mercurial/txnutil.py

narrow: introduce a config option to check if narrow is enabled or not

This patch introduces a new config option experimental.narrow which is set to
False by default and set to True by the narrow extension.

While moving narrow related logic into core, we need to know at places whether
narrow extension is enabled or not. Checking the list of extension enabled is
one solution but once narrow is inbuilt, we will definitely want a config option
to check whether narrow is turned on or not.

So this patch introduces a config option, which will evolve to the main point to
turn narrow capability on and off once all the narrow is in core.

Differential Revision: https://phab.mercurial-scm.org/D4889

# 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)