AW: Understanding behavior of git blame -M

From: Sokolov, Konstantin (ext) <konstantin.sokolov.ext@siemens.com>
Date: Aug 15, 2014, 20:54 UTC
Message-ID: <71BF70CE41AEE741896AF3A5450D86F11F28C786@DEFTHW99EH3MSX.ww902.siemens.net>
In-Reply-To: <874mxderwj.fsf@fencepost.gnu.org>

Hi David,

thank you very much for the exhaustive answer. The keyword "hunk" made me try a little bit more. So I realized that -M works as expected when at least three lines are moved.

From your answer I discern that you find the current behavior correct. In my opinion, it diverges at least from the documented behavior, as the documentation doesn't mention this "number of lines" aspect but rather speaks about "number of alphanumeric characters".

Regards Konstantin

-----Ursprüngliche Nachricht-----
Von: David Kastrup [mailto:dak@gnu.org] 
Gesendet: Freitag, 15. August 2014 16:42
An: Sokolov, Konstantin (ext)
Cc: git@vger.kernel.org
Betreff: Re: Understanding behavior of git blame -M

"Sokolov, Konstantin (ext)" <konstantin.sokolov.ext@siemens.com> writes:

Show 40 quoted lines

> Hi Folks,
>
> I'm trying to understand the behavior of git blame -M and find that 
> the actual results differ from what I understood from the 
> documentation. I've already asked longer time ago on stackoverflow and 
> on the user mailing list without any satisfactory results. So here is 
> the example:
>
> Initial content of file.txt (2cd9f7f)
>
> AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA
> BBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBB
> CCCCCCCCCCCCCCCCCCCCCCCC2222222222222222222222222
> DDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDD
> EEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEE
> GGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGG
> FFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFF
>
> Move line B to the middle (d4bbd97e):
>
> AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA
> CCCCCCCCCCCCCCCCCCCCCCCC2222222222222222222222222
> DDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDD
> BBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBB
> EEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEE
> GGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGG
> FFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFF
>
>>git blame -s -n -f -w -M20 file.txt
> ^2cd9f7f 1 1) AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA
> ^2cd9f7f 3 2) CCCCCCCCCCCCCCCCCCCCCCCC2222222222222222222222222
> ^2cd9f7f 4 3) DDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDD
> d4bbd97e 4 4) BBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBB
> ^2cd9f7f 5 5) EEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEE
> ^2cd9f7f 6 6) GGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGG
> ^2cd9f7f 7 7) FFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFF
>
> I wonder, why line B is not recognized as moved. According to the 
> documentation, I would expect git blame to report that it originates 
> from line 2 in revision 2cd9f7f. Can anybody explain the behavior?

Someone had reasons.  diff_hunks in builtin/blame.c is once called with 0 as third argument, once with 1.  Change the latter call to using 0 as well and you get your expected result:

dak@lola:/tmp/test$ /usr/local/tmp/git/git blame -s -n -f -w -M20 file.txt ^2cab496 file.txt 1 1) AAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAAA ^2cab496 file.txt 3 2) CCCCCCCCCCCCCCCCCCCCCCCC2222222222222222222222222 ^2cab496 file.txt 4 3) DDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDDD ^2cab496 file.txt 2 4) BBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBBB ^2cab496 file.txt 5 5) EEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEEE ^2cab496 file.txt 6 6) GGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGGG ^2cab496 file.txt 7 7) FFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFFF

The function diff_hunks is a wrapper for the diff engine. Putting the context length explicitly into this wrapper (rather than not passing an argument and just setting the context length to zero anyway in the function) clearly indicates that somebody _wanted_ it called with different values.

There is no documentation or rationale in the file _why_ as far as I remember.  Maybe it can crash or end up in an infinite loop.  Maybe it could do so at one point of time but no longer does.

Maybe Git is just a puzzle from genius to genius.  Good luck figuring it out.

I have not touched this when rewriting git-blame recently, and I am not interested in touching it.  I stand absolutely nothing to gain from working on Git.

-- David Kastrup