Git Kata: Reverted merge

In this kata, we explore the problems of reverting a merge commit.

The story

Your team uses this amazing library-1.2.3 for its development, which is maintained by another team.

At some point, your team integrates a new version library-1.2.4. Because you are prudent, you do this on a branch integrate-library-1.2.4.

Unfortunately, you discover after your merge that the library has a bug, which has to be fixed by this other team. To prevent the bug from being released into production, you decide to revert the merge commit.

Setup

1. Run source setup.sh (or .\setup.ps1 in PowerShell)

The task

1. Revert the merge commit. To resolve the conflict, you need to determine what features should be included in mymodule.txt.
   • You can tell whether feature X should be included or not by when it was commited.
   • You may assume that feature Y is also working with the old library version.
2. Take the role of the library team and fix the bug in the library on the integrate branch, e.g. change lib.txt.
3. Next we explore how you can get the changes from the branch into the master again. First try to merge to see what happens. The lib.txt file changes as expected, but 'mymodule.txt does not. For an in depth discussion of the reason why, consult this gist: Reverting a faulty merge.

reverting a merge commit also undoes the data that the commit changed, but it does absolutely nothing to the effects on history that the merge had.

So the merge will still exist, and it will still be seen as joining the two branches together, and future merges will see that merge as the last shared state

4. Undo the merge with a reset --hard
5. Revert the revert and try the merge again. This time it works.

Useful commands

• git revert -m 1 <merge-sha1>
• git log --oneline --graph --all
• git add <file-name>
• git revert --continue
• git switch <branch-name>
• git merge <branch-name>
• git reset --hard <sha1>
• git revert <sha1>