mercurial/txnutil.py

shelve: rewrite check for unknown shelf to delete

The code would try to delete the shelf's .patch file and if that
raised an exception, it would convert it to an `error.Abort`. This
patch rewrites it so the check is done upfront. I find it easier to
read that way. It's now clear enough that I removed the comment
explaining it as well.

As Joerg pointed out during review, another differences is that the
old code would move a `.hg` file without its `.patch` friend to backup
before it realized that the `.patch` file was missing. The new code
will error out earlier and not move the `.hg` file, which seems like
an improvement. That should only matter on corrupt `.hg/shelved/`
directories, however.

Differential Revision: https://phab.mercurial-scm.org/D9697

# 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)