错误合并后重建git分支（尊重合并提交）

Question

I have a classic situation: I merged a dev branch into master , then realised I made a mistake and reverted the merge. Then added a fix into dev and want to merge it again. Of course, I cannot merge it directly, since, from git's point of view, it was already merged.

Classic solution

Revert the revert commit, apply your fix. Move on.

• Pros : simple.

• Cons : history is spoiled. It is very hard to blame deeper than this revert of the revert.

What I want

I want to rebuild the dev branch from scratch, and merge it again. Should be easily done with git rebase --force-rebase , according to the books. But the branch history was not so simple, and contained a number of merges, including merges from master . I want to respect these.

Let's have an example already:

                                     master|
                                           v
-*------------X-----Y---------*-----*----M-W
  \          /       \         \   /    /
   \        /         \         -*-    /
    \      /  -D-      G--H--         /
     \    /  /   \    /      \       /
      A--B--C-----E--F--------I-----J
                                    ^
                                    |dev

The faulty merge is M . Its revert is W . I assume that commits on dev up to B are ok, since B was merged into master as X . I want to rebuild the branch starting from B with exactly the same topology. See the tree below. New commits are marked with the prime. You can see how commits C..J were rebased to C'..J' . Correspondingly, dev is rebased to dev' . Other commits did not change.

                                dev'|
                                    v
            C'----E'-F'-------I'----J'
           / \   /    \      /        
          |   -D'      G'-H'-           
          |           /                    |master
          |          /                     v
-*--------+---X-----Y---------*-----*----M-W
  \       |  /       \         \   /    /
   \      | /         \         -*-    /
    \     |/  -D-      G--H--         /
     \    /  /   \    /      \       /
      A--B--C-----E--F--------I-----J
                                    ^
                                    |dev

Then I'll apply my fix K and merge this new branch to master :

                                dev'|
                                    v
            C'----E'-F'-------I'----J'--K
           / \   /    \      /           \
          |   -D'      G'-H'-             \
          |           /                    \ |master
          |          /                      \v
-*--------+---X-----Y---------*-----*----M-W-M2
  \       |  /       \         \   /    /
   \      | /         \         -*-    /
    \     |/  -D-      G--H--         /
     \    /  /   \    /      \       /
      A--B--C-----E--F--------I-----J
                                    ^
                                    |dev

• Pros : good history.

• Cons : I don't know how to do this.

Ok, seems like rebase has an option for this, and it is called --rebase-merges . But it does not do what I want. It rebases all commits, that are ancestors of dev and descendants of B . Which includes commits X and Y . Which are on master already, why would I want them? (Actually, it also brings some other commits from the more ancient history, and I don't understand why yet, but it's not the point here).

Instead, I want to bring commits, that are ancestors of dev but not master (considering master before the faulty merge of course). Sounds really easy to me. Yet I don't understand how to do it, except by manually constructing this difference of commits and somehow cherry-picking them by hand, carefully preserving parents.

So, how do I rebuild my branch, containing only work done on my branch and respecting merge commits?

PS We cannot simply reset master to M^ , that would be too easy.

Answer 1

Ok, seems like I found a solution:

1. git rebase --interactive --rebase-merges B dev
2. reword C

This produces exactly the history I want. Credits to @mstrap for the hint.

I'm not exactly sure why this works the way I want though.

Answer 2

Based on https://github.com/git/git/blob/master/Documentation/howto/revert-a-faulty-merge.txt , I would

git checkout dev
$EDITOR files
git commit files -m 'Fix'
git checkout master
git revert W
git merge dev

The revert-revert is necessary to keep the changes when you re-merge.