# Copyright 2015, Google Inc.
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#
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require 'faraday'
require 'googleauth/signet'
require 'memoist'

module Google
  # Module Auth provides classes that provide Google-specific authorization
  # used to access Google APIs.
  module Auth
    # Extends Signet::OAuth2::Client so that the auth token is obtained from
    # the GCE metadata server.
    class GCECredentials < Signet::OAuth2::Client
      # The IP Address is used in the URIs to speed up failures on non-GCE
      # systems.
      COMPUTE_AUTH_TOKEN_URI = 'http://169.254.169.254/computeMetadata/v1/'\
                               'instance/service-accounts/default/token'
      COMPUTE_CHECK_URI = 'http://169.254.169.254'

      class << self
        extend Memoist

        # Detect if this appear to be a GCE instance, by checking if metadata
        # is available
        def on_gce?(options = {})
          c = options[:connection] || Faraday.default_connection
          resp = c.get(COMPUTE_CHECK_URI) do |req|
            # Comment from: oauth2client/client.py
            #
            # Note: the explicit `timeout` below is a workaround. The underlying
            # issue is that resolving an unknown host on some networks will take
            # 20-30 seconds; making this timeout short fixes the issue, but
            # could lead to false negatives in the event that we are on GCE, but
            # the metadata resolution was particularly slow. The latter case is
            # "unlikely".
            req.options.timeout = 0.1
          end
          return false unless resp.status == 200
          return false unless resp.headers.key?('Metadata-Flavor')
          return resp.headers['Metadata-Flavor'] == 'Google'
        rescue Faraday::TimeoutError, Faraday::ConnectionFailed
          return false
        end

        memoize :on_gce?
      end

      # Overrides the super class method to change how access tokens are
      # fetched.
      def fetch_access_token(options = {})
        c = options[:connection] || Faraday.default_connection
        c.headers = { 'Metadata-Flavor' => 'Google' }
        resp = c.get(COMPUTE_AUTH_TOKEN_URI)
        Signet::OAuth2.parse_credentials(resp.body,
                                         resp.headers['content-type'])
      end
    end
  end
end