Add AllowedIdentifiers configuration option

config/default.yml

@@ -44,3 +44,4 @@ ThreadSafety/RackMiddlewareInstanceVariable:
     - 'lib/middleware/**/*.rb'
     - 'app/middlewares/**/*.rb'
     - 'lib/middlewares/**/*.rb'
+  AllowedIdentifiers: []

docs/modules/ROOT/pages/cops_threadsafety.adoc

@@ -333,4 +333,8 @@ end
 | Include
 | `+app/middleware/**/*.rb+`, `+lib/middleware/**/*.rb+`, `+app/middlewares/**/*.rb+`, `+lib/middlewares/**/*.rb+`
 | Array
+
+| AllowedIdentifiers
+| `[]`
+| Array
 |===

lib/rubocop/cop/thread_safety/rack_middleware_instance_variable.rb

@@ -44,6 +44,7 @@ module ThreadSafety
       #     end
       #   end
       class RackMiddlewareInstanceVariable < Base
+        include AllowedIdentifiers
         include OperationWithThreadsafeResult
 
         MSG = 'Avoid instance variables in Rack middleware.'
@@ -68,8 +69,10 @@ def on_class(node)
 
           node.each_node(:def) do |method_definition_node|
             method_definition_node.each_node(:ivasgn, :ivar) do |ivar_node|
-              assignable, = ivar_node.to_a
-              next if assignable == application_variable || safe_variables.include?(assignable)
+              variable, = ivar_node.to_a
+              if variable == application_variable || safe_variables.include?(variable) || allowed_identifier?(variable)
+                next
+              end
 
               add_offense ivar_node
             end

spec/rubocop/cop/thread_safety/rack_middleware_instance_variable_spec.rb

@@ -316,4 +316,27 @@ def call(env)
       RUBY
     end
   end
+
+  context 'with non-empty `AllowedIdentifiers` config' do
+    let(:cop_config) do
+      { 'AllowedIdentifiers' => ['options'] }
+    end
+
+    it 'registers an offense' do
+      expect_offense(<<~RUBY)
+        class TestMiddleware
+          def call(env)
+            @app.call(env)
+          end
+
+          def initialize(app)
+            @app = app
+            @some_var = 2
+            ^^^^^^^^^^^^^ #{msg}
+            @options = 1
+          end
+        end
+      RUBY
+    end
+  end
 end