2.4. Merge Conflicts

Merge conflicts arise when people on separate branches modify the same parts of one (or multiple) files. Since Git does not know how to handle that and whose changes to consider, it prompts the user to decide instead.

Fig. 2.26 Visualization of a merge conflict

Git and GitHub assist you in solving these conflicts, as seen in the figure below:

Attribution

License: CC BY 4.0 | Copyright: © 2018-2024 The Carpentries

Fig. 2.27 Merge conflict

In this situatie, it is attempted to merge main in a conflicting branch. The conflicting lines on the main branches are preceded by <<<<<<< main, while the changes from the conflicted main branch are preceded by ======= and followed by >>>>>>> conflicting-instructions. To fix this conflict, we need to open the file in an editor and resolve the conflict by keeping the desired changes and removing the unnecessary parts. Once done, we can commit the resolved changes. Note that the current branch is now in a “MERGING” state. We have decided to take the best out of the two branches and merge their changes.

Why merge into the conflicting branch first?

Merging the main branch into the conflicting branch first allows you to resolve conflicts in a controlled environment. After that you can thoroughly test the merged changes in the conflicting branch before integrating them into the main branch, reducing the risk of introducing bugs.

Exercise

Given is the following file poem.md which is stored in the ‘main’ branch of your repository:

Roses are red,
Violets are blue,
Sugar is sweet,
And so are you.

You and your friend want to improve this poem.

You edit the file in your new branch ‘Q2’:

Roses are red,
Violets are blue,
Sugar is sweet,
Glad I made Q1 through.

Your friend edits the file in his branch ‘love’:

Roses are red,
Violets are blue,
Sugar is sweet,
And my love for MUDE is true.

What would happen when you merge the branch ‘love’ from your friend?

The fourth line in the ‘main’ branch of your repository will be replaced with the new line from your friend

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What would happen when you merge your branch ‘Q2’ into ‘main’ after merging the branch ‘love’ from your friend? Why does that happen? How do you solve that?

A merge conflict arises because now there are two conflicting changes for the fourth line of the poem. Git will not know which change to keep and will mark the conflict in the file.

To fix this conflict, you will have to open the file and manually edit the fourth line to keep the change you want.

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As instead of merging ‘Q2’ into ‘main’, after merging the branch ‘love’ from your friend, you first merge ‘main’ into ‘Q2’, what would happen when you merge ‘Q2’ into main? How would that be different from the previous question?

You will still have a merge conflict, but now you will have already resolved the conflict in ‘Q2’. If this would have been code, you could have tested the resolved conflict. Therefore, when you merge ‘Q2’ into ‘main’, you can be confident that the changes are correct and won’t introduce bugs.

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Exercise

Imagine the following time-line of event of Krisje and Reinier collaborating with Git to solve a coding challenge:

1. Krisje creates a git repository with a README.md on main and shares it with Reinier.

2. Krisje adds a file code.py on main with just two lines of code.

3. Reinier adds three lines of code to code.py in a new branch from main with the name update.

4. Reinier opens a pull request for update into main

5. Krisje adds five lines of code to code.py in a new branch from main called feature.

6. Krisje tries to merge feature into update but gets a merge conflict

7. Krisje solves the merge conflict and commits those changes to update while merging feature into update

8. Krisje merges update into main

Draw the final git history graph of the repository of Krisje and Reinier

Fig. 2.28  

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Explain what exactly Krisje might have done to solve the merge conflict in step 7.

Both branches changed code.py starting from the same two lines. update added three lines and feature added five lines, and both sets of lines are in the same place in the file. Git cannot decide which version to keep, so it stops the merge and reports a conflict. Krisje porbably manually edited the file to the intended final content. This could be keeping both sets of lines, combining them, or choosing one. She then deleted the conflict markers and continued with the merge.

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How could Krisje and Reinier have avoided the conflict by communicating or working differently? Use coding and git terminology in your answer.

Any of the following:

• Agree beforehand who works on which part of the code, so they don’t edit the same lines.

• Split the code into separate files or functions. Each person then works in their own file, and changes rarely overlap.

• Merge update into main (or main into feature) soon, so differences stay small.

• Krisje could have reviewed the PR before starting related work. Reinier’s open PR for update signals that the same file was already being changed.

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Collaborating with branches

When working together on a single branch (so in the same version), it is important to coordinate with your team to avoid conflicts and ensure smooth collaboration. Here are some best practices:

• Keep in touch with your team about who is working on what.

• Frequently pull the latest changes to keep your local branch up to date. This helps in minimizing conflicts.

• Make small, frequent commits with clear messages. This makes it easier to track changes and resolve conflicts.

• If conflicts arise, communicate with your team directly to resolve them before proceeding.

Why notebooks causes conflicts all the time

Yeah, this is really annoying. The JSON structure of a notebook causes the issue, as explained here.

Attribution

This chapter reuses material from Learn Programming for Engineers and was written by Kiril Vasilev, Riccardo Taormina, Robert Lanzafame, Tom van Woudenberg. Find out more here