server.rb

require 'sinatra'
require 'octokit'
require 'dotenv/load' # Manages environment variables
require 'json'
require 'openssl'     # Verifies the webhook signature
require 'jwt'         # Authenticates a GitHub App
require 'time'        # Gets ISO 8601 representation of a Time object
require 'logger'      # Logs debug statements

set :port, 3000
set :bind, '0.0.0.0'

class GHAapp < Sinatra::Application
  get '/' do
    'Success! Watching organization events for new repositories.'
  end

  # Converts the newlines. Expects that the private key has been set as an
  # environment variable in PEM format.
  PRIVATE_KEY = OpenSSL::PKey::RSA.new(ENV['GITHUB_PRIVATE_KEY'].gsub('\n', "\n"))

  # Your registered app must have a secret set. The secret is used to verify
  # that webhooks are sent by GitHub.
  WEBHOOK_SECRET = ENV['GITHUB_WEBHOOK_SECRET']

  # The GitHub App's identifier (type integer) set when registering an app.
  APP_IDENTIFIER = ENV['GITHUB_APP_IDENTIFIER']

  # Turn on Sinatra's verbose logging during development
  configure :development do
    set :logging, Logger::DEBUG
  end

  # Executed before each request to the `/event_handler` route
  before '/event_handler' do
    get_payload_request(request)
    verify_webhook_signature
    authenticate_app
    # Authenticate the app installation in order to run API operations
    authenticate_installation(@payload)
  end

  post '/event_handler' do
    case request.env['HTTP_X_GITHUB_EVENT']
    when 'repository'
      if @payload['action'].match?('created')
        sleep 1 # wait for creation of default branch
        protect_default_branch(@payload)
        notify_user(@payload)
      end
    end

    200 # success status
  end

  helpers do
    # Protect the default branch on new repositories
    def protect_default_branch(payload)
      @repo = payload['repository']['full_name']
      @branch = payload['repository']['default_branch']
      options = {
        # This header is necessary for beta access to the branch_protection API
        # See https://developer.github.com/v3/repos/branches/#update-branch-protection
        accept: 'application/vnd.github.luke-cage-preview+json',
        # Require at least two approving reviews on a pull request before merging
        required_pull_request_reviews: { required_approving_review_count: 2 },
        # Enforce all configured restrictions for administrators
        enforce_admins: true
      }
      logger.debug 'Protecting default branch'
      @installation_client.protect_branch(@repo, @branch, options)
    end

    # Open an issue to notify the user of branch protection rules
    def notify_user(payload)
      username = payload['sender']['login']
      help_url = 'https://help.github.com/en/articles/about-protected-branches'
      issue_title = 'Default Branch Protected 🔐'
      issue_body = <<~BODY
        @#{username}: branch protection rules have been added to the `#{@branch}` branch.
        - Collaborators cannot force push to the protected branch or delete the branch
        - All commits must be made to a non-protected branch and submitted via a pull request
        - There must be least 2 approving reviews and no changes requested before a PR can be merged
        \n **Note:** All configured restrictions are enforced for administrators.
        \n You can learn more about protected branches here: [About protected branches - GitHub Help](#{help_url})
      BODY
      logger.debug 'Creating a new issue'
      @installation_client.create_issue(@repo, issue_title, issue_body)
    end

    # Saves the raw payload and converts the payload to JSON format
    def get_payload_request(request)
      # request.body is an IO or StringIO object
      # Rewind in case someone already read it
      request.body.rewind
      # The raw text of the body is required for webhook signature verification
      @payload_raw = request.body.read
      begin
        @payload = JSON.parse @payload_raw
      rescue => e
        fail  "Invalid JSON (#{e}): #{@payload_raw}"
      end
    end

    # Instantiate an Octokit client authenticated as a GitHub App.
    # GitHub App authentication requires that you construct a
    # JWT (https://jwt.io/introduction/) signed with the app's private key,
    # so GitHub can be sure that it came from the app an not altererd by
    # a malicious third party.
    def authenticate_app
      payload = {
          # The time that this JWT was issued, _i.e._ now.
          iat: Time.now.to_i,

          # JWT expiration time (10 minute maximum)
          exp: Time.now.to_i + (10 * 60),

          # Your GitHub App's identifier number
          iss: APP_IDENTIFIER
      }

      # Cryptographically sign the JWT.
      jwt = JWT.encode(payload, PRIVATE_KEY, 'RS256')

      # Create the Octokit client, using the JWT as the auth token.
      @app_client ||= Octokit::Client.new(bearer_token: jwt)
    end

    # Instantiate an Octokit client, authenticated as an installation of a
    # GitHub App, to run API operations.
    def authenticate_installation(payload)
      @installation_id = payload['installation']['id']
      @installation_token = @app_client.create_app_installation_access_token(@installation_id)[:token]
      @installation_client = Octokit::Client.new(bearer_token: @installation_token)
    end

    # Check X-Hub-Signature to confirm that this webhook was generated by
    # GitHub, and not a malicious third party.
    #
    # GitHub uses the WEBHOOK_SECRET, registered to the GitHub App, to
    # create the hash signature sent in the `X-HUB-Signature` header of each
    # webhook. This code computes the expected hash signature and compares it to
    # the signature sent in the `X-HUB-Signature` header. If they don't match,
    # this request is an attack, and you should reject it. GitHub uses the HMAC
    # hexdigest to compute the signature. The `X-HUB-Signature` looks something
    # like this: "sha1=123456".
    # See https://developer.github.com/webhooks/securing/ for details.
    def verify_webhook_signature
      their_signature_header = request.env['HTTP_X_HUB_SIGNATURE'] || 'sha1='
      method, their_digest = their_signature_header.split('=')
      our_digest = OpenSSL::HMAC.hexdigest(method, WEBHOOK_SECRET, @payload_raw)
      halt 401 unless their_digest == our_digest

      # The X-GITHUB-EVENT header provides the name of the event.
      # The action value indicates the which action triggered the event.
      logger.debug "---- received event #{request.env['HTTP_X_GITHUB_EVENT']}"
      logger.debug "----    action #{@payload['action']}" unless @payload['action'].nil?
    end

  end

  # Finally some logic to let us run this server directly from the command line,
  # or with Rack. Don't worry too much about this code. But, for the curious:
  # $0 is the executed file
  # __FILE__ is the current file
  # If they are the same—that is, we are running this file directly, call the
  # Sinatra run method
  run! if __FILE__ == $0
end