slow down requests for dropzone uploads to avoid race conditions

lib/gitlab/testing/request_blocker_middleware.rb

@@ -7,6 +7,7 @@ module Gitlab
     class RequestBlockerMiddleware
       @@num_active_requests = Concurrent::AtomicFixnum.new(0)
       @@block_requests = Concurrent::AtomicBoolean.new(false)
+      @@slow_requests = Concurrent::AtomicBoolean.new(false)
 
       # Returns the number of requests the server is currently processing.
       def self.num_active_requests
@@ -19,9 +20,15 @@ module Gitlab
         @@block_requests.value = true
       end
 
+      # Slows down incoming requests (useful for race conditions).
+      def self.slow_requests!
+        @@slow_requests.value = true
+      end
+
       # Allows the server to accept requests again.
       def self.allow_requests!
         @@block_requests.value = false
+        @@slow_requests.value = false
       end
 
       def initialize(app)
@@ -33,6 +40,7 @@ module Gitlab
         if block_requests?
           block_request(env)
         else
+          sleep 0.2 if slow_requests?
           @app.call(env)
         end
       ensure
@@ -45,6 +53,10 @@ module Gitlab
         @@block_requests.true?
       end
 
+      def slow_requests?
+        @@slow_requests.true?
+      end
+
       def block_request(env)
         [503, {}, []]
       end

spec/features/uploads/user_uploads_file_to_note_spec.rb

@@ -21,6 +21,14 @@ feature 'User uploads file to note' do
   end
 
   context 'uploading is in progress' do
+    before do
+      Gitlab::Testing::RequestBlockerMiddleware.slow_requests!
+    end
+
+    after do
+      Gitlab::Testing::RequestBlockerMiddleware.allow_requests!
+    end
+
     it 'shows "Cancel" button on uploading', :js do
       dropzone_file([Rails.root.join('spec', 'fixtures', 'dk.png')], 0, false)