Hi Peff,

On Thu, 12 Nov 2020, Jeff King wrote:

> On Thu, Nov 12, 2020 at 03:20:38PM +0100, Johannes Schindelin wrote:
>
> > >   $ git -c interactive.difffilter='diff-so-fancy' add -p
> > >   ──────────────────────────────────────────────────────────────────────
> > >   modified: file
> > >   ──────────────────────────────────────────────────────────────────────
> > >   @ file:1 @
> > >   old
> > >   new
> > >   (1/1) Stage this hunk [y,n,q,a,d,e,?]?
> >
> > It might _seem_ that it works. But try to split a hunk. I did this with
> > the test case (interrupting it just before running `add -p`):
>
> Yes, there are definitely problems with how diff-so-fancy represents
> things (because they don't maintain a 1-to-1 correspondence). You should
> generally get a complaint like:
>
>   $ git -c interactive.difffilter='diff-so-fancy' add -p
>   fatal: mismatched output from interactive.diffFilter
>   hint: Your filter must maintain a one-to-one correspondence
>   hint: between its input and output lines.
>
> The diff-so-fancy folks have asked me about this, and I've made clear
> the problem to them, and that the solution is to have a mode which
> maintains the line correspondence. AFAIK there's no fix yet. I don't
> know how many people are actually using it in practice in its current
> buggy state.
>
> There's a big thread here:
>
>   https://github.com/so-fancy/diff-so-fancy/issues/35
>
> I don't really recommend reading the whole thing. Beyond what I just
> said, the interesting bits are:
>
>   - people recommend using the "delta" tool; it has a "--color-only"
>     option which does just diff-highlight-style coloring, but doesn't
>     break line correspondence
>
>   - somebody suggested instead of using interactive.difffilter, to add
>     an option to show each hunk in an individual pager command. So
>     add-interactive would never see the "fancy" version at all, but it
>     would be generated on the fly when the user sees it (and that filter
>     would have to be able to handle seeing individual hunks without the
>     diff header).
>
> All of which is to say that the current state is a bit of a mess, and I
> don't consider it completely necessary to fix it before the builtin
> version becomes the default. But:
>
>   - I think we should expect some possible complaints / bug reports
>     from fringe users of the already-somewhat-broken state
>
>   - IMHO the parsing of the filtered version done by the builtin is
>     going in the wrong direction (see my other mail for an elaboration)

It's a little bit of a surprise that this is the first time I hear about
this.

The way _I_ would go about fixing this is to look for a tell-tale that
we're looking at a `diff-so-fancy` style output, e.g. by looking for that
horizontal line, then adding special code to handle that.

This is not a minor undertaking, and it would currently require _two_
implementations: the Perl version and the built-in version. They would
look _very_ different from one another. Therefore, I would probably either
wait until the Perl version is retired, or selectively only adjust the
built-in version, if _I_ was to implement this.

But given that it does not work right now, and given that it has not been
working for a long time, I think it does not hurt so much if it is left in
the current state for a bit longer. I would love to focus on the patch
series that kicked off this discussion first, getting it done, before I
think about other `add -p` aspects.

Ciao,
Dscho

>
> > > While it may touch on some of the coloring code, it's otherwise
> > > orthogonal to the fixes here. And while the fix is likely to be to make
> > > render_hunk() stop re-coloring in the non-edit cases, your more-robust
> > > test here will still be checking what you expect (because it really is
> > > triggering the edit case, not relying on the extra coloring to see the
> > > effect of in-process color config).
> >
> > I don't actually think that we _can_ stop re-coloring the hunk header in
> > the general case because we need to keep this working even for split
> > hunks. It would be a very bad idea to make it work for non-split cases and
> > then something like `die()` only when the hunk is split. Better re-color
> > all of them, then, to not even risk that situation.
>
> Yeah, obviously calling die() in the split case is bad. And the fact
> that newly-created split hunk headers are not filtered the same way as
> the original hunk headers isn't ideal. But it's a pretty easy fix in the
> perl version, and not in the builtin version.
>
> -Peff
>