Re: git diff -B: splitting up complete renames

[Junio C Hamano <gitster@pobox.com>]

Jakub Narebski <jnareb@gmail.com> writes:

> ...
>   and if it detects that the file "file0" is completely rewritten,
>   it changes it to:
>
>   ------------------------------------------------
>   :100644 000000 bcd1234... 0000000... D file0
>   :000000 100644 0000000... 0123456... A file0
>   ------------------------------------------------
>
> Shouldn't the last block read for the modern git read:
>
>   ------------------------------------------------
>   :100644 000000 bcd1234... 0123456... M99 file0
>   ------------------------------------------------

Correct.  The description is based on an earlier round of the
code before 366175ef8c3b1e145f4ba846e63a1dea3ec3cacc (Rework -B
output).  "git show" of that patch has description in its
changes to Documentation/diffcore.txt.

The original motivation of -B was not about helping three way
renames it allows you when used with -M/-C.  It was about
showing textual diff more readably for quite dissimilar
filepairs.

> Third: Do "git diff --no-index" (filesystem diff) can show breaking / 
> use dissimilarity? I couldn't make it work...

It doesn't, for two reasons.

 * Look at diffcore_break().  It breaks filepairs with identical
   paths and nothing else.

   The reason for this check is that the caller can call
   diffcore_rename() _before_ diffcore_break() [*1*] and a
   filepair that was detected as renamed pair ought to be
   already found similar enough.  These days, I think you can
   replace the check with p->renamed_pair.

 * Look at diffcore_merge_broken(), which is responsible for
   re-merging a broken pair.  Because we match them by looking
   at their names (iow, we do not use pointers to link halves of
   a broken pair pointing at each other).

   This was correct before "no-index" stuff because "git diff"
   was about comparing pairs of blobs found at corresponding
   paths in two tree-like things (i.e. "a/one" and "b/one"
   corresponds to each other when you do "git diff -- one").
   The modification to introduce no-index forgot to update this
   logic.

If you wanted to fix this, you can change p->broken_pair from a
boolean to a pointer that points at the other half of the broken
pair (and record that when we break a pair in diffcore_break()),
and look at that pointer to decide which one to match up with
which other one in diffcore_merge_broken().  Together with a
change to look at p->renamed_pair instead of paths I mentioned,
I think it would work more like the regular git diff for
"no-index" case.

I consider that "no-index" frontend more or less a bolted on
half-baked hack that covers only minimally necessary cases to
serve as non-git "diff" replacement, without sharing enough with
the real "git diff" internals; I would not be surprised at all
if there are more corner cases like this that does not work.
But I do not think it is fundamentally unfixable.  The change to
add "no-index" support just needed to be more careful, and it is
not too late to make it so.


[Footnote]

*1* diffcore was designed as a generic library to allow
experimenting more combinations of transformations than what
then-current git used.  All of our existing callers ended up
calling the transformations in the same order (i.e. the order
diffcore_std() calls them), but individual transformations were
written not to assume too much about the order they would be
called.