mercurial/txnutil.py

phabricator: rework phabread to reduce memory usage and round-trips

This patch reworked phabread a bit so it fetches the lightweight
"Differential Revision" metadata for a stack first. Then read other data.

This allows the code to:

  a) send 1 request to get all `hg:meta` data instead of N requests
  b) patches are read in desired order, no need to buffer the output

"b)" reduces the memory usage from O(N^2) to O(N) since we no longer keep
old patch contents in memory.

The above `N` means the number of patches in the stack.

# 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)