fix accepting one more request after worker shutdown command is sent

lib/puma/server.rb

@@ -400,6 +400,9 @@ def handle_servers
                     end
                   else
                     pool.wait_until_not_full
+
+                    # A stop may have arrived while waiting for a thread
+                    break if check.wait_readable(0) && handle_check
                   end
                 end
 

test/test_puma_server.rb

@@ -1962,6 +1962,41 @@ def test_not_drain_on_shutdown
     test_drain_on_shutdown false
   end
 
+  def test_no_accept_after_stop_while_waiting_for_thread
+    started_connections = Queue.new
+    wait = Queue.new
+
+    server_run(queue_requests: false, max_threads: 1, drain_on_shutdown: false) do
+      started_connections << true
+      wait.pop
+      [200, {}, ["DONE"]]
+    end
+
+    busy = send_http GET_10
+    started_connections.pop
+
+    waiting_for_thread = Queue.new
+    @pool.define_singleton_method(:wait_until_not_full) do
+      waiting_for_thread << true
+      super()
+    end
+
+    pending = send_http GET_10
+    waiting_for_thread.pop
+
+    @server.stop
+    wait.close
+    assert_match 'DONE', busy.read_body
+
+    served = begin
+      pending.wait_readable(1) && pending.read_body.include?('DONE')
+    rescue Errno::ECONNRESET, EOFError
+      false
+    end
+
+    refute served, 'Connection accepted after stop command was served'
+  end
+
   def test_remote_address_header
     server_run(remote_address: :header, remote_address_header: 'HTTP_X_REMOTE_IP') do |env|
       [200, {}, [env['REMOTE_ADDR']]]