Experiment 2

You should set up the required code repositories in 13.2 Prepare Source Code if you have not done it yet.

In this section, you are going to start by creating a pull request for person a’s changes, followed by a separate pull request for person b’s. Once both pull requests are up, you will wait for the CI workflows to run. Since person a’s changes have not been merged into the main branch yet, person b’s CI check completes successfully without incorporating person a’s updates. Feeling confident, you merge person a’s code changes into the main branch. Subsequently, you go ahead and merge person b’s pull request, and it should be successful. But then — boom! The codebase is now broken, despite both pull requests passing all their tests.

1. Make sure you are still in the right project folder.

cd path/to/experiment-2

2. Switch to person-a branch.

git checkout person-a

3. Push your changes to the remote repository on the corresponding branch.

git push --set-upstream origin person-a

4. On your remote repository, click Compare & pull request.

5. Click Create pull request.

6. On your local repository, switch to person-b branch.

git checkout person-b

7. Push your changes to the remote repository on the corresponding branch.

git push --set-upstream origin person-b

8. On your remote repository, click Compare & pull request.

9. Click Create pull request.

10. You should now have passed CI workflow executions of both pull requests. Do not click Merge pull request yet.

• Console for the person a’s pull request.

• Console for the person b’s pull request.

11. On the person a’s pull request console, click Merge pull request.

12. Click Confirm merge.

13. Click Delete branch.

14. Click the Code tab and then open the main.py file.

15. You might notice that the default value of n is now 7.

16. On the person b’s pull request console, click Merge pull request.

12. Click Confirm merge.

13. Click Delete branch.

14. Click the Code tab and then open the main.py file.

15. In addition to the default value of n is 7, you might notice that the return value of the function has changed.

Boom!!! Your main branch is now broken. Even if each pull request passes its CI checks, code changes made to different lines — without any visible conflicts — can still break your codebase.

16. On your local repository, switch to the main branch.

git checkout main

17. Pull the lasted codes from the remote main branch.

git pull

18. Run the test.

python test.py

By now, your test cases are expected to fail.

Just like in the first experiment, even with successful local testing on each branch and passing CI workflows for individual pull requests, your main codebase can still end up broken. In this case, it gets worse — since the person b’s pull request did not catch the bad merge early on. To help avoid this, let’s take the next step: set up a basic branch protection rule to strengthen your workflow and prevent these issues from slipping through.