Protect against cyclical references

lib/propshaft/compiler/css_asset_urls.rb

@@ -9,17 +9,17 @@ def compile(asset, input)
     input.gsub(ASSET_URL_PATTERN) { asset_url resolve_path(asset.logical_path.dirname, $1), asset.logical_path, $2, $1 }
   end
 
-  def referenced_by(asset)
-    Set.new.tap do |references|
-      asset.content.scan(ASSET_URL_PATTERN).each do |referenced_asset_url, _|
-        referenced_asset = load_path.find(resolve_path(asset.logical_path.dirname, referenced_asset_url))
-
-        unless references.include?(referenced_asset)
-          references << referenced_asset
-          references.merge referenced_by(referenced_asset)
-        end
+  def referenced_by(asset, references: Set.new)
+    asset.content.scan(ASSET_URL_PATTERN).each do |referenced_asset_url, _|
+      referenced_asset = load_path.find(resolve_path(asset.logical_path.dirname, referenced_asset_url))
+
+      unless references.include?(referenced_asset)
+        references << referenced_asset
+        references.merge referenced_by(referenced_asset, references: references)
       end
     end
+
+    references
   end
 
   private

lib/propshaft/load_path.rb

@@ -12,7 +12,7 @@ def find(asset_name)
   end
 
   def find_referenced_by(asset)
-    compilers.referenced_by(asset)
+    compilers.referenced_by(asset).delete(self)
   end
 
   def assets(content_types: nil)

test/fixtures/assets/first_path/dependent/c.css

@@ -1,3 +1,5 @@
+@import url('a.css')
+
 p {
   color: red;
 }