¬ just serendipity 🍀 (not just serendipity)

Sometimes you want to send a pull request for review to your teammates and want to ensure they do not accidentally prematurely merge it.

One way to do so is to note in the commit message / PR description that it should not be merged. Nonetheless, humans don’t always read instructions, our attention span is quite limited.

It would be more robust to employ a mechanism to prevent the PR from being submitted, even if someone attempts to do so.

You could leave the PR as a draft. However, this could have the side effect that not all CI checks may run. You don’t want surprising failures later on, that could slow you down.

I like the following approach, which is partly inspired by an internal presubmit check at Google: write “DO NOT SUBMIT” in the PR title, and have a CI check (github action¹) fail in this situation. Mark the action as required:

If status checks are required for a repository, the required status checks must pass before you can merge your branch into the protected branch. For more information, see About protected branches.

Use the following github action (.github/workflows/pr-checks.yml):

name: PR Checks

permissions: read-all

on:
  pull_request:
    types: [opened, edited, synchronize]

concurrency:
  group: ${{ github.workflow }}-${{ github.ref }}
  cancel-in-progress: true

jobs:
  pr-checks:
    runs-on: ubuntu-latest
    steps:
      - name: '"DO NOT SUBMIT" check'
        env:
          PR_TITLE: ${{ github.event.pull_request.title }}
        run: |
          echo "Title: $PR_TITLE"

          if [[ "${PR_TITLE,,}" =~ "do not submit" ]]; then
            echo "::error::PR title '$PR_TITLE' must not contain 'DO NOT SUBMIT'"
            exit 1
          fi

${PR_TITLE,,} converts the PR title to lowercase so that the comparison is case insensitive.

=~ "do not submit" is a regular expression to check if the title includes the “DO NOT SUBMIT” string.

With the action in place, I can just update my PR title by prepending DO NOT SUBMIT: to it. The prefix doubles down as a documentation of my intent.