Fix custom trace mode using default base class

lib/graphql/schema.rb

@@ -171,9 +171,9 @@ def trace_class(new_class = nil)
       end
 
       # @return [Class] Return the trace class to use for this mode, looking one up on the superclass if this Schema doesn't have one defined.
-      def trace_class_for(mode)
+      def trace_class_for(mode, build: true)
         own_trace_modes[mode] ||
-          (superclass.respond_to?(:trace_class_for) ? superclass.trace_class_for(mode) : (own_trace_modes[mode] = build_trace_mode(mode)))
+          (superclass.respond_to?(:trace_class_for) ? superclass.trace_class_for(mode, build: build) : (build ? (own_trace_modes[mode] = build_trace_mode(mode)) : nil))
       end
 
       # Configure `trace_class` to be used whenever `context: { trace_mode: mode_name }` is requested.
@@ -218,7 +218,7 @@ def build_trace_mode(mode)
         else
           # First, see if the superclass has a custom-defined class for this.
           # Then, if it doesn't, use this class's default trace
-          base_class = (superclass.respond_to?(:trace_class_for) && superclass.trace_class_for(mode)) || trace_class_for(:default)
+          base_class = (superclass.respond_to?(:trace_class_for) && superclass.trace_class_for(mode, build: false)) || trace_class_for(:default)
           # Prepare the default trace class if it hasn't been initialized yet
           base_class ||= (own_trace_modes[:default] = build_trace_mode(:default))
           mods = trace_modules_for(mode)

spec/graphql/tracing/trace_modes_spec.rb

@@ -138,6 +138,31 @@ def execute_query(query:)
     end
   end
 
+  describe "inheriting from a custom default trace class" do
+    class CustomBaseTraceParentSchema < GraphQL::Schema
+      class CustomTrace < GraphQL::Tracing::Trace
+      end
+
+      trace_class CustomTrace
+    end
+
+    class CustomBaseTraceSubclassSchema < CustomBaseTraceParentSchema
+      trace_with Module.new, mode: :special_with_base_class
+    end
+
+    it "uses the default trace class for default mode" do
+      assert_equal CustomBaseTraceParentSchema::CustomTrace, CustomBaseTraceParentSchema.trace_class_for(:default)
+      assert_equal CustomBaseTraceParentSchema::CustomTrace, CustomBaseTraceSubclassSchema.trace_class_for(:default).superclass
+
+      assert_instance_of CustomBaseTraceParentSchema::CustomTrace, CustomBaseTraceParentSchema.new_trace
+      assert_kind_of CustomBaseTraceParentSchema::CustomTrace, CustomBaseTraceSubclassSchema.new_trace
+    end
+
+    it "uses the default trace class for special modes" do
+      assert_includes CustomBaseTraceSubclassSchema.trace_class_for(:special_with_base_class).ancestors, CustomBaseTraceParentSchema::CustomTrace
+      assert_kind_of CustomBaseTraceParentSchema::CustomTrace, CustomBaseTraceSubclassSchema.new_trace(mode: :special_with_base_class)
+    end
+  end
 
   describe "custom default trace mode" do
     class CustomDefaultSchema < TraceModesTest::ParentSchema