get-github-app-token: Retrieve Auth Token for GitHub App

GitHub Apps can be assigned more granular permissions than personal access tokens (PATs), but authenticating as a GitHub App is more complicated than just using a PAT. The authentication process works like this:

1. You download a private key for your app from GitHub.
2. You use that private key to sign a JSON Web Token (JWT) and present that JWT to GitHub to retrieve an installation token.
3. Use that installation token to authenticate API calls to GitHub, for example to operate on repositories.

For more details, see GitHub's documentation.

get-github-app-token simplifies using GitHub Apps in GitHub Actions workflows by handling step 2 for you. You put your GitHub App's ID and private key into your repository's secrets. Then in your workflow, you use get-github-app-token to get an installation token that you can use later in your workflow.

Note that your GitHub App must be installed on the repository you want to perform privileged operations on.

Usage on GitHub Actions

You can use this action in a GitHub Actions workflow like this:

  - name: Retrieve token
    id: get-token
    uses: U8NWXD/[email protected]
    with:
      app_id: ${{ secrets.APP_ID }}
      private_key: ${{ secrets.APP_PRIVATE_KEY }}
  - name: Checkout private repository
    uses: actions/checkout@v2
    with:
      repository: user/my-private-repo
      token: ${{ steps.get-token.outputs.token }}

Note that you MUST specify a release branch after @ on the uses: line. The main branch does not contain the bundled distribution files that GitHub Actions requires. Tags like v0.0.1 live on the main branch and so also lack the bundled distribution files. These files are only present on the release branches.

Because we rely on release branches to keep the distribution files out of the main branch, you cannot specify major or major-minor version numbers (e.g. v1 or v1.1). Instead, you must specify a full version number (e.g. v1.0.1) as part of the release branch name (e.g. release-v1.0.1). Alternatively, you can specify a commit hash.

Developing

Set Up

Setting up the project is as easy as running yarn install to install the dependencies.

Demo

To test out this GitHub Action, create a GitHub app and install it on a repository. Then download its private key and save the file as privateKey.pem. Then create a demo.js file with the following content:

const fs = require('fs');
const utils = require('./utils');

async function main() {
  const privateKey = (await fs.promises.readFile('privateKey.pem')).toString();

  const token = await utils.getToken(
    '<YOUR APP ID>',
    privateKey,
    '<YOUR USER NAME>/<YOUR REPOSITORY NAME>');
  console.log(token);
}

main();

Then when you run node demo.js you should see a token printed that starts with ghs_.

Create a New Release

To create a new release, you need to first:

• Make sure you have all your changes in the main branch. This includes updates to the version number in package.json.

• Pick a version number. We follow semantic versioning.

• Checkout the main branch and run the release script with your version number (in this example v0.0.1):

  $ ./release.sh 0.0.1
  ...

The release.sh script takes care of everything else for you. It:

• Runs some simple checks to make sure you remembered to update the package.json version and that you have a clean working directory.
• Creates a new version tag on main and pushes it.
• Creates a new distribution with yarn dist. This command creates a bundled dist/index.json file with all the dependencies, and it copies over action.yml into dist/.
• Creates a new git repository in dist and pushes the contents to the release branch. Note that this means that we do not expect to track commit history on the release branches. Each branch will have just one commit with the distribution files.