view mercurial/txnutil.py @ 32001:c85f19c66e8d

tests: add tests for poorly behaving HTTP server

I've spent several hours over the past few weeks investigating
networking failures involving hg.mozilla.org. As part of this, it
has become clear that the Mercurial client's error handling when
it encounters network failures is far from robust.

To prove this is true, I've devised a battery of tests simulating
various network failures, notably premature connection closes. To
achieve this, I've implemented an extension that monkeypatches the
built-in HTTP server and hooks in at the socket level and allows
various events to occur based on config options. For example, you
can refuse to accept() a client socket or you can close() the socket
after N bytes have been sent or received. The latter effectively
simulates an unexpected connection drop (and these occur all the
time in the real world).

The new test file launches servers exhibiting various "bad" behaviors
and points a client at them. As the many TODO comments in the test
call attention to, Mercurial often displays unhelpful errors when
network-related failures occur. This makes it difficult for users
to understand what's going on and difficult for server administrators
to pinpoint root causes without packet tracing.

Upcoming patches will attempt to fix these error handling
deficiencies.

# txnutil.py - transaction related utilities
#
#  Copyright FUJIWARA Katsunori <[e-mail]> and others
#
# 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 errno

from . import (
    encoding,
)

def mayhavepending(root):
    '''return whether 'root' may have pending changes, which are
    visible to this process.
    '''
    return root == encoding.environ.get('HG_PENDING')

def trypending(root, vfs, filename, **kwargs):
    '''Open  file to be read according to HG_PENDING environment variable

    This opens '.pending' of specified 'filename' only when HG_PENDING
    is equal to 'root'.

    This returns '(fp, is_pending_opened)' tuple.
    '''
    if mayhavepending(root):
        try:
            return (vfs('%s.pending' % filename, **kwargs), True)
        except IOError as inst:
            if inst.errno != errno.ENOENT:
                raise
    return (vfs(filename, **kwargs), False)