Re: [PATCH v2 2/3] blame: add the ability to ignore commits and their changes

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

René Scharfe <l.s.r@web.de> writes:

> Am 17.01.2019 um 21:29 schrieb Barret Rhoden:
>> The blame_entry will get passed up the tree until we find a commit that
>> has a diff chunk that affects those lines.  If an ignored commit added
>> more lines than it removed, the blame will fall on a commit that made a
>> change nearby.  There is no general solution here, just a best-effort
>> approach.  For a trivial example, consider ignoring this commit:
>>
>> Z: "Adding Lines"
>>  foo
>> +No commit
>> +ever touched
>> +these lines
>>  bar
>
> Wouldn't it make more sense to assign such lines to unknown, perhaps
> represented by an all-zero commit ID, instead of blaming a semi-random
> bystander?

I share the sentiment.

Isn't it, however, showing a bigger problem in the "feature"?

Your "a change that adds lines without removing any" is an obvious
case where these added lines have no corresponding lines in the
preimage, but most of the time it is unclear what line corresponds
to what previous line.  If a commit being "ignored" brought a change
like this:

     1
    -four
    -three
    +3
    +4
     5

did "+3" come from "-three"?

Or did "+4" (read: "added '4'") come from "-three" (read: "removed
'three'")?  Did it come from "-four"?  Or was it genuinely added by
that ignored commit?  Your suggestion deals with the case where we
decide that "+4" had no corresponding lines in the preimage (and
paint it as "no blame can be assigned").  But when we decide that
"+4" came from "-four" or "-three", we continue drilling down from
that ignored commit and assign the blame to either the commit that
introduced "four" or the commit that introduced "three", which would
obviously give us different result.  Worse yet, if a reader expected
us to consider "+4" came from "-four" at that ignored commit, but
the algorithm decided that "+4" corresponded to "-three", when we
show the commit that eventually gets blamed for that line that has
"4" has no "four" (it has "three"), which I suspect would confuse
the reader of the output.

So... I dunno.