Subject: Re: Locating merge that dropped a change

From: Kevin Bracey <[email-masked]>
Date: Thu, 11 Apr 2013 20:28:37 +0300

On 09/04/2013 21:00, Kevin Bracey wrote:
>
> So, how to automatically find a merge that ignored a known change?

I think I've found the problem. It only doesn't work _if you specify the 
file_.

Specifically, if I was missing an addition, my first attempt to find it 
would be

   git log -p -m -S<addition> <file>

If the addition was lost in a merge, that doesn't even show the 
addition, which is surprising, but intentional. The addition isn't part 
of the HEAD version of <file>, so no point going down that path of the 
merge. Fine. However, I expected this to work:

   git log --full-history -p -m -S<addition> <file>

But it doesn't. It finds the addition, but _not_ the loss in the merge 
commit.

But this does work:

   git log -p -m -S<addition>

That really feels like a bug to me. By specifying a file, I've made it 
miss the change, and I can see no way to get the change without making 
it a full-tree operation.

Looking at try_to_simplify_commit() it appears the merge that removed 
the addition is excluded because it's TREESAME to its _other_ parent. 
But with --full-history, we should only be eliminating a merge if its 
TREESAME to all parents, surely? Otherwise we have this case that we 
show a commit but not its reversion.

And the code doing this looks broken, or at least illogical - the parent 
loop sets "tree_same" for a same parent in --full-history, rather than 
immediately setting the TREESAME flag, so maybe previous authors were 
thinking about this. But setting tree_same guarantees that TREESAME is 
always set on exit anyway, so it's pointless (unless I'm missing something).

It does appear this is documented behaviour in the manual: "full-history 
without parent rewriting" .. "P and M were excluded because they are 
TREESAME to _a_ parent." I would say that they should have been excluded 
due to being TREESAME to _all_ parents. I really don't want a merge 
where one version of my file was chosen over another excluded from its 
so-called "full history".

Now, obviously in a sane environment, most merges won't be that 
interesting, as they'll just be propagating wanted patches from the real 
commits already in the log. But I'd like some way to find merges that 
drop code in a specified file, and surely "--full-history" is it?

Kevin