FIX: CDN wasn't properly applied to category background images when using S3

lib/stylesheet/importer.rb

@@ -132,12 +132,8 @@ COMMENT
       @theme == :nil ? nil : @theme
     end
 
-    def apply_cdn(url)
-      "#{GlobalSetting.cdn_url}#{url}"
-    end
-
     def category_css(category)
-      "body.category-#{category.full_slug} { background-image: url(#{apply_cdn(category.uploaded_background.url)}) }\n"
+      "body.category-#{category.full_slug} { background-image: url(#{upload_cdn_path(category.uploaded_background.url)}) }\n"
     end
 
     def imports(asset, parent_path)

spec/components/stylesheet/importer_spec.rb

@@ -0,0 +1,31 @@
+require 'rails_helper'
+require 'stylesheet/importer'
+
+describe Stylesheet::Importer do
+
+  def compile_css(name)
+    Stylesheet::Compiler.compile_asset(name)[0]
+  end
+
+  it "applies CDN to background category images" do
+    expect(compile_css("category_backgrounds")).to_not include("background-image")
+
+    background = Fabricate(:upload)
+    category = Fabricate(:category, uploaded_background: background)
+
+    expect(compile_css("category_backgrounds")).to include("body.category-#{category.full_slug}{background-image:url(#{background.url})}")
+
+    GlobalSetting.expects(:cdn_url).returns("//awesome.cdn")
+    expect(compile_css("category_backgrounds")).to include("body.category-#{category.full_slug}{background-image:url(//awesome.cdn#{background.url})}")
+  end
+
+  it "applies S3 CDN to background category images" do
+    SiteSetting.s3_cdn_url = "https://s3.cdn"
+
+    background = Fabricate(:upload_s3)
+    category = Fabricate(:category, uploaded_background: background)
+
+    expect(compile_css("category_backgrounds")).to include("body.category-#{category.full_slug}{background-image:url(https://s3.cdn/uploads")
+  end
+
+end

spec/fabricators/upload_fabricator.rb

@@ -8,6 +8,10 @@ Fabricator(:upload) do
   url { sequence(:url) { |n| "/uploads/default/#{n}/1234567890123456.png" } }
 end
 
+Fabricator(:upload_s3, from: :upload) do
+  url { sequence(:url) { |n| "#{Discourse.store.absolute_base_url}/uploads/default/#{n}/1234567890123456.png" } }
+end
+
 Fabricator(:attachment, from: :upload) do
   id 42
   user