# txnutil.py - transaction related utilities

#

#  Copyright FUJIWARA Katsunori <foozy@lares.dti.ne.jp> 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.

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)