mercurial/txnutil.py

cachevfs: add a vfs dedicated to cache

Most of the cache content lives in '.hg/cache/'. Moreover they are computed
exclusively from data in the '.hg/store' directory. This creates issues with
the share extension as the '.hg/store' directory is shared but the '.hg/cache'
is not. On large repositories, this makes this prevent some usage of the share
extension inefficient as some caches can take minutes to be recomputed.

To improve the situation, we introduce a new 'cachevfs' that will be dedicated
to cache reading and writing. In the next patches of this series, we'll
migrate the 4 existing caches to it and update the share extension.

# 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)