Re-run GitHub workflows for incoming Pull Requests

In order to prevent "semantic merge conflicts", each pull request needs to be tested based on its potential merge commit with its target branch. A full-fledged solution to this would be to use a proper merge pipeline. While a merge pipeline is a really powerful tool that becomes a game changer once a project reaches a certain size, it brings its own complexity.

Small projects can get along with a simpler solution. When using GitHub Actions triggered by the pull_request event, GitHub will use the potential merge commit by default. But those events aren't triggered when the target branch changes. This is the problem that this action tries to address. Every time a push to some repository branch happens, this GitHub Action may be used to find all the open and mergeable pull requests targeting the branch being pushed to, and trigger a re-run of a certain workflow on those pull requests.

Note that this is not entirely bullet-proof. There are several edge cases in which bad pull requests can slip through. If that's not acceptable, please consider a more elaborate approach than this one. This is a proof-of-concept that might be good enough for some folks. Some edge cases are:

• Pull requests get merged in quick succession:
  Pull requests that were passing all of their status checks before the update of the target branch will remain in a mergeable state until the GitHub Action for the target branch triggered their respective builds. Usually, this time window is quite small, say, under a minute, but if the triggering GitHub action fails or gets stuck in the execution queue, this can go unnoticed for quite a while.
• Stale pull requests:
  GitHub's docs mention that a GitHub workflow run can only be re-run up to 30 days after the triggering event event has occurred. So whenever a pull request isn't touched for longer than that period, the re-run cannot be triggered anymore and some other action needs to be taken.

Usage

In order to use this action, you'll need to add a new workflow file to be triggered when branches are updated. You'll also need the name of the workflow your project uses for running tests on pull requests.

E.g.:

The workflow to trigger re-running PR workflows:

# .github/re-run-pr-workflows.yml
name: Re-run workflows for incoming PRs

on:
  push:
    branches:
    - '**'

jobs:
  trigger_ci:
    name: Trigger PR tests run
    runs-on: ubuntu-20.04
    steps:
    - name: Trigger PR tests run
      uses: twz123/rerun-pr-workflows-action@v0.3
      with:
        workflow: Run PR Tests

Where Run PR Tests is the name set within the file that runs the PR tests. E.g.:

# .github/run-pr-tests.yml
name: Run PR Tests

on:
  pull_request:
    branches: [ main, staging ]

  workflow_dispatch:

jobs:
  test:
    name: Run Tests
    runs-on: ubuntu-20.04

    steps:
      - uses: actions/checkout@v4

      # Testing steps go here.
      # ...

Code in Main

First, you'll need to have a reasonably modern version of node handy. The CI builds use Node 20, currently.

Install the dependencies

$ npm install

Build the typescript, package it for distribution, lint, run tests ... all in one go

$ npm run all