Adds an implementation of Application Default Credentials

- supports two initial scenarios
  - 2LO with service accounts
  - Compute Engine

- performs all 3 'sniffs' for the service account credentials
  - from environment
  - from a well known file
  - from GCE

lib/googleauth.rb

@@ -0,0 +1,62 @@
+# Copyright 2015, Google Inc.
+# All rights reserved.
+#
+# Redistribution and use in source and binary forms, with or without
+# 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.
+#     * Neither the name of Google Inc. nor the names of its
+# 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
+# "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
+# LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS FOR
+# A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE COPYRIGHT
+# OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT, INCIDENTAL,
+# SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING, BUT NOT
+# LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES; LOSS OF USE,
+# DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER CAUSED AND ON ANY
+# THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT LIABILITY, OR TORT
+# (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN ANY WAY OUT OF THE USE
+# OF THIS SOFTWARE, EVEN IF ADVISED OF THE POSSIBILITY OF SUCH DAMAGE.
+
+require 'googleauth/service_account'
+require 'googleauth/compute_engine'
+
+module Google
+  # Module Auth provides classes that provide Google-specific authorization
+  # used to access Google APIs.
+  module Auth
+    NOT_FOUND = <<END
+Could not load the default credentials. Browse to
+https://developers.google.com/accounts/docs/application-default-credentials
+for more information
+END
+
+    # Obtains the default credentials implementation to use in this
+    # environment.
+    #
+    # If supplied, scope is used to create the credentials instance, when it
+    # can applied.  E.g, on compute engine, the scope is ignored.
+    #
+    # @param scope [string|array] the scope(s) to access
+    # @param options [hash] allows override of the connection being used
+    def get_application_default(scope, options = {})
+      creds = ServiceAccountCredentials.from_env(scope)
+      return creds unless creds.nil?
+      creds = ServiceAccountCredentials.from_well_known_path(scope)
+      return creds unless creds.nil?
+      fail NOT_FOUND unless GCECredentials.on_gce?(options)
+      GCECredentials.new
+    end
+
+    module_function :get_application_default
+  end
+end