Re: [PATCH] diff: fix up SHA-1 abbreviations outside of repository

[Jeff King <peff@peff.net>]

On Sun, Dec 04, 2016 at 11:19:46PM -0800, Junio C Hamano wrote:

> > -	if (no_index)
> > +	if (no_index) {
> >  		/* If this is a no-index diff, just run it and exit there. */
> > +		startup_info->have_repository = 0;
> >  		diff_no_index(&rev, argc, argv);
> > +	}
> 
> This kind of change makes me nervous (partly because I am not seeing
> the whole code but only this part of the patch).
> 
> Some code may react to "have_repository" being zero and do the right
> thing (which I think is what you are using from your previous "we
> did one of the three cases" change here), but the codepath that led
> to "have_repository" being set to non-zero previously must have done
> a lot more than just flipping that field to non-zero, and setting
> zero to this field alone would not "undo" what it did.

I _think_ it's OK because the only substantive change would be the
chdir() to the top of the working tree. But that information is carried
through by revs->prefix, which we act on regardless of the value of
startup_info->have_repository when we call prefix_filename().

I agree that it may be an accident waiting to happen, though, as soon as
some buried sub-function needs to care about the distinction.

> I wonder if we're better off if we made sure that diff_no_index()
> works the same way regardless of the value of "have_repository"
> field?

If you mean adding a diffopt flag like "just abbreviate everything to
FALLBACK_DEFAULT_ABBREV even if we're in a repository", and then setting
that in diff_no_index(), I agree that is a lot cleaner.

I'm still not 100% convinced that it's actually the correct behavior,
but at least doing a more contained version wouldn't take away other
functionality like reading config.

-Peff