Subject: Re: [PATCH v2 0/3] rev-parse and "--"

From: Jonathan Nieder <[email-masked]>
Date: Mon, 9 Dec 2013 12:56:21 -0800

Junio C Hamano wrote:

> I do not share the "with --verify is better hence no problem"
> reasoning.  My "not so much worth worrying about" comes solely from
> a hunch that nobody has "HEAD~3..HEAD" in their working directory,

That's what makes it a problem.  This change makes it very easy to
make a general-purpose script that breaks in an edge case that the
script's author is not likely to run into.  Then as soon as someone
adds a file with such a name to the test data in their repo, their
favorite general-purpose repo munger just breaks.

If we wanted to forbid git from tracking files named HEAD~3..HEAD
altogether, that would be a different story.

> and if somebody has one, then they must be using "--verify" (or a
> clarifying "--"), because their "git log" and whatever they use "git
> rev-parse HEAD~3..HEAD" for would behave very differently otherwise.

Isn't protecting against this kind of thing the reason we ask authors
of general-purpose scripts to use "simple, do what I say and not what
I mean" plumbing commands?

Another relevant detail is that using rev-parse with "--" is more
painful than without, since it includes the "--" in its output.
Without this change, it seems much more likely to me that someone
would do

	git rev-parse <commits> |
	while read commit
	do
		...
	done

than

	git rev-parse <commits> -- |
	while read commit
	do
		if test "$commit" = "--"
		then
			continue
		fi

		...
	done

> So it is not merely "--verify is better"---in a situation where the
> backward incompatibility matters, I doubt the existing behaves
> sensibly in the first place.

What in the former of the above two loops is broken?

> But if we cook it for a while, I suspect that we will find more and
> more breakages of expectations in the existing scripts in and out of
> the tree;

Alas, probably no, because nobody has "HEAD~3..HEAD" in their working
directory.  That's exactly the problem --- it creates an edge case
that nobody is likely to test until the once-in-a-few-years moment
when they need it.

Jonathan