Add specs for caching on timeout

Fixes logstash-plugins#40
Fixes logstash-plugins#25

lib/logstash/filters/dns.rb

@@ -69,6 +69,9 @@ class LogStash::Filters::DNS < LogStash::Filters::Base
   # Use custom hosts file(s). For example: `["/var/db/my_custom_hosts"]`
   config :hostsfile, :validate => :array
 
+  attr_reader :hit_cache
+  attr_reader :failed_cache
+
   public
   def register
     if @nameserver.nil? && @hostsfile.nil?
@@ -90,7 +93,6 @@ def register
 
   public
   def filter(event)
-
     if @resolve
       return if resolve(event).nil?
     end

spec/filters/dns_spec.rb

@@ -322,10 +322,11 @@
 
     describe "retries" do
 
+      let(:host) { "unknownhost" }
       let(:subject) { LogStash::Filters::DNS.new(config) }
-      let(:event) { LogStash::Event.new("message" => "unkownhost") }
+      let(:event) { LogStash::Event.new("message" => host) }
       let(:max_retries) { 3 }
-      let(:config) { { "resolve" => ["message"], "max_retries" => max_retries } }
+      let(:config) { { "resolve" => ["message"], "max_retries" => max_retries, "failed_cache_size" => 10 } }
 
       before(:each) { subject.register }
 
@@ -338,6 +339,12 @@
           expect(subject).to receive(:getaddress).exactly(max_retries+1).times
           subject.filter(event)
         end
+
+        it "should cache the timeout" do
+          expect do
+            subject.filter(event)
+          end.to change { subject.failed_cache[host] }.from(nil).to(true)
+        end
       end
 
       context "when failing temporarily" do