view mercurial/txnutil.py @ 44592:7cd5c0968139

templater: add subsetparents(rev, revset) function

Naming suggestions are welcome. And this could be flagged as an (ADVANCED)
function since the primary use case is to draw a graph.

This provides all data needed for drawing revisions graph filtered by revset,
and allows us to implement a GUI graph viewer in some languages better than
Python. A frontend grapher will be quite similar to our graphmod since
subsetparents() just returns parent-child relations in the filtered sub graph.

Frontend example:

  https://hg.sr.ht/~yuja/hgv/browse/default/core/hgchangesetgrapher.cpp

However, the resulting graph will be simpler than the one "hg log -G" would
generate because redundant edges are eliminated. This should be the same graph
rendering strategy as TortoiseHg.

This function could be implemented as a revset predicate, but that would mean
the scanning state couldn't be cached and thus slow.

Test cases are split to new file since test-template-functions.t is quite
big and we'll need a new branchy repository anyway.

# 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)