annotate hgdemandimport/__init__.py @ 36875:16499427f6de

hgweb: refactor fake file object proxy for archiving

Python's zip file writer operates on a file object. When doing work,
it periodically calls write(), flush(), and tell() on that object.

In WSGI contexts, the start_response function returns a write()
function. That's a function to write data, not a full file object.
So, when the archival code was first introduced in 2b03c6733efa in
2006, someone invented a proxy "tellable" type that wrapped a file
object like object and kept track of write count so it could
implement tell() and satisfy zipfile's needs.

When our archival code runs, it attempts to tell() the destination
and if that fails, converts it to a "tellable" instance. Our WSGI
application passes the "wsgirequest" instance to the archival
function. It fails the tell() test and is converted to a "tellable."
It's worth noting that "wsgirequest" implements flush(), so
"tellable" doesn't.

This hackery all seems very specific to the WSGI code. So this commit
moves the "tellable" type and the conversion of the destination file
object into the WSGI code. There's a chance some other caller may be
passing a file object like object that doesn't implement tell(). But
I doubt it.

As part of the refactor, our new type implements flush() and doesn't
implement __getattr__. Given the intended limited use of this type,
I want things to fail fast if there is an attempt to access attributes
because I think it is important to document which attributes are being
used for what purposes.

Differential Revision: https://phab.mercurial-scm.org/D2791

# hgdemandimport - 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.

'''demandimport - automatic demand-loading of modules'''

# This is in a separate package from mercurial because in Python 3,
# demand loading is per-package. Keeping demandimport in the mercurial package
# would disable demand loading for any modules in mercurial.

from __future__ import absolute_import

import os
import sys

if sys.version_info[0] >= 3:
    from . import demandimportpy3 as demandimport
else:
    from . import demandimportpy2 as demandimport

# Extensions can add to this list if necessary.
ignore = [
    '__future__',
    '_hashlib',
    # ImportError during pkg_resources/__init__.py:fixup_namespace_package
    '_imp',
    '_xmlplus',
    'fcntl',
    'nt', # pathlib2 tests the existence of built-in 'nt' module
    'win32com.gen_py',
    'win32com.shell', # 'appdirs' tries to import win32com.shell
    '_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',
    '__builtin__',
    'builtins',
    'urwid.command_map', # for pudb
    ]

_pypy = '__pypy__' in sys.builtin_module_names

if _pypy:
    ignore.extend([
        # _ctypes.pointer is shadowed by "from ... import pointer" (PyPy 5)
        '_ctypes.pointer',
    ])

demandimport.init(ignore)

# Re-export.
isenabled = demandimport.isenabled
disable = demandimport.disable
deactivated = demandimport.deactivated

def enable():
    # chg pre-imports modules so do not enable demandimport for it
    if ('CHGINTERNALMARK' not in os.environ
        and os.environ.get('HGDEMANDIMPORT') != 'disable'):
        demandimport.enable()