fix s3_cdn_url when the s3 bucket contains a folder

lib/file_store/s3_store.rb

@@ -85,7 +85,8 @@ module FileStore
     def cdn_url(url)
       return url if SiteSetting.Upload.s3_cdn_url.blank?
       schema = url[/^(https?:)?\/\//, 1]
-      url.sub("#{schema}#{absolute_base_url}", SiteSetting.Upload.s3_cdn_url)
+      folder = @s3_helper.s3_bucket_folder_path.nil? ? "" : "#{@s3_helper.s3_bucket_folder_path}/"
+      url.sub("#{schema}#{absolute_base_url}/#{folder}", "#{SiteSetting.Upload.s3_cdn_url}/")
     end
 
     def cache_avatar(avatar, user_id)

lib/s3_helper.rb

@@ -4,7 +4,7 @@ class S3Helper
 
   class SettingMissing < StandardError; end
 
-  attr_reader :s3_bucket_name
+  attr_reader :s3_bucket_name, :s3_bucket_folder_path
 
   def initialize(s3_bucket_name, tombstone_prefix = '', options = {})
     @s3_options = default_s3_options.merge(options)

spec/components/pretty_text_spec.rb

@@ -725,6 +725,29 @@ describe PrettyText do
 
       test_s3_cdn
     end
+
+    def test_s3_with_subfolder_cdn
+      raw = <<~RAW
+        <img src='https:#{Discourse.store.absolute_base_url}/subfolder/original/9/9/99c9384b8b6d87f8509f8395571bc7512ca3cad1.jpg'>
+      RAW
+
+      html = <<~HTML
+        <p><img src="https://awesome.cdn/subfolder/original/9/9/99c9384b8b6d87f8509f8395571bc7512ca3cad1.jpg"></p>
+      HTML
+
+      expect(PrettyText.cook(raw)).to eq(html.strip)
+    end
+
+    it 'can substitute s3 with subfolder cdn when added via global setting' do
+
+      global_setting :s3_access_key_id, 'XXX'
+      global_setting :s3_secret_access_key, 'XXX'
+      global_setting :s3_bucket, 'XXX/subfolder'
+      global_setting :s3_region, 'XXX'
+      global_setting :s3_cdn_url, 'https://awesome.cdn/subfolder'
+
+      test_s3_with_subfolder_cdn
+    end
   end
 
   describe "emoji" do