Don't re-lock the sender mutex when #add can't restart the thread

lib/datadog/statsd/sender.rb

@@ -50,6 +50,8 @@ def flush(sync: false)
       def rendez_vous
         # could happen if #start hasn't be called
         return unless message_queue
+        # no companion thread to answer, e.g. it could not be re-created
+        return unless sender_thread && sender_thread.alive?
 
         # Initialize and get the thread's sync queue
         queue = (@thread_class.current[:statsd_sync_queue] ||= @queue_class.new)
@@ -81,6 +83,9 @@ def add(message)
             @message_queue = nil
             message_buffer.reset
             start
+            # the companion thread could not be re-created, don't queue
+            # messages that nothing will ever send
+            return if @done
             @flush_timer.start if @flush_timer && @flush_timer.stop?
           }
         end
@@ -106,7 +111,13 @@ def start
           @sender_thread.name = "Statsd Sender" unless Gem::Version.new(RUBY_VERSION) < Gem::Version.new('2.3')
         rescue ThreadError => e
           @logger.debug { "Statsd: Failed to start sender thread: #{e.message}" } if @logger
-          @mx.synchronize { @done = true }
+          # #add calls #start while already holding @mx, and Ruby mutexes
+          # are not re-entrant
+          if @mx.owned?
+            @done = true
+          else
+            @mx.synchronize { @done = true }
+          end
         end
 
         @flush_timer.start if @flush_timer

spec/statsd/sender_spec.rb

@@ -239,6 +239,35 @@
         end
       end
     end
+
+    context 'when the sender thread died and a new one cannot be started' do
+      let(:thread_class) { class_double(Thread) }
+      let(:dead_thread) { Thread.new {}.tap(&:join) }
+
+      before do
+        allow(thread_class).to receive(:new).and_return(dead_thread)
+        subject.start
+        allow(thread_class).to receive(:new).and_raise(ThreadError, "can't create Thread")
+        allow(message_buffer).to receive(:reset)
+      end
+
+      after do
+        subject.stop
+      end
+
+      it 'drops the messages without raising' do
+        expect do
+          subject.add('message')
+          subject.add('message')
+        end.not_to raise_error
+        expect(subject.send(:message_queue).length).to eq 0
+      end
+
+      it 'does not wait for the missing companion thread in rendez_vous' do
+        subject.add('message')
+        Timeout.timeout(1) { subject.rendez_vous }
+      end
+    end
   end
 
   describe '#flush' do