Re: Keeping changes in sync with upstream projects

[Stefan Monnier <monnier@iro.umontreal.ca>]

These are very good questions, thanks for asking.
The Gnus guys have their own answer to that (not sure how advanced it
is by now), so maybe they could say how well it works for them.

The 2-way sync is the harder part.  And doing it *with* differences
(aka "discarded/held changesets"), it's even worse.
So I think the right course of action is something like:

- start with 4 version of the code:

  [upstream] <-> [clean upstream] <-> [clean Emacs] <-> [Emacs]

  where "clean upstream" means "upstream without the patches that
  shouldn't make it into Emacs" and "clean Emacs" means "Emacs without
  the changes that shouldn't make it into upstream".  I.e. [clean
  upstream] and [clean Emacs] should be virtually identical, except
  maybe for file layout and VCS.

- first simplification: reject Emacs patches that can't make it
  into upstream.  I.e. enforce [clean Emacs] == [Emacs].
  That means that when an Emacs patch is unacceptable upstream, it needs
  to be resolved somehow: either one side convinces the other, or the
  patch needs to be reworked.  Typically such changes are linked to
  features that don't exist/work in XEmacs or in older Emacsen, so the
  problem can resolved by adding compatibility code.

- second simplification: don't install into [upstream] patches that
  should make it into Emacs; instead install them into [clean upstream].

- the flow is now limited:

  [upstream] <- [clean upstream] <-> [Emacs]

- so [upstream] is basically a normal feature-branch that tracks the
  "trunk" aka [clean upstream].  Any DVCS will handle that just fine.

- so all that's left is the core 2-way sync.  I don't know of any
  easy/right way to do that.

- of course, the best is to add a third simplification: let upstream use
  [Emacs] for development, so the "2-way" sync is a nop.


        Stefan