bug#20637: incompatible, undocumented change to vc-working-revision

[Michael Albinus]

Dmitry Gutov <dgutov@yandex.ru> writes:

> Hey Michael,

Hi Dmitry,

> Problem: vc-registered is slower than vc-backend. Like, orders of
> magnitude slower. vc-backend caches the result of the previous
> vc-registered invocation in vc-file-prop-obarray. But if we call
> vc-registered directly, we go the whole way each time, including
> calling vc-BACKEND-registered.

I haven't thought too much about performance. But you are right, we
shouldn't add serious performance penalties to the code. And improving
performance for the 25.1 release is much too late.

So we might revert the patch for vc-state and vc-working-revision indeed
for the emacs-25 branch, going back to using vc-backend.

In the master branch we might apply my proposed patch using
vc-registered or something similar, and start to improve performance. In
parallel, we shall start to write a VCS section for the elisp manual,
describing vc-* functionality in more detail. We could start with
vc-backend and vc-responsible-backend and their intended use. I'm
missing such documentation for years.

I'll come back later today with the patch for emacs-25, if you agree.

Best regards, Michael.