Fix infinite loop when mounting endpoint with same superclass

Subclass APIs were being assigned the same top level inheritable
settings from the superclass. When an API attempted to mount another
API with the same superclass, the same setting object would be added to
the setting stack. When an attempt was made to retrieve a value from the
settings, it would sit in loop calling itself.

.rubocop_todo.yml

@@ -34,7 +34,7 @@ Metrics/AbcSize:
 # Offense count: 282
 # Configuration parameters: CountComments, ExcludedMethods.
 Metrics/BlockLength:
-  Max: 3117
+  Max: 3125
 
 # Offense count: 9
 # Configuration parameters: CountComments.

CHANGELOG.md

@@ -14,6 +14,7 @@
 * [#1724](https://github.com/ruby-grape/grape/pull/1724): Optional nested array validation - [@ericproulx](https://github.com/ericproulx).
 * [#1725](https://github.com/ruby-grape/grape/pull/1725): Fix `rescue_from :all` documentation - [@Jelkster](https://github.com/Jelkster).
 * [#1726](https://github.com/ruby-grape/grape/pull/1726): Improved startup performance during API method generation - [@jkowens](https://github.com/jkowens).
+* [#1727](https://github.com/ruby-grape/grape/pull/1727): Fix infinite loop when mounting endpoint with same superclass - [@jkowens](https://github.com/jkowens).
 * Your contribution here.
 
 ### 1.0.1 (9/8/2017)

lib/grape/dsl/settings.rb

@@ -165,13 +165,13 @@ def within_namespace(&_block)
 
       private
 
-      # Builds the current class :inheritable_setting. If available, it returns the superclass's :inheritable_setting.
-      # Otherwise, a clean :inheritable_setting is returned.
+      # Builds the current class :inheritable_setting. If available, it inherits from
+      # the superclass's :inheritable_setting.
       def build_top_level_setting
-        if defined?(superclass) && superclass.respond_to?(:inheritable_setting) && superclass != Grape::API
-          superclass.inheritable_setting
-        else
-          Grape::Util::InheritableSetting.new
+        Grape::Util::InheritableSetting.new.tap do |setting|
+          if defined?(superclass) && superclass.respond_to?(:inheritable_setting) && superclass != Grape::API
+            setting.inherit_from superclass.inheritable_setting
+          end
         end
       end
     end

spec/grape/api_spec.rb

@@ -3129,6 +3129,16 @@ def static
         get '/two/v1/world'
         expect(last_response.status).to eq 200
       end
+
+      context 'when mounting class extends a subclass of Grape::API' do
+        it 'mounts APIs with the same superclass' do
+          base_api = Class.new(Grape::API)
+          a = Class.new(base_api)
+          b = Class.new(base_api)
+
+          expect { a.mount b }.to_not raise_error
+        end
+      end
     end
   end