Re: why "git rebase" searching the duplicate patches in <upstream branch> rather than in <new base branch>?

[Andy Zhang <zhgdrx@gmail.com>]

On Tue, Jul 20, 2021 at 6:23 AM Junio C Hamano <gitster@pobox.com> wrote:
>
> Andy Zhang <zhgdrx@gmail.com> writes:
>
> > why "git rebase" searching the duplicate patches in <upstream
> > branch> rather than in <new base branch>?
> >
> > hi, all:
> >
> >  I am reading the help of "git rebase", it says:
> >     "If the upstream branch already contains a change you have made
> > (e.g., because you mailed a patch which was applied upstream), then
> > that commit will be skipped. "
> >
> >  But, because we are applying commits to <new base branch> rather than
> > to <upstream branch>, I really don't understand why we are searching
> > the duplicate patches in <upstream branch> rather than in <new base
> > branch>?
>
> It is either a design bug or a documentation bug, or both ;-)
>
It should NOT be a documentation bug, because, in my experiment, I
observe that, "git"'s behavior is exactly the same as documentation.
So, it should be "work as design".

> I do think it makes sense to skip commits from the branch we are
> rebasing that have equivalent commits in the upstream, as it is
> expected that upstream might have already applied/cherry-picked some
> of the changes you are rebasing, and you do not want to use the same
> change twice.
>
> When we are transplanting a series of commits from an old base to
> totally unrelated base using the --onto option, e.g. when replaying
> the contents of 'topic' relative to 'next' down to 'master' in your
> topology, however,
>
> > Old tree is:
> >
> > o---o---o---o---o  master
> >     \
> >      o---o---o---o---o  next
> >                       \
> >                        o---o---o  topic
>
> it is not necessarily obvious where to stop digging back at.  In the
> above picture where 'master' and 'next' have ancestry relationship,
> we could try to see if the three commits on 'topic' branch being
> replayed match any of the commits in next..master range, but when
> using the --onto option, there does not have to be any relationship
> between the <upstream> and <new base> (they do not have to share a
> root commit).  So from that point of view, it probably makes sense
> to default to --no-reapply-cherry-picks when --onto is used, while
> defaulting --reapply-cherry-picks when --onto is not used.
>
>