hgext/narrow/narrowrepo.py

commandserver: add IPC channel to teach repository path on command finished

The idea is to load recently-used repositories first in the master process,
and fork(). The forked worker can reuse a warm repository if it's preloaded.

There are a couple of ways of in-memory repository caching. They have pros
and cons:

 a. "preload by master"
    pros: can use a single cache dict, maximizing cache hit rate
    cons: need to reload a repo in master process (because worker process
          dies per command)
 b. "prefork"
    pros: can cache a repo without reloading (as worker processes persist)
    cons: lower cache hit rate since each worker has to maintain its own cache
 c. "shared memory" (or separate key-value store server)
    pros: no need to reload a repo in master process, ideally
    cons: need to serialize objects to sharable form

Since my primary goal is to get rid of the cost of loading obsstore without
massive rewrites, (c) doesn't work. (b) isn't ideal since it would require
much more SDRAMs than (a). So I take (a).

The idea credits to Jun Wu.

# narrowrepo.py - repository which supports narrow revlogs, lazy loading
#
# Copyright 2017 Google, Inc.
#
# 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

from mercurial import (
    wireprototypes,
)

from . import (
    narrowdirstate,
)

def wraprepo(repo):
    """Enables narrow clone functionality on a single local repository."""

    class narrowrepository(repo.__class__):

        def _makedirstate(self):
            dirstate = super(narrowrepository, self)._makedirstate()
            return narrowdirstate.wrapdirstate(self, dirstate)

        def peer(self):
            peer = super(narrowrepository, self).peer()
            peer._caps.add(wireprototypes.NARROWCAP)
            peer._caps.add(wireprototypes.ELLIPSESCAP)
            return peer

    repo.__class__ = narrowrepository