Using git merge, how can I merge a bugfix into multiple long-term branches?

Question

Recently at work, we've switched from using SVN to using git. We have to maintain multiple versions of our software, and we had them setup as branches before. So after using git svn clone, we ended up with 4 branches: master, 5.0, 5.1, and 5.2.

Note: We use master as our development branch. 5.0, 5.1, and 5.2 all branch from master and are used as production branches.

A few days have gone by and quite a few changes have gone into master. A bug was just found in 5.2 and we want to fix it. We've tried the the solution from Merging one change to multiple branches in Git with partial luck. We can always merge back into master easily, but whenever we merge back into 5.2 we get a bunch of conflicts. Everything we've seen that shows this process (creating a new branch for bugfix and merging it back into development and production) shows that it should be as simple as:

(master)$ git checkout -b bugfix
# fixed bug and committed

(bugfix)$ git checkout master
(master)$ git merge bugfix
# successful merge

(master)$ git checkout 5.2
(5.2)$ git merge bugfix
# successful merge

However for us when we get to that final line git merge bugfix we get tons and tons of conflicts. Conflicts for files that we haven't even touched in the bugfix. What are we doing wrong?

Note: We have also tried doing this process by starting the bugfix branch from 5.2. Then merging the bugfix into 5.2 is easy but merging the bugfix into master generates the conflicts.

Other places that show the process:

• http://nvie.com/posts/a-successful-git-branching-model/#hotfix-branches
• https://www.atlassian.com/git/tutorials/comparing-workflows/gitflow-workflow (scroll to "End-user discovers a bug")

Answer 1

git merge is of all changes since a common base. If you want to record a merge of just one change to multiple histories, you have to make it the only change to some ancestral content in all the histories, like so:

git checkout -b bugfix `git merge-base 5.0 5.1 5.2 master`
# so now bugfix is an ancestor of all of the above

# fix the bug here, then:
git commit -m 'bug fixed'

git checkout 5.0; git merge bugfix     # bugfix has just the one change
git checkout 5.1; git merge bugfix
git checkout 5.2; git merge bugfix
git checkout master; git merge bugfix

It might be better to go all the way back and branch bugfix off the commit that introduced the bug in the first place, but that's not always going to make for a clean merge if it's been around long enough.