mercurial/demandimport.py

help: internals topic for wire protocol

The Mercurial wire protocol is under-documented. This includes a lack
of source docstrings and comments as well as pages on the official
wiki.

This patch adds the beginnings of "internals" documentation on the
wire protocol.

The documentation should have nearly complete coverage on the
lower-level parts of the protocol, such as the different transport
mechanims, how commands and arguments are sent, capabilities, and,
of course, the commands themselves.

As part of writing this documentation, I discovered a number of
deficiencies in the protocol and bugs in the implementation. I've
started sending patches for some of the issues. I hope to send a lot
more.

This patch starts with the scaffolding for a new internals page.

# demandimport.py - global demand-loading of modules for Mercurial
#
# Copyright 2006, 2007 Matt Mackall <[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.

'''
demandimport - automatic demandloading of modules

To enable this module, do:

  import demandimport; demandimport.enable()

Imports of the following forms will be demand-loaded:

  import a, b.c
  import a.b as c
  from a import b,c # a will be loaded immediately

These imports will not be delayed:

  from a import *
  b = __import__(a)
'''

from __future__ import absolute_import

import contextlib
import os
import sys

# __builtin__ in Python 2, builtins in Python 3.
try:
    import __builtin__ as builtins
except ImportError:
    import builtins

contextmanager = contextlib.contextmanager

_origimport = __import__

nothing = object()

# Python 3 doesn't have relative imports nor level -1.
level = -1
if sys.version_info[0] >= 3:
    level = 0
_import = _origimport

def _hgextimport(importfunc, name, globals, *args, **kwargs):
    try:
        return importfunc(name, globals, *args, **kwargs)
    except ImportError:
        if not globals:
            raise
        # extensions are loaded with "hgext_" prefix
        hgextname = 'hgext_%s' % name
        nameroot = hgextname.split('.', 1)[0]
        contextroot = globals.get('__name__', '').split('.', 1)[0]
        if nameroot != contextroot:
            raise
        # retry to import with "hgext_" prefix
        return importfunc(hgextname, globals, *args, **kwargs)

class _demandmod(object):
    """module demand-loader and proxy

    Specify 1 as 'level' argument at construction, to import module
    relatively.
    """
    def __init__(self, name, globals, locals, level):
        if '.' in name:
            head, rest = name.split('.', 1)
            after = [rest]
        else:
            head = name
            after = []
        object.__setattr__(self, "_data",
                           (head, globals, locals, after, level, set()))
        object.__setattr__(self, "_module", None)
    def _extend(self, name):
        """add to the list of submodules to load"""
        self._data[3].append(name)

    def _addref(self, name):
        """Record that the named module ``name`` imports this module.

        References to this proxy class having the name of this module will be
        replaced at module load time. We assume the symbol inside the importing
        module is identical to the "head" name of this module. We don't
        actually know if "as X" syntax is being used to change the symbol name
        because this information isn't exposed to __import__.
        """
        self._data[5].add(name)

    def _load(self):
        if not self._module:
            head, globals, locals, after, level, modrefs = self._data
            mod = _hgextimport(_import, head, globals, locals, None, level)
            if mod is self:
                # In this case, _hgextimport() above should imply
                # _demandimport(). Otherwise, _hgextimport() never
                # returns _demandmod. This isn't intentional behavior,
                # in fact. (see also issue5304 for detail)
                #
                # If self._module is already bound at this point, self
                # should be already _load()-ed while _hgextimport().
                # Otherwise, there is no way to import actual module
                # as expected, because (re-)invoking _hgextimport()
                # should cause same result.
                # This is reason why _load() returns without any more
                # setup but assumes self to be already bound.
                mod = self._module
                assert mod and mod is not self, "%s, %s" % (self, mod)
                return

            # load submodules
            def subload(mod, p):
                h, t = p, None
                if '.' in p:
                    h, t = p.split('.', 1)
                if getattr(mod, h, nothing) is nothing:
                    setattr(mod, h, _demandmod(p, mod.__dict__, mod.__dict__,
                                               level=1))
                elif t:
                    subload(getattr(mod, h), t)

            for x in after:
                subload(mod, x)

            # Replace references to this proxy instance with the actual module.
            if locals and locals.get(head) == self:
                locals[head] = mod

            for modname in modrefs:
                modref = sys.modules.get(modname, None)
                if modref and getattr(modref, head, None) == self:
                    setattr(modref, head, mod)

            object.__setattr__(self, "_module", mod)

    def __repr__(self):
        if self._module:
            return "<proxied module '%s'>" % self._data[0]
        return "<unloaded module '%s'>" % self._data[0]
    def __call__(self, *args, **kwargs):
        raise TypeError("%s object is not callable" % repr(self))
    def __getattribute__(self, attr):
        if attr in ('_data', '_extend', '_load', '_module', '_addref'):
            return object.__getattribute__(self, attr)
        self._load()
        return getattr(self._module, attr)
    def __setattr__(self, attr, val):
        self._load()
        setattr(self._module, attr, val)

_pypy = '__pypy__' in sys.builtin_module_names

def _demandimport(name, globals=None, locals=None, fromlist=None, level=level):
    if not locals or name in ignore or fromlist == ('*',):
        # these cases we can't really delay
        return _hgextimport(_import, name, globals, locals, fromlist, level)
    elif not fromlist:
        # import a [as b]
        if '.' in name: # a.b
            base, rest = name.split('.', 1)
            # email.__init__ loading email.mime
            if globals and globals.get('__name__', None) == base:
                return _import(name, globals, locals, fromlist, level)
            # if a is already demand-loaded, add b to its submodule list
            if base in locals:
                if isinstance(locals[base], _demandmod):
                    locals[base]._extend(rest)
                return locals[base]
        return _demandmod(name, globals, locals, level)
    else:
        # There is a fromlist.
        # from a import b,c,d
        # from . import b,c,d
        # from .a import b,c,d

        # level == -1: relative and absolute attempted (Python 2 only).
        # level >= 0: absolute only (Python 2 w/ absolute_import and Python 3).
        # The modern Mercurial convention is to use absolute_import everywhere,
        # so modern Mercurial code will have level >= 0.

        # The name of the module the import statement is located in.
        globalname = globals.get('__name__')

        def processfromitem(mod, attr):
            """Process an imported symbol in the import statement.

            If the symbol doesn't exist in the parent module, it must be a
            module. We set missing modules up as _demandmod instances.
            """
            symbol = getattr(mod, attr, nothing)
            if symbol is nothing:
                mn = '%s.%s' % (mod.__name__, attr)
                if mn in ignore:
                    importfunc = _origimport
                else:
                    importfunc = _demandmod
                symbol = importfunc(attr, mod.__dict__, locals, level=1)
                setattr(mod, attr, symbol)

            # Record the importing module references this symbol so we can
            # replace the symbol with the actual module instance at load
            # time.
            if globalname and isinstance(symbol, _demandmod):
                symbol._addref(globalname)

        def chainmodules(rootmod, modname):
            # recurse down the module chain, and return the leaf module
            mod = rootmod
            for comp in modname.split('.')[1:]:
                if getattr(mod, comp, nothing) is nothing:
                    setattr(mod, comp, _demandmod(comp, mod.__dict__,
                                                  mod.__dict__, level=1))
                mod = getattr(mod, comp)
            return mod

        if level >= 0:
            if name:
                # "from a import b" or "from .a import b" style
                rootmod = _hgextimport(_origimport, name, globals, locals,
                                       level=level)
                mod = chainmodules(rootmod, name)
            elif _pypy:
                # PyPy's __import__ throws an exception if invoked
                # with an empty name and no fromlist.  Recreate the
                # desired behaviour by hand.
                mn = globalname
                mod = sys.modules[mn]
                if getattr(mod, '__path__', nothing) is nothing:
                    mn = mn.rsplit('.', 1)[0]
                    mod = sys.modules[mn]
                if level > 1:
                    mn = mn.rsplit('.', level - 1)[0]
                    mod = sys.modules[mn]
            else:
                mod = _hgextimport(_origimport, name, globals, locals,
                                   level=level)

            for x in fromlist:
                processfromitem(mod, x)

            return mod

        # But, we still need to support lazy loading of standard library and 3rd
        # party modules. So handle level == -1.
        mod = _hgextimport(_origimport, name, globals, locals)
        mod = chainmodules(mod, name)

        for x in fromlist:
            processfromitem(mod, x)

        return mod

ignore = [
    '__future__',
    '_hashlib',
    # ImportError during pkg_resources/__init__.py:fixup_namespace_package
    '_imp',
    '_xmlplus',
    'fcntl',
    'win32com.gen_py',
    '_winreg', # 2.7 mimetypes needs immediate ImportError
    'pythoncom',
    # imported by tarfile, not available under Windows
    'pwd',
    'grp',
    # imported by profile, itself imported by hotshot.stats,
    # not available under Windows
    'resource',
    # this trips up many extension authors
    'gtk',
    # setuptools' pkg_resources.py expects "from __main__ import x" to
    # raise ImportError if x not defined
    '__main__',
    '_ssl', # conditional imports in the stdlib, issue1964
    '_sre', # issue4920
    'rfc822',
    'mimetools',
    'sqlalchemy.events', # has import-time side effects (issue5085)
    # setuptools 8 expects this module to explode early when not on windows
    'distutils.msvc9compiler'
    ]

def isenabled():
    return builtins.__import__ == _demandimport

def enable():
    "enable global demand-loading of modules"
    if os.environ.get('HGDEMANDIMPORT') != 'disable':
        builtins.__import__ = _demandimport

def disable():
    "disable global demand-loading of modules"
    builtins.__import__ = _origimport

@contextmanager
def deactivated():
    "context manager for disabling demandimport in 'with' blocks"
    demandenabled = isenabled()
    if demandenabled:
        disable()

    try:
        yield
    finally:
        if demandenabled:
            enable()