hg

util: add a file object proxy that can notify observers

There are various places in Mercurial where we may want to
instrument low-level I/O. The use cases I can think of all
involve development-type activities like monitoring the raw
bytes passing through a file (for testing and debugging),
counting the number of I/O function calls (for performance
monitoring), and changing the behavior of I/O function calls
(e.g. simulating a failure) (to facilitate testing).

This commit invents a mechanism to wrap a file object so we
can observe activity on it. We have similar functionality in
badserverext.py. But that's a test-only extension and is pretty
specific to the HTTP server. I would like a mechanism in core
that is sufficiently generic so it can be used by multiple
consumers, including `hg debug*` commands.

The added code consists of a proxy type for file objects.
It is bound to an "observer," which receives callbacks whenever
I/O methods are called.

We also add an implementation of an observer that logs specific
I/O events. This observer will be used in an upcoming commit
to record low-level wire protocol activity.

A helper function to convert a file object into an observed
file object has also been implemented.

I don't anticipate any critical functionality in core using
these types. So I don't think explicit test coverage is
worth implementing.

Differential Revision: https://phab.mercurial-scm.org/D2462

#!/usr/bin/env python
#
# mercurial - scalable distributed SCM
#
# Copyright 2005-2007 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.
from __future__ import absolute_import

import os
import sys

if os.environ.get('HGUNICODEPEDANTRY', False):
    try:
        reload(sys)
        sys.setdefaultencoding("undefined")
    except NameError:
        pass

libdir = '@LIBDIR@'

if libdir != '@' 'LIBDIR' '@':
    if not os.path.isabs(libdir):
        libdir = os.path.join(os.path.dirname(os.path.realpath(__file__)),
                              libdir)
        libdir = os.path.abspath(libdir)
    sys.path.insert(0, libdir)

# enable importing on demand to reduce startup time
try:
    if sys.version_info[0] < 3 or sys.version_info >= (3, 6):
        import hgdemandimport; hgdemandimport.enable()
except ImportError:
    sys.stderr.write("abort: couldn't find mercurial libraries in [%s]\n" %
                     ' '.join(sys.path))
    sys.stderr.write("(check your install and PYTHONPATH)\n")
    sys.exit(-1)

from mercurial import dispatch
dispatch.run()