),
    )

ignores = set()

def init(ignoreset):
    global ignores
    ignores = ignoreset

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():
    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