mercurial/demandimport.py

protocol: send application/mercurial-0.2 responses to capable clients

With this commit, the HTTP transport now parses the X-HgProto-<N>
header to determine what media type and compression engine to use for
responses. So far, we only compress responses that are already being
compressed with zlib today (stream response types to specific
commands). We can expand things to cover additional response types
later.

The practical side-effect of this commit is that non-zlib compression
engines will be used if both ends support them. This means if both
ends have zstd support, zstd - not zlib - will be used to compress
data!

When cloning the mozilla-unified repository between a local HTTP
server and client, the benefits of non-zlib compression are quite
noticeable:

  engine     server CPU (s)   client CPU (s)    bundle size
zlib (l=6)      174.1            283.2         1,148,547,026
zstd (l=1)       99.2            267.3         1,127,513,841
zstd (l=3)      103.1            266.9         1,018,861,363
zstd (l=7)      128.3            269.7           919,190,278
zstd (l=10)     162.0               -            894,547,179
none             95.3            277.2         4,097,566,064

The default zstd compression level is 3. So if you deploy zstd
capable Mercurial to your clients and servers and CPU time on
your server is dominated by "getbundle" requests (clients cloning
and pulling) - and my experience at Mozilla tells me this is often
the case - this commit could drastically reduce your server-side
CPU usage *and* save on bandwidth costs!

Another benefit of this change is that server operators can install
*any* compression engine. While it isn't enabled by default, the
"none" compression engine can now be used to disable wire protocol
compression completely. Previously, commands like "getbundle" always
zlib compressed output, adding considerable overhead to generating
responses. If you are on a high speed network and your server is under
high load, it might be advantageous to trade bandwidth for CPU.
Although, zstd at level 1 doesn't use that much CPU, so I'm not
convinced that disabling compression wholesale is worthwhile. And, my
data seems to indicate a slow down on the client without compression.
I suspect this is due to a lack of buffering resulting in an increase
in socket read() calls and/or the fact we're transferring an extra 3 GB
of data (parsing HTTP chunked transfer and processing extra TCP packets
can add up). This is definitely worth investigating and optimizing. But
since the "none" compressor isn't enabled by default, I'm inclined to
punt on this issue.

This commit introduces tons of tests. Some of these should arguably
have been implemented on previous commits. But it was difficult to
test without the server functionality in place.

# 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, and if the
            parent module is a package, it must be a module. We set missing
            modules up as _demandmod instances.
            """
            symbol = getattr(mod, attr, nothing)
            nonpkg = getattr(mod, '__path__', nothing) is nothing
            if symbol is nothing:
                if nonpkg:
                    # do not try relative import, which would raise ValueError,
                    # and leave unknown attribute as the default __import__()
                    # would do. the missing attribute will be detected later
                    # while processing the import statement.
                    return
                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',
    'nt', # pathlib2 tests the existence of built-in 'nt' module
    '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',
    '__builtin__',
    'builtins',
    ]

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

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()