Always normalize the result of the relative_url filter (#6185)

Merge pull request 6185

lib/jekyll/filters/url_filters.rb

@@ -23,9 +23,9 @@ module Jekyll
       # Returns a URL relative to the domain root as a String.
       def relative_url(input)
         return if input.nil?
-        return ensure_leading_slash(input.to_s) if sanitized_baseurl.nil?
+        parts = [sanitized_baseurl, input]
         Addressable::URI.parse(
-          ensure_leading_slash(sanitized_baseurl) + ensure_leading_slash(input.to_s)
+          parts.compact.map { |part| ensure_leading_slash(part.to_s) }.join
         ).normalize.to_s
       end
 

test/test_filters.rb

@@ -491,6 +491,15 @@ class TestFilters < JekyllUnitTest
         })
         assert_equal "/", filter.relative_url(page_url)
       end
+
+      should "not return the url by reference" do
+        filter = make_filter_mock({ :baseurl => nil })
+        page = Page.new(filter.site, test_dir("fixtures"), "", "front_matter.erb")
+        assert_equal "/front_matter.erb", page.url
+        url = filter.relative_url(page.url)
+        url << "foo"
+        assert_equal "/front_matter.erb", page.url
+      end
     end
 
     context "strip_index filter" do