[BUG?] git checkout -b removes MERGE_HEAD

[BUG?] git checkout -b removes MERGE_HEAD

[Jan Hudec]

Hello all,

I am not sure whether it is a bug or not, but it definitely did
surprise me a lot. The use-case was as follows:

 - I checked out master and ran a merge from branch (let's call
   it "branch"). It had a lot of conflicts, so I needed to test
   the result well.
 - During the testing I found a problem, but it may have already
   existed on master before the merge.
 - Therefore I needed to return to clean master, test it and return
   to the merge. So I thought I'd commit the merge to a temporary
   branch, test master again and merge the temporary to master if
   the problem is not from the merge. So I did:

   $ git checkout -b temp
   $ git commit

   OOPS! It forgot it was a merge.

Yes, I could have simply commited to master and reset --hard it
back. But I didn't expect it to forget it was a merge in the first
place. After all, it does NOT touch the index nor the working tree,
so why should it clear the MERGE_HEAD and MERGE_MSG?

So should this behaviour be changed?

Thanks,
Jan

-- 
                                        - Jan Hudec <bulb@ucw.cz>

Re: [BUG?] git checkout -b removes MERGE_HEAD

[Junio C Hamano]

"Jan Hudec" <bulb@ucw.cz> writes:

>  - During the testing I found a problem, but it may have already
>    existed on master before the merge.
>  - Therefore I needed to return to clean master, test it and return
>    to the merge. So I thought I'd commit the merge to a temporary
>    branch, test master again and merge the temporary to master if
>    the problem is not from the merge. So I did:
>
>    $ git checkout -b temp
>    $ git commit
>
>    OOPS! It forgot it was a merge.

I think it was an oversight by the author of 91dcdfd (Make "git checkout"
create new branches on demand, 2005-07-11) who forgot that he also did
ef0bfa2 (Remove MERGE_HEAD in "git checkout/reset", 2005-06-21).  The
rationale for the removal of MERGE_HEAD from the earlier commit is:

    [this command] will end up resetting the index to some specific
    head, and any unresolved merge will be forgotten [hence there is no
    point recording the result as a merge].

but when used with -b no such "resetting of the index" happens.

Having said that, changing it not to remove MERGE_HEAD and MERGE_MSG is
probably not sufficient, as MERGE_MSG would likely to already record to
which branch you are recording the merge (I didn't check, though).

Re: [BUG?] git checkout -b removes MERGE_HEAD

[Jan Hudec]

On Thu, Nov 18, 2010 at 10:39:31 -0800, Junio C Hamano wrote:
> "Jan Hudec" <bulb@ucw.cz> writes:
> 
> >  So I did:
> >
> >    $ git checkout -b temp
> >    $ git commit
> >
> >    OOPS! It forgot it was a merge.
> 
> I think it was an oversight by the author of 91dcdfd (Make "git checkout"
> create new branches on demand, 2005-07-11) who forgot that he also did
> ef0bfa2 (Remove MERGE_HEAD in "git checkout/reset", 2005-06-21).  The
> rationale for the removal of MERGE_HEAD from the earlier commit is:
> 
>     [this command] will end up resetting the index to some specific
>     head, and any unresolved merge will be forgotten [hence there is no
>     point recording the result as a merge].
> 
> but when used with -b no such "resetting of the index" happens.
> 
> Having said that, changing it not to remove MERGE_HEAD and MERGE_MSG is
> probably not sufficient, as MERGE_MSG would likely to already record to
> which branch you are recording the merge (I didn't check, though).

I believe it indeed does. However since it's now just a template for the
commit message, it can be fixed up by the user. If he actually wants to, that
is.

In my case, I only needed to stash the changes, so I could switch back to
'master' (left merge parent) to test the bug I was seeing was already there
and than I merged the temporary branch to 'master' (which was fast forward).
So the message saying I merged to 'master' was actually appropriate even when
I was commiting to some other ref.

-- 
						 Jan 'Bulb' Hudec <bulb@ucw.cz>

Re: [BUG?] git checkout -b removes MERGE_HEAD

[Jeff King]

On Thu, Nov 18, 2010 at 10:39:31AM -0800, Junio C Hamano wrote:

> I think it was an oversight by the author of 91dcdfd (Make "git checkout"
> create new branches on demand, 2005-07-11) who forgot that he also did
> ef0bfa2 (Remove MERGE_HEAD in "git checkout/reset", 2005-06-21).  The
> rationale for the removal of MERGE_HEAD from the earlier commit is:
> 
>     [this command] will end up resetting the index to some specific
>     head, and any unresolved merge will be forgotten [hence there is no
>     point recording the result as a merge].
> 
> but when used with -b no such "resetting of the index" happens.

It is not necessarily about "-b" is it? It is about moving to a
different commit. So I would think that on branch "master":

  # should clear MERGE_HEAD
  git checkout foo

  # should not clear MERGE_HEAD; it's a no-op, and we have in the past
  # encouraged people to run this to see the ahead/behind count
  git checkout
  # or
  git checkout master

  # should clear MERGE_HEAD; we are moving to a new commit
  git checkout -b foo some_commit

  # should not clear MERGE_HEAD; this is Jan's case
  git checkout -b foo
  # or
  git checkout -b foo master

-Peff