[PATCH 1 of 4 RFC] chgcache: new experimental extension

[Jun Wu]

Excerpts from Yuya Nishihara's message of 2017-02-13 22:46:23 +0900:
> On Wed, 8 Feb 2017 17:41:01 -0800, Jun Wu wrote:
> > # HG changeset patch
> > # User Jun Wu <quark at fb.com>
> > # Date 1486601732 28800
> > #      Wed Feb 08 16:55:32 2017 -0800
> > # Node ID 138f7ba58a70de9610713b8bd55d1ba0ac468fa6
> > # Parent  a68510b69f413545722c086eaeb840dd5e8305b4
> > # Available At https://bitbucket.org/quark-zju/hg-draft 
> > #              hg pull https://bitbucket.org/quark-zju/hg-draft  -r 138f7ba58a70
> > chgcache: new experimental extension
> > 
> > This patch starts a new experimental extension - chgcache, which aims to
> > make chg stateful and be able to preload repo objects like the changelog,
> > the dirstate, the obsstore etc.
> > 
> > The feature is being developed as a new extension, instead of directly at
> > core chg, because its APIs and implementation details are still subject to
> > change.
> 
> I don't get why we're starting it as an extension. Can you elaborate?
> 
> My random thoughts:
> 
>  a) because the feature is unstable
>     => could be gated by config knob.
>  b) for faster experiment of ideas
>     => out-of-core extension might be better.
>  c) to dump the whole implementation in case it turns out bad
>     => maybe.

I actually prefer doing it in core directly, but I don't have enough
confidence on the final API. I guess it's fine as the users of the API would
be limited.

Another slight concern is putting all the API code in chgserver.py will make
it much longer - probably also fine as long as different sections are
separated by comments clearly.