Re: [PATCH 4/4] git-rebase -i: New option to support rebase with merges

[Junio C Hamano <gitster@pobox.com>]

Jörg Sommer <joerg@alea.gnuu.de> writes:

> Me too, but I think it's not possible to do what I want with -p. -p
> misses a definition of the (new) parent of a commit. It tries to preserve
> all commits from all branches. But going through the _list_ of commands
> couldn't preserve this structure.
>
> o--A--B
>  \     \
>   C--D--M--E
>
> How should the graph look like after these commands:
>
> pick A
> pick C
> squash E
> # pick D
> pick B
> pick M

I am beginning to suspect that the root cause of this is that the todo
language is not expressive enough to reproduce a merge _and_ allow end
user editing.

Let's step back a bit.

If you have this history:

      o---o---o---o---o---Z
     /
    X---Y---A---B
         \       \
          C---D---M---E

and you want to transplant the history  between X..E on top of Z, from the
command line you would say:

	$ git rebase --interactive -p --onto Z X E

First let's think what you would do if you want to do this by hand.  The
sequence would be:

	$ git checkout Z^0 ;# detach at Z

        $ git cherry-pick Y
        $ git tag new-Y ;# remember it
        $ git cherry-pick A
        $ git cherry-pick B
        $ git tag new-B ;# remember it
        $ git checkout new-Y
        $ git cherry-pick C
        $ git cherry-pick D
        $ git merge new-B ;# this reproduces M
        $ git cherry-pick E

	$ git branch -f $the_branch && git checkout $the_branch


Now how does the todo file before you edit look like?

	pick Y
        pick A
        pick B
        pick C
        pick D
        pick M
        pick E

The todo file expects the initial detaching and the final switching back
outside of its control, so it is Ok that the first "checkout Z^0" and the
last "branch && checkout" do not appear, but it should be able to express
the remainder and let you tweak.  Is it expressive enough to do so?  

Most of the "pick" from the above list literally translate to the
"cherry-pick", and if you change any of them to "edit", that is
"cherry-pick --no-edit" followed by a return of control to you with insn
to "rebase --continue" to resume.  There appears nothing magical.

Not really.  There already are two gotchas even without talking about
end-user editing.

First, "pick M" is not "cherry-pick M".  You do not want to end up with
merging an old parent before rewriting.  It has to be something like
"merge rewritten-Y".

Second, before you start picking C, if you want to preserve merges, you
have to switch to rewritten Y.  The original sequence left in todo does
not have that information to begin with.  We need, before the "pick C", to
say the equivalent of "git checkout new-Y" in the manual sequence
illustrated above.  The lack of "checkout new-Y" is perfectly fine if
rebase is meant to linearlize the history, but if you want to preserve the
shape of the history, you would need to give a clue that the sequence that
begins with the "pick C" starts from somewhere else.

You also need to make sure that "pick M" moved elsewhere still merges the
tips of two forked histories.  Moving "pick M" before "pick C" or "pick A"
would not make much sense.  So you would need some kind of "barrier" that
says "do not move this 'pick M' beyond this point".

Perhaps we can make it clearer by introducing a few more primitives to the
todo language: mark, reset and merge.  The above illustrated history would
then become:

	pick Y
        mark #0
	pick A
        pick B
        mark #1
        reset #0
        pick C
        pick D
	mark #2
        merge #1 #2
        pick E

You can change any of the "pick" to "edit, or drop it, and you can reorder
"pick" in a sequence of "pick", but you cannot change "mark", "reset",
"merge", or move "pick" across insn that was not originally "pick".