Initialize monitor and copy internal filters list on FilterChain#dup

lib/rackstash/filter_chain.rb

@@ -273,6 +273,15 @@ module Rackstash
 
     private
 
+    def initialize_copy(orig)
+      super
+
+      mon_initialize
+      synchronize do
+        @filters = orig.to_a
+      end
+    end
+
     def index_at(index)
       case index
       when Integer, ->(o) { o.respond_to?(:to_int) }

spec/rackstash/filter_chain_spec.rb

@@ -225,6 +225,16 @@ describe Rackstash::FilterChain do
     end
   end
 
+  describe '#dup' do
+    it 'duplicates the filters array' do
+      filter_chain << a_filter
+      dupped = filter_chain.dup
+
+      expect(filter_chain.length).to eql dupped.length
+      expect { filter_chain << a_filter }.not_to change { dupped.length }
+    end
+  end
+
   describe '#each' do
     it 'yields each filter' do
       filter_chain << -> {}