Re: [PATCH] log: use true parents for diff even when rewriting

[Thomas Rast <trast@inf.ethz.ch>]

Junio C Hamano <gitster@pobox.com> writes:

> Thomas Rast <trast@inf.ethz.ch> writes:
>
>> When using pathspec filtering in combination with diff-based log
>> output, parent simplification happens before the diff is computed.
>> The diff is therefore against the *simplified* parents.
>>
>> This works okay, arguably by accident, in the normal case: the pruned
>> commits did not affect the paths being filtered, so the diff against
>> the prune-result is the same as against the diff against the true
>> parents.
>>
>> However, --full-diff breaks this guarantee, and indeed gives pretty
>> spectacular results when comparing the output of
>>
>>   git log --graph --stat ...
>>   git log --graph --full-diff --stat ...
>>
>> (--graph internally kicks in parent simplification, much like
>> --parents).

Hmm, I stopped writing the message midway through.  There should be
another two paragraphs here about storing the original parent list on
the side for later use when showing the diff.

>> Perhaps like this.  It's getting a bit late, so I'm not sure if I'm
>> missing another user of the "true" parent list, but it does fix the
>> issue you reported.
>
> Conceptually I can see how this will change the history
> simplification in the vertical direction (skipping the ancestry
> chain and jumping directly to the closest grandparent that touched
> the specified path), but I am not sure how well this interacts with
> history simplification in the horizontal direciton (culling
> irrelevant side branches from the merge).

But isn't that similarly confusing for the user as Uwe's original
problem?  Suddenly we'd be showing a merge commit as an ordinary one,
simply because the merged history did not affect the filtered
pathspecs.  Thus we would show everything that has been merged on the
*other* files as a big diff.  Would that be useful?  It would certainly
be a big difference in how the commit is shown.

> I also have to wonder if we always want to incur this save-parents
> overhead, or we are better off limiting it to only when --full-diff
> is in effect.

I haven't quite convinced myself that it is 100% safe to use the
rewritten parents when --full-diff is not in effect...

-- 
Thomas Rast
trast@{inf,student}.ethz.ch