concepts-guidelines.md

Concepts, guidelines and advanced usage

This document covers terminology, how the action works, general usage guidelines, and advanced usage.

• Terminology
• Events and checkout
• How the action works
• Guidelines
  • Providing a consistent base
  • Events which checkout a commit
  • Restrictions on repository forks
  • Triggering further workflow runs
  • Security
• Advanced usage
  • Creating pull requests in a remote repository
  • Push using SSH (deploy keys)
  • Push pull request branches to a fork
  • Authenticating with GitHub App generated tokens
  • Running in a container or on self-hosted runners

Terminology

Pull requests are proposed changes to a repository branch that can be reviewed by a repository's collaborators before being accepted or rejected.

A pull request references two branches:

• The base of a pull request is the branch you intend to change once the proposed changes are merged.
• The branch of a pull request represents what you intend the base to look like when merged. It is the base branch plus changes that have been made to it.

Events and checkout

This action expects repositories to be checked out with the official GitHub Actions checkout action. For each event type there is a default GITHUB_SHA that will be checked out.

The default can be overridden by specifying a ref on checkout.

      - uses: actions/checkout@v2
        with:
          ref: develop

How the action works

Unless the base input is supplied, the action expects the target repository to be checked out on the pull request base—the branch you intend to modify with the proposed changes.

Workflow steps:

1. Checkout the base branch
2. Make changes
3. Execute create-pull-request action

The following git diagram shows how the action creates and updates a pull request branch.

Guidelines

Providing a consistent base

For the action to work correctly it should be executed in a workflow that checks out a consistent base branch. This will be the base of the pull request unless overridden with the base input.

This means your workflow should be consistently checking out the branch that you intend to modify once the PR is merged.

In the following example, the push and create events both trigger the same workflow. This will cause the checkout action to checkout inconsistent branches and commits. Do not do this. It will cause multiple pull requests to be created for each additional base the action is executed against.

on:
  push:
  create:
jobs:
  example:
    runs-on: ubuntu-latest
    steps:
      - uses: actions/checkout@v2

There may be use cases where it makes sense to execute the workflow on a branch that is not the base of the pull request. In these cases, the base branch can be specified with the base action input. The action will attempt to rebase changes made during the workflow on to the actual base.

Events which checkout a commit

The default checkout for the majority of events will leave the repository checked out on a branch. However, some events such as release and pull_request will leave the repository in a "detached HEAD" state. This is because they checkout a commit, not a branch. In these cases, you must supply the base input so the action can rebase changes made during the workflow for the pull request.

Workflows triggered by pull_request events will by default check out a merge commit. Set the base input as follows to base the new pull request on the current pull request's branch.

      - uses: peter-evans/create-pull-request@v3
        with:
          base: ${{ github.head_ref }}

Workflows triggered by release events will by default check out a tag. For most use cases, you will need to set the base input to the branch name of the tagged commit.

      - uses: peter-evans/create-pull-request@v3
        with:
          base: master

Restrictions on repository forks

GitHub Actions have imposed restrictions on workflow runs triggered by public repository forks. Private repositories can be configured to enable workflows from forks to run without restriction.

The restrictions apply to the pull_request event triggered by a fork opening a pull request in the upstream repository.

• Events from forks cannot access secrets, except for the default GITHUB_TOKEN.

  With the exception of GITHUB_TOKEN, secrets are not passed to the runner when a workflow is triggered from a forked repository.

  GitHub Actions: Using encrypted secrets in a workflow

• The GITHUB_TOKEN has read-only access when an event is triggered by a forked repository.

  GitHub Actions: Permissions for the GITHUB_TOKEN

These restrictions mean that during a pull_request event triggered by a forked repository, actions have no write access to GitHub resources and will fail on any attempt.

A job condition can be added to prevent workflows from executing when triggered by a repository fork.

on: pull_request
jobs:
  example:
    runs-on: ubuntu-latest
    # Check if the event is not triggered by a fork
    if: github.event.pull_request.head.repo.full_name == github.repository

Triggering further workflow runs

Pull requests created by the action using the default GITHUB_TOKEN cannot trigger other workflows. If you have on: pull_request or on: push workflows acting as checks on pull requests, they will not run.

When you use the repository's GITHUB_TOKEN to perform tasks on behalf of the GitHub Actions app, events triggered by the GITHUB_TOKEN will not create a new workflow run.

GitHub Actions: Events that trigger workflows

Workarounds to trigger further workflow runs

There are a number of workarounds with different pros and cons.

• Use the default GITHUB_TOKEN and allow the action to create pull requests that have no checks enabled. Manually close pull requests and immediately reopen them. This will enable on: pull_request workflows to run and be added as checks.

• Use a repo scoped Personal Access Token (PAT) created on an account that has write access to the repository that pull requests are being created in. This is the standard workaround and recommended by GitHub. However, the PAT cannot be scoped to a specific repository so the token becomes a very sensitive secret. If this is a concern, the PAT can instead be created for a dedicated machine account that has collaborator access to the repository. Also note that because the account that owns the PAT will be the creator of pull requests, that user account will be unable to perform actions such as request changes or approve the pull request.

• Use SSH (deploy keys) to push the pull request branch. This is arguably more secure than using a PAT because deploy keys can be set per repository. However, this method will only trigger on: push workflows.

• Use a machine account that creates pull requests from its own fork. This is the most secure because the PAT created only grants access to the machine account's fork, not the main repository. This method will trigger on: pull_request workflows to run. Workflows triggered on: push will not run because the push event is in the fork.

• Use a GitHub App to generate a token that can be used with this action. GitHub App generated tokens are more secure than using a PAT because GitHub App access permissions can be set with finer granularity and are scoped to only repositories where the App is installed. This method will trigger both on: push and on: pull_request workflows.

Security

From a security perspective it's good practice to fork third-party actions, review the code, and use your fork of the action in workflows. By using third-party actions directly the risk exists that it could be modified to do something malicious, such as capturing secrets.

Alternatively, use the action directly and reference the commit hash for the version you want to target.

  - uses: thirdparty/foo-action@172ec762f2ac8e050062398456fccd30444f8f30

This action uses ncc to compile the Node.js code and dependencies into a single JavaScript file under the dist directory.

Advanced usage

Creating pull requests in a remote repository

Checking out a branch from a different repository from where the workflow is executing will make that repository the target for the created pull request. In this case, a repo scoped Personal Access Token (PAT) is required.

      - uses: actions/checkout@v2
        with:
          token: ${{ secrets.PAT }}
          repository: owner/repo

      # Make changes to pull request here

      - uses: peter-evans/create-pull-request@v3
        with:
          token: ${{ secrets.PAT }}

Push using SSH (deploy keys)

Deploy keys can be set per repository and so are arguably more secure than using a repo scoped Personal Access Token (PAT). Allowing the action to push with a configured deploy key will trigger on: push workflows. This makes it an alternative to using a PAT to trigger checks for pull requests. Note that you cannot use deploy keys alone to create a pull request in a remote repository because then using a PAT would become a requirement. This method only makes sense if creating a pull request in the repository where the workflow is running.

How to use SSH (deploy keys) with create-pull-request action:

1. Create a new SSH key pair for your repository. Do not set a passphrase.
2. Copy the contents of the public key (.pub file) to a new repository deploy key and check the box to "Allow write access."
3. Add a secret to the repository containing the entire contents of the private key.
4. As shown in the example below, configure actions/checkout to use the deploy key you have created.

    steps:
      - uses: actions/checkout@v2
        with:
          ssh-key: ${{ secrets.SSH_PRIVATE_KEY }}

      # Make changes to pull request here

      - name: Create Pull Request
        uses: peter-evans/create-pull-request@v3

Push pull request branches to a fork

Instead of pushing pull request branches to the repository you want to update, you can push them to a fork of that repository. This allows you to employ the principle of least privilege by using a dedicated user acting as a machine account. This user has no access to the main repository. It will use their own fork to push code and create the pull request.

1. Create a new GitHub user and login.
2. Fork the repository that you will be creating pull requests in.
3. Create a Personal Access Token (PAT).
4. Logout and log back into your main user account.
5. Add a secret to your repository containing the above PAT.
6. As shown in the following example workflow, set the push-to-fork input to the full repository name of the fork.

      - uses: actions/checkout@v2

      # Make changes to pull request here

      - uses: peter-evans/create-pull-request@v3
        with:
          token: ${{ secrets.MACHINE_USER_PAT }}
          push-to-fork: machine-user/fork-of-repository

Authenticating with GitHub App generated tokens

A GitHub App can be created for the sole purpose of generating tokens for use with GitHub actions. These tokens can be used in place of GITHUB_TOKEN or a Personal Access Token (PAT). GitHub App generated tokens are more secure than using a PAT because GitHub App access permissions can be set with finer granularity and are scoped to only repositories where the App is installed.

1. Create a minimal GitHub App, setting the following fields:

   • Set GitHub App name.
   • Set Homepage URL to anything you like, such as your GitHub profile page.
   • Uncheck Active under Webhook. You do not need to enter a Webhook URL.
   • Under Repository permissions: Contents select Access: Read & write.
   • Under Repository permissions: Pull requests select Access: Read & write.

2. Create a Private key from the App settings page and store it securely.

3. Install the App on any repository where workflows will run requiring tokens.

4. Set secrets on your repository containing the GitHub App ID, and the private key you created in step 2. e.g. APP_ID, APP_PRIVATE_KEY.

5. The following example workflow shows how to use tibdex/github-app-token to generate a token for use with this action.

    steps:
      - uses: actions/checkout@v2

      - uses: tibdex/github-app-token@v1
        id: generate-token
        with:
          app_id: ${{ secrets.APP_ID }}
          private_key: ${{ secrets.APP_PRIVATE_KEY }}

      # Make changes to pull request here

      - name: Create Pull Request
        uses: peter-evans/create-pull-request@v3
        with:
          token: ${{ steps.generate-token.outputs.token }}

Running in a container or on self-hosted runners

This action can be run inside a container, or on self-hosted runners, by installing the necessary dependencies.

This action requires git to be installed and on the PATH. Note that actions/checkout requires Git 2.18 or higher to be installed, otherwise it will just download the source of the repository instead of cloning it.

The following examples of running in a container show the dependencies being installed during the workflow, but they could also be pre-installed in a custom image.

Alpine container example:

jobs:
  createPullRequestAlpine:
    runs-on: ubuntu-latest
    container:
      image: alpine
    steps:
      - name: Install dependencies
        run: apk --no-cache add git

      - uses: actions/checkout@v2

      # Make changes to pull request here

      - name: Create Pull Request
        uses: peter-evans/create-pull-request@v3

Ubuntu container example:

jobs:
  createPullRequestAlpine:
    runs-on: ubuntu-latest
    container:
      image: ubuntu
    steps:
      - name: Install dependencies
        run: |
          apt-get update
          apt-get install -y software-properties-common
          add-apt-repository -y ppa:git-core/ppa
          apt-get install -y git

      - uses: actions/checkout@v2

      # Make changes to pull request here

      - name: Create Pull Request
        uses: peter-evans/create-pull-request@v3