Mark sender thread as fork safe for Puma

Author: joshuay03

lib/datadog/statsd/sender.rb

@@ -104,6 +104,8 @@ def start
           # start background thread
           @sender_thread = @thread_class.new(&method(:send_loop))
           @sender_thread.name = "Statsd Sender" unless Gem::Version.new(RUBY_VERSION) < Gem::Version.new('2.3')
+          # advise multi-threaded app servers to ignore this thread for the purposes of fork safety warnings
+          @sender_thread.thread_variable_set(:fork_safe, true)
         rescue ThreadError => e
           @logger.debug { "Statsd: Failed to start sender thread: #{e.message}" } if @logger
           @mx.synchronize { @done = true }

spec/statsd/sender_spec.rb

@@ -48,6 +48,13 @@
           |thds| thds.any? { |t| t.name == "Statsd Sender" }
         }
       end
+
+      it 'marks the sender thread as fork safe' do
+        subject.start
+        expect(Thread.list).to satisfy {
+          |thds| thds.any? { |t| t.name == "Statsd Sender" && t.thread_variable_get(:fork_safe) }
+        }
+      end
     end
 
     context 'when the sender is started' do
@@ -204,9 +211,9 @@
 
         let(:thread_class) do
           if Thread.instance_methods.include?(:name=)
-            fake_thread = instance_double(Thread, { "alive?" => true, "name=" => true, "join" => true })
+            fake_thread = instance_double(Thread, { "alive?" => true, "name=" => true, "join" => true, "thread_variable_set" => true })
           else
-            fake_thread = instance_double(Thread, { "alive?" => true, "join" => true })
+            fake_thread = instance_double(Thread, { "alive?" => true, "join" => true, "thread_variable_set" => true })
           end
           class_double(Thread, new: fake_thread)
         end