class Restforce::Middleware::Authentication

the oauth access token (if a refresh token is present).
will attempt to either reauthenticate (username and password) or refresh
When a request fails (a status of 401 is returned), the middleware
Faraday middleware that allows for on the fly authentication of requests.

def authenticate!

Internal: Performs the authentication and returns the response body. 
def authenticate!
  response = connection.post '/services/oauth2/token' do |req|
    req.body = encode_www_form(params)
  end
  if response.status >= 500
    raise Restforce::ServerError, error_message(response)
  elsif response.status != 200
    raise Restforce::AuthenticationError, error_message(response)
  end
  @options[:instance_url] = response.body['instance_url']
  @options[:oauth_token]  = response.body['access_token']
  @options[:authentication_callback]&.call(response.body)
  response.body
end

def call(env)

can reissue the request.
Rescue from 401's, authenticate then raise the error again so the client 
def call(env)
  @app.call(env)
rescue Restforce::UnauthorizedError
  authenticate!
  raise
end

def connection

Internal: Faraday connection to use when sending an authentication request. 
def connection
  @connection ||= Faraday.new(faraday_options) do |builder|
    builder.use Faraday::Request::UrlEncoded
    builder.use Restforce::Middleware::Mashify, nil, @options
    builder.response :json
    if Restforce.log?
      builder.use Restforce::Middleware::Logger,
                  Restforce.configuration.logger,
                  @options
    end
    builder.adapter @options[:adapter]
  end
end

def encode_www_form(params)

URI in 1.8 does not include encode_www_form
Featured detect form encoding. 
def encode_www_form(params)
  if URI.respond_to?(:encode_www_form)
    URI.encode_www_form(params)
  else
    params.map do |k, v|
      k = CGI.escape(k.to_s)
      v = CGI.escape(v.to_s)
      "#{k}=#{v}"
    end.join('&')
  end
end

def error_message(response)

Internal: The parsed error response. 
def error_message(response)
  return response.status.to_s unless response.body
  "#{response.body['error']}: #{response.body['error_description']} " \
    "(#{response.status})"
end

def faraday_options 

def faraday_options
  { url: "https://#{@options[:host]}",
    proxy: @options[:proxy_uri],
    ssl: @options[:ssl] }
end

def params

Internal: The params to post to the OAuth service. 
def params
  raise NotImplementedError
end