view mercurial/txnutil.py @ 51234:59183a19954e

rust-index: use interior mutability in head revs and caches

For upcoming changes in `hg-cpython` switching to the `hg-core` index in
ancestors iterators, we will need to avoid excessive mutability, restricting
the use of mutable references on `hg::index::Index` to methods that actually
logically mutate it, whereas the maintenance of caches such as `head_revs`
clearly does not. We illustrate that immediately by switching to immutable
borrows in the corresponding methods of `hg-cpython::MixedIndex`

# 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 . 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 FileNotFoundError:
            pass
    return (vfs(filename, **kwargs), False)