bug#21998: Run 'make change-history' on release branch

[Eli Zaretskii <eliz@gnu.org>]

> From: Glenn Morris <rgm@gnu.org>
> Cc: John Wiegley <johnw@gnu.org>,  21998@debbugs.gnu.org
> Date: Tue, 08 Mar 2016 02:32:12 -0500
> 
> > I think if we care at all about having ChangeLog in the releases, we
> > should simply reinstate the file and maintain it in the repository.
> 
> FWIW, that's not what I was hoping would come from this, and I think
> that would be a retrograde step.

Can you tell why?  It solves all the problems you mention in your
mail, at negligible costs.  So it seems to be a clear winner.

> For 1) and 2), experience shows that few people will bother to make
> corrections.

Is it an important drawback that few people bother to make
corrections?  If it is, then any solution that involves such
corrections is not what we want.

Also, is the situation with corrections worse or better than what it
was when we maintained ChangeLog files in the repository?

> PS For the record, to explain the actual merging issue as I see it:
> 
> Suppose emacs-25 and master are synced at revision aaa and ChangeLog.2
> is up-to-date. emacs-25 advances to rev xxx, and independently master to
> rev xxx'. emacs-25 gets ChangeLog.2 updated, and merged to master. Now
> the footer of master ChangeLog.2 reports that it is up-to-date to rev
> xxx. What does this mean for the changes on master between aaa and xxx'?
> Because xxx on master is "after" xxx', I suspect it means they end up
> missing from ChangeLog.2 forever, which is bad.

No, they don't end up missing.  They will in general be in the "wrong"
order, time-wise, though.  But what is the "right" order for this
situation, where changes are made in parallel on several branches?  Do
we want them interwoven, in strict order of their commit times?  Or do
we want all the entries of a merge from the branch be kept together
with the time stamp of the merge?  If we want to continue keeping a
single generated ChangeLog file on master and on the branch, we need
to decide what is the desired result.

> But maybe there's no such issue, or it is fixable with some git
> trivia.

The only "git trivia" that's possible is a custom merge driver (which
AFAIK doesn't exist).  Git itself is not aware of the special meaning
of the time stamps in ChangeLog entries.

We could also have some Lisp rearranging the entries in whatever order
we decide we want it, after git-merge-changelog puts them in the order
it thinks is right.

> I don't know. That's why I made this bug report. AFAICS, the adopted
> "solution" was simply to ignore the issue, which means
> master/ChangeLog.2 is (probably) messed up at the moment.

It is not messed, but it isn't in chronological order, either.  And it
looks like no one ran "make change-history" on master for several
moons, so problems might become worse when they do.