Exclude json content type from workhorse interception

app/controllers/application_controller.rb

@@ -116,7 +116,7 @@ class ApplicationController < ActionController::Base
   def render(*args)
     super.tap do
       # Set a header for custom error pages to prevent them from being intercepted by gitlab-workhorse
-      if response.content_type == 'text/html' && (400..599).cover?(response.status)
+      if workhorse_excluded_content_types.include?(response.content_type) && (400..599).cover?(response.status)
         response.headers['X-GitLab-Custom-Error'] = '1'
       end
     end
@@ -124,6 +124,10 @@ class ApplicationController < ActionController::Base
   protected
+  def workhorse_excluded_content_types
+    @workhorse_excluded_content_types ||= %w(text/html application/json)
+  end
+
   def append_info_to_payload(payload)
     super

spec/controllers/application_controller_spec.rb

@@ -641,24 +641,32 @@ describe ApplicationController do
         end
       end
-      it 'does not set a custom header' do
+      it 'sets a custom header' do
         get :index, format: :json
-        expect(response.headers['X-GitLab-Custom-Error']).to be_nil
+        expect(response.headers['X-GitLab-Custom-Error']).to eq '1'
       end
-    end
-    context 'given a json response for an html request' do
-      controller do
-        def index
-          render json: {}, status: :unprocessable_entity
+      context 'for html request' do
+        it 'sets a custom header' do
+          get :index
+
+          expect(response.headers['X-GitLab-Custom-Error']).to eq '1'
         end
       end
-      it 'does not set a custom header' do
-        get :index
-        expect(response.headers['X-GitLab-Custom-Error']).to be_nil
+      context 'for 200 response' do
+        controller do
+          def index
+            render json: {}, status: :ok
+          end
+        end
+        it 'does not set a custom header' do
+          get :index, format: :json
+
+          expect(response.headers['X-GitLab-Custom-Error']).to be_nil
+        end
       end
     end
   end