add support for refresh tokens

[zendesk-mradmacher]

Add support for refreshing OAuth access token.

When you need some inspiration, consult how the token refresh flow is done by Restforce, the public gem for Salesforce REST API.

I add a new ZendeskAPI::TokenRefresher service, to obtain new access and refresh tokens. It takes client configuration as a parameter. When tokens get refreshed, the client configuration is updated, so the next requests are issued using the newly obtained access token. Also a block is yielded with new access and refresh tokens, so they could be stored for further use.

The alternative solution is to use ZendeskAPI::Middleware::Response::TokenRefresher, to start the process of token refreshing each time when the API responds with 401. The middleware also updates the client configuration. It requires a callback to be configured. The called is called whenever new tokens are obtained to allow storing them for further use.
There is a configuration option to enable middleware usage.

How to test?

In Admin Center, Apps and Integrations / APIs / OAuth Clients create a new client. Note client id and secret and redirect URL. The redirect URL can be any address.

• Login to Zendesk as an user.
• Assumming your Zendesk subdomain is "testing", open the following URL in browser:

https://testing.zendesk.com/oauth/authorizations/new?client_id=<your client id>&response_type=code&redirect_uri=<encoded redirect URL>&scope=read

The encoded URL looks like https%3A%2F%2Fexample.com.

• Accept the authorization and intercept the code issued to the redirect URL.
• Request for access and refresh tokens using the intercepted code:

curl https://testing.zendesk.com/oauth/tokens \
  -H "Content-Type: application/json" \
  -d '{"grant_type": "authorization_code", "code": "<here goes the code>", "client_id": "<your client id>", "client_secret": "<your client secret>", "redirect_uri": "<your redirect URL", "scope": "read", "expires_in": 300 }' \
  -X POST

expires_in is chosen to be 5 minutes (the lowest possible value), to make the access token expire quickly, for easier testing.

• Save tokens in a file named tokens in the following format:

<access token>,<refresh token>

• Use the following Ruby script to make requests to your Zendesk. Observe how tokens are refreshed upon expiring.

require 'zendesk_api'

# `tokens` is a one line file that stores comma separated tokens:
# <access token>,<refresh token>
access_token, refresh_token = File.open("tokens") do |file|
  file.read.strip.split(",")
end

client = ZendeskAPI::Client.new do |config|
  # Mandatory:
  config.url = "https://testing.zendesk.com/api/v2"

  config.client_id = "<your client id>"
  config.client_secret = "<your client secret>"
  config.access_token_expiration = 300 # expire quickly to observe token refreshing

  config.access_token = access_token
  config.refresh_token = refresh_token

  # In case of using the middleware for refreshing tokens, update the storage with new tokens
  # config.auto_refresh_tokens = true
  # config.refresh_tokens_callback = lambda do |access_token, refresh_token|
  #   File.open("tokens", "w") do |file|
  #     file.write("#{access_token},#{refresh_token}")
  #   end
  # end

  require 'logger'
  config.logger = Logger.new(STDOUT)
end

users = client.users.per_page(1)
begin
  # A request with an expired access token is made.
  users.fetch!
  # The request is rejected with 401 status code.
rescue ZendeskAPI::Error::Unauthorized
  # Refresh tokens and store them securely
  ZendeskAPI::TokenRefresher.new(client.config).refresh_token do |access_token, refresh_token|
    File.open("tokens", "w") do |file|
      file.write("#{access_token},#{refresh_token}")
    end
  end
  # Issue the request again.
  users.fetch!
end