fixup! Add url_relative_to method to ZencoderAttachment

app/models/pageflow/zencoder_attachment.rb

@@ -1,3 +1,5 @@
+require 'uri'
+
 module Pageflow
   class ZencoderAttachment
     cattr_accessor :default_options
@@ -44,11 +46,13 @@ def url(url_options = {})
     end
 
     def url_relative_to(attachment)
-      result = url
-      result.gsub!("#{File.dirname(attachment.url)}/", '') ||
-        raise("Could not generate relative url for #{result} based on #{attachment.url}.")
+      dir_path = File.dirname(URI.parse(attachment.url).path)
+
+      unless URI.parse(url).path.start_with?(dir_path)
+        raise("Could not generate relative url for #{url} based on #{attachment.url}.")
+      end
 
-      result
+      url.split("#{dir_path}/", 2).last
     end
 
     private

spec/models/pageflow/zencoder_attachment_spec.rb

@@ -186,6 +186,17 @@ def file_double(id:)
         expect(result).to eq('high/rendition.mpd')
       end
 
+      it 'handles protocol relative urls correctly' do
+        Pageflow.config.zencoder_options[:s3_protocol] = ''
+        file = file_double(id: 5)
+        manifest = ZencoderAttachment.new(file, 'dash/manifest.mpd')
+        attachment = ZencoderAttachment.new(file, 'dash/high/rendition.mpd')
+
+        result = attachment.url_relative_to(manifest)
+
+        expect(result).to eq('high/rendition.mpd')
+      end
+
       it 'fails if other attachment is in other directory' do
         file = file_double(id: 5)
         manifest = ZencoderAttachment.new(file, 'other/manifest.mpd')