view mercurial/txnutil.py @ 43137:81efc4a295e7

sidedatacopies: add a new requirement for storing copies into sidedata

The end goal is to have changesets centric sidedata information stored into
changelog sidedata. This make it possible to use the changeset based copy
tracing algorithm on any repository without affecting hashes.

The actual implementation is coming. The feature is marked as experimental
(do not use in production) until we stabilise details about the format.

Differential Revision: https://phab.mercurial-scm.org/D6945

# 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)