threads / discuss / 33993

preventing evil merges

Subject: preventing evil merges

## tl;dr

3 messages between May 30, 2013 and Jun 3, 2013.

replies: 2people: 3as markdown or json

Sandro Santilli· May 30, 2013, 16:34 UTC · lore

Hey all, I've be burnt by what someone on IRC referred to as "evil merges", that is loss of history after amending a merge commit:

 git merge anotherbranch
 git add something
 git commit --amend

After the steps above the addition of "something" can't be found in the history anymore, but the file is there. Moreover (I think but didn't try to reproruce) a further rebase to a common ancestore of my working branch and "anotherbranch" resulted in further loss of the changes. The only way for me to get them back has been by checking out from reflog.

I've no idea about what was going on but this experience reminded me of another one I had in the past in which we could not figure out when some changes were added into a repository (!).

If amending a merge is so dangerous, would it make sense to require and hard-to-type switch in order for it to really do anything ?

--strk;

Junio C Hamano· Jun 3, 2013, 17:20 UTC · re: Sandro Santilli · lore

Re: preventing evil merges

Sandro Santilli <strk@keybit.net> writes:

Show 6 quoted lines

>  git merge anotherbranch
>  git add something
>  git commit --amend
>
> After the steps above the addition of "something" can't be found in
> the history anymore, but the file is there.

This is a very common and sensible thing to do when dealing with semantic conflict. Imagine that you changed the name of a global variable in the code on your current branch since the anotherbranch you are pulling from forked from you. Then imagine further that the anotherbranch added one location that refers to that variable.

Since they are not aware of the name change, they added the new reference with the old variable name. The part they added is a new code, so it is very likely that there is no textual conflict when you did "git merge anotherbranch". But now the result is broken.

And you fix that semantic conflict by editing the file they added the new reference to the variable under the old name and make it use the variable with the new name. You "git add something" and amend the merge.

"git show" of the result will show you what happened, I think.

Antoine Pelisse· Jun 3, 2013, 17:37 UTC · re: Junio C Hamano · lore

Re: preventing evil merges

On Mon, Jun 3, 2013 at 7:20 PM, Junio C Hamano <gitster@pobox.com> wrote:

Show 26 quoted lines

> Sandro Santilli <strk@keybit.net> writes:
>
>>  git merge anotherbranch
>>  git add something
>>  git commit --amend
>>
>> After the steps above the addition of "something" can't be found in
>> the history anymore, but the file is there.
>
> This is a very common and sensible thing to do when dealing with
> semantic conflict.  Imagine that you changed the name of a global
> variable in the code on your current branch since the anotherbranch
> you are pulling from forked from you.  Then imagine further that the
> anotherbranch added one location that refers to that variable.
>
> Since they are not aware of the name change, they added the new
> reference with the old variable name.  The part they added is a new
> code, so it is very likely that there is no textual conflict when
> you did "git merge anotherbranch".  But now the result is broken.
>
> And you fix that semantic conflict by editing the file they added
> the new reference to the variable under the old name and make it use
> the variable with the new name.  You "git add something" and amend
> the merge.
>
> "git show" of the result will show you what happened, I think.

Also, you need to use --cc option of log to see the change in history (in addition to -p):

    git log --cc -p