Mark sender thread as fork safe for Puma

Author: joshuay03

lib/datadog/statsd/sender.rb

@@ -51,7 +51,7 @@ def rendez_vous
         # could happen if #start hasn't be called
         return unless message_queue
 
-        # Initialize and get the thread's sync queue
+        # initialize and get the thread's sync queue
         queue = (@thread_class.current[:statsd_sync_queue] ||= @queue_class.new)
         # tell sender-thread to notify us in the current
         # thread's queue
@@ -104,6 +104,9 @@ 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
+          # see Puma's implementation for `:fork_safe`: https://github.com/puma/puma/blob/v7.2.0/lib/puma/cluster.rb#L374
+          @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