Undo a fast-forward merge

Question

I've made some changes to my git repo that I wish to undo.

My git repo looked like this:

A-B----     master
   \   /
    C-D     * develop

I was on the develop branch, forgot that it differed from the master branch, made a change on develop, merged it into master, and then pushed to my remote (called publish).

Because there were no changes on master since B (common ancestor), git did a fast-forward merge.

Now, my repo looks like this:

A-B-C-D   master, develop, remotes/publish/master, remotes/publish/develop.

I wanted to revert the last merge, restoring master to B.

From what I read in How to undo last commit(s) in Git?, I used git reset sha-of-B to restore my master branch to revision B.

Questions:

• How do I restore develop to revision D?
• How do I then push these changes back to remote/publish?

Answer 1

If you reset the branch master, it won't touch develop branch. In order to reput all in order, you should do:

git checkout master
git reset --hard sha-of-B
git checkout develop
git reset --hard sha-of-D
git checkout master
git merge develop --no-ff

Answer 2

• imagine you are still on master where you merged and pushed
• git reset --hard @{1}
  • this resets branch “master” to where it was one step back on your computer (i.e. at “B”)
• for develop do nothing, because it should still be at “D”
• git push publish develop
  • this pushes branch “develop” in the usual way
• git push publish master --force-with-lease
  • pushes branch “master” backwards, with the permission gained via --force-with-lease in a safe way
  • the use of --force might silently overwrite work from others, so I never use that
• or both push operations together with this surprising syntax:
  • git push publish develop --force-with-lease=master

Answer 3

If seems that you were checked out to develop when you ran git reset, and this is what messed you up.

You can fix develop as follows:

git checkout develop
git merge --ff-only D

Now, you can restore master back to B:

git checkout master
git reset --hard B

Get back to develop and push it:

git checkout develop
git push develop