Re: `git blame` Line Number Off-by-one

From: Jeff King <peff@peff.net>
Date: Aug 7, 2020, 22:35 UTC
Message-ID: <20200807223522.GB3750245@coredump.intra.peff.net>
In-Reply-To: <20200807222630.GA3750245@coredump.intra.peff.net>

On Fri, Aug 07, 2020 at 06:26:30PM -0400, Jeff King wrote:

Show 17 quoted lines

> On Fri, Aug 07, 2020 at 03:05:27PM -0700, Junio C Hamano wrote:
> 
> > Jeff King <peff@peff.net> writes:
> > 
> > > Dropping it entirely, as below, doesn't break any tests. Junio, do you
> > > know of a case this is meant to improve?
> > 
> > I think the only conceivable case is that in the middle of a single
> > block of text in an ancient version, another block of lines gets
> > inserted during the evolution of the file, but in the end these
> > intermediate edits all go away and the same original text remains.
> > 
> > In such a case, without coalescing, we would not treat the original
> > single block of text as a single unit.
> 
> Yeah, that makes sense, and it should be possible to construct a case
> based on that.

It was even easier than I expected. :)

Try this:

  git init repo
  cd repo
  
  seq 20 >file
  git add file
  git commit -m base
  
  { seq 10; echo foo; seq 11 20; } >file
  git commit -am cruft
  
  seq 20 >file
  git commit -am revert

  git blame --root --porcelain file

With the coalescing, we get a single 20-line block:

  25d93ad4c4b27570ccd4b6412ccaa549ae6f9ff7 1 1 20
  author Jeff King
  ...
          1
  ...
  25d93ad4c4b27570ccd4b6412ccaa549ae6f9ff7 11 11
          11

and without it, we get two 10-line blocks:

  25d93ad4c4b27570ccd4b6412ccaa549ae6f9ff7 1 1 10
  author Jeff King
  ...
          1
  ...
  25d93ad4c4b27570ccd4b6412ccaa549ae6f9ff7 11 11 10
          11

I doubt it really matters all that much in practice, but it's possible some consumer of --porcelain relies on us giving minimal blocks.

So that argues for fixing blame_coalesce() to make sure that we coalesce only when the blocks are adjacent in both the source and the result. Otherwise we're losing information.

-Peff