# Copyright 2015, Google Inc.
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require 'googleauth/signet'
require 'googleauth/credentials_loader'
require 'multi_json'

module Google
  # Module Auth provides classes that provide Google-specific authorization
  # used to access Google APIs.
  module Auth
    # Authenticates requests using Google's Service Account credentials.
    #
    # This class allows authorizing requests for service accounts directly
    # from credentials from a json key file downloaded from the developer
    # console (via 'Generate new Json Key').
    #
    # cf [Application Default Credentials](http://goo.gl/mkAHpZ)
    class ServiceAccountCredentials < Signet::OAuth2::Client
      TOKEN_CRED_URI = 'https://www.googleapis.com/oauth2/v3/token'
      extend CredentialsLoader

      # Reads the private key and client email fields from the service account
      # JSON key.
      def self.read_json_key(json_key_io)
        json_key = MultiJson.load(json_key_io.read)
        fail 'missing client_email' unless json_key.key?('client_email')
        fail 'missing private_key' unless json_key.key?('private_key')
        [json_key['private_key'], json_key['client_email']]
      end

      # Initializes a ServiceAccountCredentials.
      #
      # @param scope [string|array] the scope(s) to access
      # @param json_key_io [IO] an IO from which the JSON key can be read
      def initialize(scope, json_key_io)
        private_key, client_email = self.class.read_json_key(json_key_io)
        super(token_credential_uri: TOKEN_CRED_URI,
              audience: TOKEN_CRED_URI,
              scope: scope,
              issuer: client_email,
              signing_key: OpenSSL::PKey::RSA.new(private_key))
      end
    end
  end
end