Detect cyclic soruces by distributed search planner

lib/droonga/plugin/distributor/distributed_search_planner.rb

@@ -36,6 +36,10 @@ def initialize(search_request_message)
     UNLIMITED = -1
 
     def build_messages
+      if Searcher.have_circular_source?(@queries)
+        raise Searcher::CircularReferenceSource.new(@queries)
+      end
+
       ensure_unifiable!
 
       @queries.each do |input_name, query|

lib/droonga/searcher.rb

@@ -44,6 +44,22 @@ def initialize(queries)
       end
     end
 
+    class << self
+      def have_circular_source?(queries)
+        query_sorter = QuerySorter.new
+        queries.each do |name, query|
+          source = query["source"]
+          query_sorter.add(name, [source])
+        end
+        begin
+          sorted_queries = query_sorter.tsort
+        rescue TSort::Cyclic
+          return true
+        end
+        false
+      end
+    end
+
     def initialize(context)
       @context = context
     end