mercurial/txnutil.py

localrepo: document and test bug around opening shared repos

As part of refactoring this code, I realized that we don't
validate the requirements of a shared repository. This commit
documents that next to the requirements validation code and adds a
test demonstrating the buggy behavior.

I'm not sure if I'll fix this. But it is definitely a bug that
users could encounter, as LFS, narrow, and potentially other
extensions dynamically add requirements on first use. One part
of this I'm not sure about is how to handle loading the .hg/hgrc
of the shared repo. We need to do that in order to load extensions.
But we don't want that repo's hgrc to overwrite the current repo's.

Differential Revision: https://phab.mercurial-scm.org/D4572

# 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)