Re: how to rebase backwards

bill lam [off-list ref] writes:

I want to rebase a new branch onto an earlier commit 

original:  
E is a newly created branch from D and not yet modified

                    E
                   /
-- A -- B -- C -- D

I sense a grave misconception here.  If you have "not yet modified"
anything at all since D, then E does *not* even exist.

Remember, a branch is just a pointer pointing at a commit.  You can have
more than one such pointers pointing at the same commit.  So, if you have
been working on 'master' branch, building up to D, your history would look
like this:

    ---A---B---C---D
                   ^ master

And you would "create a branch", say "hack", like this:

    $ git checkout -b hack

What happens to your history when you do so is this:

                   v hack
    ---A---B---C---D
                   ^ master

The same commit "D", pointed by two branches, 'master' and 'hack'.

If you start editing files at this point, before making a commit, nothing
in the history changes.  But once you make a commit E while on branch
'hack' (since you ran "git checkout -b hack" above), the history changes
into this:

                     E
                    /^ hack
    ---A---B---C---D
                   ^ master

new: what I wanted

     B'-- C' -- E     for deployment
    /          
-- A -- B -- C -- D    trunk

so that I can continue to change all commits from B to C without
affecting E

I try (not sure if correct) doing it by
  git rebase --onto A B E

This asks to replay C, D and E in this order on top of A, to create

      C'--D'--E'
     /
 -- A

which is different from what you described earlier.

Assuming that you did actually commit to create E, after branching 'hack'
off of 'master', i.e. your history looks like this:

                     E
                    /^ hack
    ---A---B---C---D
                   ^ master

the way to create what you want would actually be

    $ git checkout -b deploy master^ ;# branch deploy at "C"
    $ git cherry-pick hack           ;# replay E on top of it

which would make a history of this shape.

                 v deploy
                 E'  E
                /   /^ hack
    ---A---B---C---D
                   ^ master

If you need to later tweak B and C (only) for deployment branch, you could
further rewrite B and/or C with

    $ git rebase -i A

to rewrite everything since A leading to the tip of your current branch
(i.e. E'), to result in

                   v deploy
           B'--C'--E"
          /
         /       E'  E
        /       /   /^ hack
    ---A---B---C---D
                   ^ master

Now, you may not want to keep both 'hack' and 'deploy'.  Perhaps you
forked 'deploy' and modified for E but it was done on top of D that you
did not want to include in 'deploy' by mistake.  IOW, you might have done
this:

                     E
                    /^ deploy
    ---A---B---C---D
                   ^ master

Another way to make 'deploy' point at the result of replaying E on top of
C when your history looks like this is (assuming you are on 'deploy'):

    $ git reset --hard HEAD^^ ;# reset to "C"
    $ git cherry-pick @{1}    ;# cherry-pick "E"

That would give you a history of this shape:

                 v deploy
                 E'  E
                /   /
    ---A---B---C---D
                   ^ master

Note that the original commit "E" becomes dangling and it will eventually
be garbage-collected.

Thank you for detail explanation.  From what you described, I begin
with master commit D, 
 $ git checkout -b deploy
 $ git commit --allow-empty -m deploy

 
                      E
                     /^ deploy
     ---A---B---C---D
                    ^ master
 
 $ git rebase -i A

                      v deploy
          B'--C'--D'--E'
         /
     ---A---B---C---D
                    ^ master

since E is an empty commit, I suppose content of D' E' and D are
identical at this point.  Is that correct?

If several months later, I forget which is the common ancestor for
master and deploy, how do I generate the above graph or identify
commit A as the common ancestor for these two branches?

-- 
regards,
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