Automerge Dependabot PRs on GitHub

Whether I’m trying out a new tool or just prototyping with a familiar stack, I usually create a new project on GitHub and run all the experiments there. Some examples of these are:

• rubric: linter config initializer for Python
• exert: declaratively apply converter functions to class attributes
• hook-slinger: generic service to send, retry, and manage webhooks
• think-async: exploring cooperative concurrency primitives in Python
• epilog: container log aggregation with Elasticsearch, Kibana & Filebeat

While many of these prototypes become full-fledged projects, most end up being just one-time journies. One common theme among all of these endeavors is that I always include instructions in the readme.md on how to get the project up and running—no matter how small it is. Also, I tend to configure a rudimentary CI pipeline that runs the linters and tests. GitHub Actions and Dependabot¹ make it simple to configure a basic CI workflow. Dependabot keeps the dependencies fresh and makes pull requests automatically when there’s a new version of a dependency used in a project.

Things can get quickly out of hand if you’ve got a large collection of repos where the automated CI runs periodically. Every now and then, I get a sizable volume of PRs in these fairly stale repos that I still want to keep updated. Merging these manually is a chore. Luckily, there are multiple ways² that GitHub offers to automatically merge PRs. The workflow that is documented here is the one I happen to like the most. I also think that this process leads to the path of the least surprise. Instead of depending on a bunch of GitHub settings, we’ll write a GitHub action workflow³ to automate the process.

First, you’ll need to turn on the auto-merge option from the repository settings. To do so, go to the repo’s settings tab and turn on the Allow auto-merge option from the Pull Requests section:

Now, you probably don’t want to mindlessly merge every pull request Dependabot throws at you. You most likely want to make sure that a pull request triggers certain tests and it’ll be merged only if all of those checks pass. To do so, you can turn on branch protection⁴. From the settings panel, select Branches on the left panel:

Once you’ve selected the tab, add a branch protection rule to the target branch against which Dependabot will send the pull requests:

In this case, I’m adding the protection layer to the main branch. I’ve turned on the Require status checks to pass before merging toggle and added the build step to the list of status checks that are required. Here, you can select any job from your CI files in the .github/workflows directory:

Once this is done, you can drop the following CI file in the .github/workflows directory of your repo. It’s the same file⁵ that’s currently living inside this site’s CI folder.

# .github/workflows/automerge.yml

name: Dependabot auto-merge

on: pull_request

permissions:
  contents: write
  pull-requests: write  # Needed if in a private repository

jobs:
  dependabot:
    runs-on: ubuntu-latest
    if: ${{ github.actor == 'dependabot[bot]' }}
    steps:
      - name: Enable auto-merge for Dependabot PRs
        run: gh pr merge --auto --merge "$PR_URL"
        env:
          PR_URL: ${{github.event.pull_request.html_url}}
          # GitHub provides this variable in the CI env. You don't
          # need to add anything to the secrets vault.
          GITHUB_TOKEN: ${{ secrets.GITHUB_TOKEN }}

From now on, every time Dependabot sends a merge request, the checks will be triggered and if all the mandatory checks pass, the automerge.yml workflow will merge it into the target branch.

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