Optimise upload path calls

String#underscore isn't particularly slow, but it's possible for us to
call it many times in a users autocomplete request, with mostly-static
values ('User', 'Group', etc.). We can memoise this and save a
surprising amount of time (around 10% of the total request time in some
cases).

app/controllers/uploads_controller.rb

@@ -45,7 +45,7 @@ class UploadsController < ApplicationController
       when Appearance
         true
       else
-        permission = "read_#{model.class.to_s.underscore}".to_sym
+        permission = "read_#{model.class.underscore}".to_sym
 
         can?(current_user, permission, model)
       end

app/models/application_record.rb

@@ -41,4 +41,8 @@ class ApplicationRecord < ActiveRecord::Base
       find_or_create_by(*args)
     end
   end
+
+  def self.underscore
+    Gitlab::SafeRequestStore.fetch("model:#{self}:underscore") { self.to_s.underscore }
+  end
 end

app/uploaders/attachment_uploader.rb

@@ -9,6 +9,6 @@ class AttachmentUploader < GitlabUploader
   private
 
   def dynamic_segment
-    File.join(model.class.to_s.underscore, mounted_as.to_s, model.id.to_s)
+    File.join(model.class.underscore, mounted_as.to_s, model.id.to_s)
   end
 end

app/uploaders/avatar_uploader.rb

@@ -25,6 +25,6 @@ class AvatarUploader < GitlabUploader
   private
 
   def dynamic_segment
-    File.join(model.class.to_s.underscore, mounted_as.to_s, model.id.to_s)
+    File.join(model.class.underscore, mounted_as.to_s, model.id.to_s)
   end
 end

app/uploaders/personal_file_uploader.rb

@@ -20,7 +20,7 @@ class PersonalFileUploader < FileUploader
   def self.model_path_segment(model)
     return 'temp/' unless model
 
-    File.join(model.class.to_s.underscore, model.id.to_s)
+    File.join(model.class.underscore, model.id.to_s)
   end
 
   def object_store

changelogs/unreleased/make-autocomplete-faster-with-lots-of-results.yml

@@ -0,0 +1,5 @@
+---
+title: Improve performance of users autocomplete when there are lots of results
+merge_request:
+author:
+type: performance

spec/controllers/concerns/send_file_upload_spec.rb

@@ -1,3 +1,4 @@
+# coding: utf-8
 # frozen_string_literal: true
 
 require 'spec_helper'
@@ -13,7 +14,7 @@ describe SendFileUpload do
 
       # user/:id
       def dynamic_segment
-        File.join(model.class.to_s.underscore, model.id.to_s)
+        File.join(model.class.underscore, model.id.to_s)
       end
     end
   end

spec/factories/uploads.rb

@@ -13,7 +13,7 @@ FactoryBot.define do
     end
 
     # this needs to comply with RecordsUpload::Concern#upload_path
-    path { File.join("uploads/-/system", model.class.to_s.underscore, mount_point.to_s, 'avatar.jpg') }
+    path { File.join("uploads/-/system", model.class.underscore, mount_point.to_s, 'avatar.jpg') }
 
     trait :personal_snippet_upload do
       uploader "PersonalFileUploader"

spec/models/application_record_spec.rb

@@ -52,4 +52,10 @@ describe ApplicationRecord do
       expect { Suggestion.find_or_create_by!(note: nil) }.to raise_error(ActiveRecord::RecordInvalid)
     end
   end
+
+  describe '.underscore' do
+    it 'returns the underscored value of the class as a string' do
+      expect(MergeRequest.underscore).to eq('merge_request')
+    end
+  end
 end

spec/uploaders/object_storage_spec.rb

@@ -12,7 +12,7 @@ class Implementation < GitlabUploader
 
   # user/:id
   def dynamic_segment
-    File.join(model.class.to_s.underscore, model.id.to_s)
+    File.join(model.class.underscore, model.id.to_s)
   end
 end