D808: config: use copy-on-write to improve copy performance

[quark (Jun Wu)]

quark added inline comments.

INLINE COMMENTS

> mbthomas wrote in util.py:602
> Using a boolean means both sides have to copy when they each write - the original dict becomes frozen and can't be re-used.  If we use a count of outstanding copies instead, the last copy can just be used as-is when the count goes to 0.
> 
> I don't know how much modification of the original occurs after copy, so maybe this isn't important.

Doing refcount adds 10% overhead for `ui.copy()` benchmark. I guess it's fine. If we really care about it, we can use a native `cow` class.

> mbthomas wrote in util.py:633
> Any way we can check/enforce this?  I'm guessing not without impacting perf.

I think we want performance here. If it becomes a concern, we can add extra checks if `devel.something` is set.

Side note: having a native class in inheritance hierarchy is important for CPython performance according to what I learned. I thought `pure_obj.__contains__ = native_obj.__contains__` may have a reasonable perf, but the former is about 4x slower. So `class pure(native)` is preferred for perf (but probably not design pattern's point of view).

REPOSITORY
  rHG Mercurial

REVISION DETAIL
  https://phab.mercurial-scm.org/D808

To: quark, #hg-reviewers, mbthomas
Cc: mbthomas, mercurial-devel