How to Automatically Rebase All Your Merge Requests on Gitlab with Jbang

In our current project we debated (too much :) ) about our merge strategy.

We finally settled with a fast-forward strategy. On GitLab, when you merge a MR on main, you must rebase all the other MRs afterwards. If your CI takes more than a couple of minutes, it is cumbersome and take unnecessary time to integrate our code.

I found this elegant solution on Medium to automate rebase everytime someone merges on the main branch. If you’re looking for a quick solution, be sure to check that out.

While we liked this solution, we preferred using a label to identify MRs we wished to auto rebase.

I thought it would be fun to replicate it using Jbang. The full code available on Github.

Jbang

Jbang is a launcher script, that makes getting started with java very simple. It’s like the standard Jshell (since JDK 11), but with super powers:

• Dependency declaration
• Include multiple files
• Easy IDE integration

For more information check out these articles or the reference documentation.

The Code

Let’s create our script via jbang, using the cli template:

1

jbang init --template=cli autorebase.java

This enables to power of picocli in our script (parameter parsing/validation). Once the file is created we are going to use a gitlab java api client, so we must add the Gitlab4j dependency.

1

//DEPS org.gitlab4j:gitlab4j-api:5.0.1

From there the code is pretty straightforward:

 1
 2
 3
 4
 5
 6
 7
 8
 9
10
11
12
13
14
15
16
17
18
19
20
21
22

  @Override
  public Integer call() throws Exception {
    try (GitLabApi gitLabApi = buildGitlabApi()) {
      List<MergeRequest> mergeRequestList = gitLabApi.getMergeRequestApi()
          .getMergeRequests(projectId, Constants.MergeRequestState.OPENED);

      List<MergeRequest> openedMRList = mergeRequestList.stream()
          .filter(mr -> !Boolean.TRUE.equals(mr.getRebaseInProgress()))
          .filter(mr -> mr.getLabels().contains(AUTOREBASE_LABEL))
          .toList();

      for (MergeRequest mr : openedMRList) {
        Long mrIid = mr.getIid();
        System.out.println("Rebasing open MR [%s,%s]".formatted(mrIid, mr.getTitle()));

        if (!dryRun) {
          gitLabApi.getMergeRequestApi().rebaseMergeRequest(projectId, mrIid);
        }
      }
    }
    return 0;
  }

• 1-4 : We look for all opened MR on a given project
• 7-10: We filter by a specific label (but you can choose whatever fits your needs)
• 10+: We iterate over the MR list to rebase them

Gitlab CI

The final part is integrating this script on your pipeline .gitlab-ci.yml

1
2
3
4
5
6

🔄 gitlab-auto-rebase-mr:
  extends: .jbang-job
  script:
    - jbang autorebase.java --gitlab-url ${GITLAB_URL} --project-id ${CI_PROJECT_ID} -t ${ACCESS_TOKEN}
  rules:
    - if: $CI_COMMIT_BRANCH == "main"

Of couse, your job image should contain a java + jbang.

Conclusion

In this blog post we demonstrated how to integrate a simple java script in your gitlab pipeline using jbang.

While java is still verbose compared to the bash equivalent I feel the java counterpart offers many advantages:

• easier debugging
• better ide integration
• massive ecosystem (libraries and frameworks)

Hopefully your next custom script will integrate a bit of java in it, I know mine will!