Add hot restart integration test

This test demonstrates that it's possible for clients to receive
unexpected ECONNRESET errors during hot restarts.

CI shows that the test fails on various platforms/rubies, such as JRuby
and TruffleRuby on Ubuntu as well as MRI on macOS. No failures for MRI
on Ubuntu or MRI on Windows.

test/test_integration_single.rb

@@ -10,6 +10,59 @@ def test_usr2_restart
     assert_equal "Hello World", new_reply
   end
 
+  def test_hot_restart_does_not_drop_connections
+    skip_unless_signal_exist? :USR2
+
+    replies = Hash.new 0
+    num_requests = 500
+    refused = thread_run_refused unix: false
+    message = 'A' * 2**14
+
+    cli_server "-q test/rackup/hello.ru"
+
+    client_thread = Thread.new do
+      num_requests.times do
+        begin
+          socket = TCPSocket.new(HOST, @tcp_port)
+          @ios_to_close << socket
+          socket << "POST / HTTP/1.1\r\nContent-Length: #{message.bytesize}\r\n\r\n#{message}"
+          true until socket.gets == "\r\n"
+          body = read_body(socket, 20)
+          if body == "Hello World"
+            replies[:success] += 1
+          else
+            replies[:unexpected_response] += 1
+          end
+        rescue Errno::ECONNRESET
+          # connection was accepted but then closed
+          # client would see an empty response
+          replies[:reset] += 1
+        rescue *refused
+          replies[:refused] += 1
+        end
+      end
+    end
+
+    run = true
+
+    restart_thread = Thread.new do
+      while run
+        Process.kill :USR2, @pid
+        wait_for_server_to_boot
+        sleep 1
+      end
+    end
+
+    client_thread.join
+    run = false
+    restart_thread.join
+
+    assert_equal 0, replies[:unexpected_response], "Unexpected response"
+    assert_equal 0, replies[:reset], "Expected no reset errors"
+    assert_equal 0, replies[:refused], "Expected no refused connections"
+    assert_equal num_requests, replies[:success]
+  end
+
   # It does not share environments between multiple generations, which would break Dotenv
   def test_usr2_restart_restores_environment
     # jruby has a bug where setting `nil` into the ENV or `delete` do not change the