[Discussion] cherry-picking a merge

[Discussion] cherry-picking a merge

[Junio C Hamano]

Earlier "git cherry-pick" learned the "-m <parent-number>"
option to allow cherry-picking a merge commit.

When you have this history:

  ---o---o---C---A---M
      \             /
       o---o-------B

You can replay the change between A and M (in other words, the
effect of merging B into A) on top of C to create a new commit,
with:

        $ git cherry-pick -m 1 M

In the current implementation, the resulting commit has a single
parent C.  This is quite similar to a squash merge of B into C.

When you think about it, as long as the topological relationship
between A and B is very similar to that of C and B (iow,
"merge-base A B" and "merge-base C B" are the same), the effect
should be the same as a real merge between B and C, shouldn't it?

  ---o---o---C---A---M
      \       \     /
       o---o---\---B
                \   \
                 `---X

I am wondering if it makes sense to record the result of
"cherry-pick -m" as a real merge between the current HEAD and
all the other parents of the cherry-picked merge except the one
that is named with the <parent-number>.

Re: [Discussion] cherry-picking a merge

[Shawn O. Pearce]

Junio C Hamano <gitster@pobox.com> wrote:
> You can replay the change between A and M (in other words, the
> effect of merging B into A) on top of C to create a new commit,
> with:
> 
>         $ git cherry-pick -m 1 M
> 
> In the current implementation, the resulting commit has a single
> parent C.  This is quite similar to a squash merge of B into C.
> 
> When you think about it, as long as the topological relationship
> between A and B is very similar to that of C and B (iow,
> "merge-base A B" and "merge-base C B" are the same), the effect
> should be the same as a real merge between B and C, shouldn't it?
> 
>   ---o---o---C---A---M
>       \       \     /
>        o---o---\---B
>                 \   \
>                  `---X
> 
> I am wondering if it makes sense to record the result of
> "cherry-pick -m" as a real merge between the current HEAD and
> all the other parents of the cherry-picked merge except the one
> that is named with the <parent-number>.

Yes.

Then `rebase -i` might be able to learn how to "pick" merge commits.
And then my coworkers can stop bothering me about that.  And just
do it themselves.

I used to think of merges as something special.  I now really only
look at them as being special *during* the merge process, as you
may need to generate that recursive base.  But otherwise its just
"diff M^1..M".  So why isn't it?

:-)

-- 
Shawn.

Re: [Discussion] cherry-picking a merge

[Junio C Hamano]

"Shawn O. Pearce" <spearce@spearce.org> writes:

> Junio C Hamano <gitster@pobox.com> wrote:
> ...
>> When you think about it, as long as the topological relationship
>> between A and B is very similar to that of C and B (iow,
>> "merge-base A B" and "merge-base C B" are the same), the effect
>> should be the same as a real merge between B and C, shouldn't it?
>> 
>>   ---o---o---C---A---M
>>       \       \     /
>>        o---o---\---B
>>                 \   \
>>                  `---X
>> 
>> I am wondering if it makes sense to record the result of
>> "cherry-pick -m" as a real merge between the current HEAD and
>> all the other parents of the cherry-picked merge except the one
>> that is named with the <parent-number>.
>
> Yes.

Actually, this turns to be a trick question ;-)

If there was an accidental clean merge between A and B, and a
part of the duplicated same changes in the accidental clean
merge sits between C and A, replaying the difference from A to M
on top of C would not yield the same result as merging B into C.