# Copyright 2015, Google Inc.
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#
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# modification, are permitted provided that the following conditions are
# met:
#
#     * Redistributions of source code must retain the above copyright
# notice, this list of conditions and the following disclaimer.
#     * Redistributions in binary form must reproduce the above
# copyright notice, this list of conditions and the following disclaimer
# in the documentation and/or other materials provided with the
# distribution.
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# contributors may be used to endorse or promote products derived from
# this software without specific prior written permission.
#
# THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
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# LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
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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
    NO_METADATA_SERVER_ERROR = <<END
Error code 404 trying to get security access token
from Compute Engine metadata for the default service account. This
may be because the virtual machine instance does not have permission
scopes specified.
END
    UNEXPECTED_ERROR_SUFFIX = <<END
trying to get security access token from Compute Engine metadata for
the default service account
END

    # 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)
        case resp.status
        when 200
          Signet::OAuth2.parse_credentials(resp.body,
                                           resp.headers['content-type'])
        when 404
          fail(Signet::AuthorizationError, NO_METADATA_SERVER_ERROR)
        else
          msg = "Unexpected error code #{resp.status}" + UNEXPECTED_ERROR_SUFFIX
          fail(Signet::AuthorizationError, msg)
        end
      end
    end
  end
end
