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Splitting up a large Merge Request

It is common when developing a new feature on a branch to end up fixing a number of bugs or extending functionality elsewhere in the code. After you are done, however, the code reviewers may well ask that the bug fixes be separated into a separate merge request. This makes their life easier. They can review the bug fixes and get these merges into master quickly. Meanwhile, your new feature may need a more in-depth review. It would not be sensible to hold up fixing bugs based on the review of new code.

In this note, I'll describe the method I use to break such large merge requests up. Advice on merge requests in general can be found here.

Figure out where the changes are

Let's say you are developing on feature-branch and want to merge into main-branch. First, let's figure out which files have been modified:

$ git diff --name-only main-branch feature-branch
src/utils.py
src/magic.py

Here we can see that both utils.py and magic.py have been modified. Let's say that the changes to utils.py where all bug fixes while magic.py is a new file introducing new features. The code reviewers want to split up our merge request into two: one which only adds the bug fixes to utils.py and a second one which adds the new functionality in magic.py.

The Problem and Solution

The problem, from our perspective, is that the changes to magic.py depend on the changes to utils.py! Of course, we fixed the bugs so that it would run.

To satisfy the code reviewers while not breaking our branch, we need to:

  1. Create a new merge request with only the changes to utils.py included
  2. Request the new merge request be merged into master
  3. Rebase our feature-branch to master (bringing with it the changes to utils.py)
  4. Now we have a feature-branch with only magic.py being changed, but all the while feature-branch never got broken.

Technical how-to

The steps above can be done as follows:

Create a new merge request with only the changes to utils.py included 

$ (main-branch) git checkout main-branch
$ (main-branch) git checkout -b fix-utils  # Create branch of main-branch
$ (fix-utils) git diff main-branch feature-branch -- src/utils.py > utils_changes.patch  # Write the diff between the main and feature branches to a patch
$ (fix-utils) git apply utils_changes.patch  # Apply the patch
$ (fix-utils) git commit -m "Fixing utils"
Now you can push this branch and create a merge request.

Once fix-utils gets merged into main-branch, all you need to do is rebase your feature-branch to main-branch and then your merge request will only contain changes to magic.py. Magic.