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GitHub Pull Request Plugin

A Buildkite plugin to build the merged state of pull requests and trigger new builds on PR branches after the default branch is changed.

This means that builds will run against what the merged commit will be, rather than what is in the PR, reducing the risk of bad merges breaking master/trunk.

Why would I want this?

  1. Parity with other CI setups. Including TeamCity, Travis, and Jenkins.
  2. Can help avoid building stale branches, as CI is done against the target branch at the time of the PR being raised rather than when the source branch was created
  3. Reduce the risk of failed builds from reaching master, due to bad merges (See the example at the bottom of this document)

While this approach doesn't mitigate breaking changes made to the target branch branch after the PR is raised, it does help reduce the [time] window in which bad merges can appear -- the thinking being the age of a PR is clearly visible on GitHub unlike the age of the parent commit for a PR branch.

Modes

The plugin has two modes:

  • mode: checkout to merge the PR after checking out source; and
  • mode: trigger to async trigger another build (of the current pipeline)

Example

Ensure Skip pull request builds for existing commits is set to false in your Pipeline settings, as BuildKite will build the branch and skip the PR build.

pr-settings

Checkout Mode

When using checkout mode, the plugin should be specified on each build step (e.g. where a checkout happens).

plugins: &plugins
  - seek-oss/github-merged-pr#v1.1.2:
      mode: checkout

steps:
  - label: 'Make something'
    commands:
      - make something
    plugins:
      <<: *plugins

  - label: 'Make something else'
    commands:
      - make something-else
    plugins:
      <<: *plugins

Trigger Mode

In trigger mode the plugin should only be specified on one step, to prevent triggering builds multiple times in a pipeline. When update_prs is set to true in this mode, pipelines on the default branch (e.g. master) will trigger new builds on all PR branches that still exist in the repository. If oldest_pr is set, any PR with an issue number lower than specified will be ignored when triggering new builds.

steps:
  - label: 'Make something'
    commands:
      - make something
    plugins:
      - seek-oss/github-merged-pr#v1.1.2:
          mode: trigger
          update_prs: true
          oldest_pr: 12

  - label: 'Make something else'
    commands:
      - make something-else

Tests

To run the tests of this plugin, run

docker-compose run --rm tests

License

MIT (see LICENSE)

Bad Merge Example

An example of why it may be desirable to build the merged commit. Let's say you're raising a PR to implement an add function to a calculator that can already subtract:

origin/master (commit 1abcdef): calculator.js

class Calculator {
  constructor() {
    this.currentAnswer = 0;
  }

  subtract(value) {
    this.currentAnswer -= value;
  }
}

(Tests omitted for brevity).

You create a branch locally to implement your feature

feature/add (commit feba123) (parent commit 1abcdef): calculator.js

class Calculator {
  constructor() {
    this.currentAnswer = 0;
  }

  subtract(value) {
    this.currentAnswer -= value;
  }

  add(value) {
    this.currentAnswer += value;
  }
}

In the meantime, origin/master is updated to rename currentAnswer to answer. origin/master (commit 56789ab) (parent commit 1abcdef): calculator.js

class Calculator {
  constructor() {
    this.answer = 0;
  }

  subtract(value) {
    this.answer -= value;
  }
}

You then raise your PR for feature/add, but:

  1. CI will run against the HEAD of the branch (commit feba123); and
  2. The PR will likely be merged (as tests pass) -- unless someone on your team notices; and
  3. The git merge will succeed (as there's no text conflicts); and
  4. The target branch (master) will be broken :(