如何撤消两个要素分支之间的合并？

Question

OK, here is the situation: Two of my colleagues worked on two new features ( feature_1 and feature_2 ). Now there were some changes in one branch feature_1 , the other developer want to see in his branch feature_2 . So he merged the branch feature_1 into his branch feature_2 . It can be illustrated as so:

Before the merge:

P--o--o--o    master
\\     
 \\B--C--D    feature_1
  \
   \A--E      feature_2

After the merge:

P--o--o--o    master
\\     
 \\B--C--D    feature_1
  \       \
   \A--E---M  feature_2

He pushes the merge commit and both developers are going to continue working on their branches.

P--o--o--o--o--o--o    master
\\     
 \\B--C--D--o--o--o    feature_1
  \       \
   \A--E---M---o--o    feature_2

Now it's time to merge feature_2 back into master. But if we would just do that, all changes from feature_1 (at least those from the last merge from feature_1 into feature_2 ) would be merged back into master. But as feature_1 is not ready yet, that should not happen.

So how can we "unmerge" feature_1 from feature_2 , so the commits from feature_1 will not be merged into master.

I tried to do a git rebase master on feature_2 , but this would only remove the merge commit M, but leave the commits B, C and D (or more precice B', C' and D') on feature_2 .

Any ideas on how to get the commits from feature_1 out of feature_2 ? We wouldn't mind to rebase and force push or create a new branch with only the commits from feature_2 .

UPDATE (my solution):

Thanks to @kan, who provided a nice solution for my merge issue. In the comments we also discussed, how to solve it, when multiple merges from feature_1 to feature_2 happend. I have found a solutions in a single command, that would to the trick. So let's assume, we have the following scenario:

P--o--o--o--o--o--o--o    master
\\     
 \\B--C--D--o--o--o--o    feature_1
  \       \     \
   \A--E---M--o--M2--o    feature_2

So we have merged twice. No a rebase also have to be done twice. The first one -with --onto M2^ M2 and the second one with --onto M^ M . This can be done with this single command:

git rev-list --ancestry-path master..HEAD --merges | xargs -I % git rebase --onto %^ % feature_2

This will basically get all the merge commits from the range master to the HEAD of feature_2 two and runs the rebase on "from right to left". So it will first use M2 and than M in the rebase. This command can also be used, if there is only one merge commit, but than you can simply use the shorter git rebase --onto M^ M command.

Answer 1

The most visible way to do it - do rebase -i and manually drop feature_1's commits. If there are too many commits to filter manually, let me know, I'll try think harder...

Ok. Thinking harder... ;)

I suggest, there is only one merge point M. In this case you should rebase all commits after merge commit onto the commit right before merge point. Use git rebase --onto EM feature_2 .

Answer 2

Since this particular merge has been pushed, you are going to want to revert the merge commit .

git revert -m 1 <SHA-of-M>

Should you want to reintroduce any of the changes from feature_1 again, you will have to revert the revert , but this is the safest option, as it preserves the history of your commits.

To clarify a bit on "revert the revert": suppose that you've merged this branch to master.

P--o--o--o--o--o--o---F2 master
\\                    /
 \\B--C--D--o--o--o  /   feature_1
  \       \         /
   \A--E---M---o--R/     feature_2

If you want to merge feature_1 into master , you will have to first revert the revert commit that came along with the merge into master (so, you'd have to revert the R commit).

P--o--o--o--o--o--o---F2--R'-F1   master
\\                    /     /
 \\                  /     /
  \\                /     /
   \A--E---M---o--R/     /        feature_2
    \       \           /
     \B--C---D--o--o--o/          feature_1