view mercurial/txnutil.py @ 46393:66e8e279133b

hghave: list the module needed for the `vcr` check

I'm tired of having to look up modules each time I setup a system, and try to
distinguish between similar package names to get the right one.  Now that the
search API has been disabled, it's even harder.  There are other python packages
here that should be listed like this, but this is the one that came up missing
today, so it's a start.

Differential Revision: https://phab.mercurial-scm.org/D9879

# 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(b'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(b'%s.pending' % filename, **kwargs), True)
        except IOError as inst:
            if inst.errno != errno.ENOENT:
                raise
    return (vfs(filename, **kwargs), False)