Retry handler destroys the socket on error status code

[fatal10110]

Bug Description

On getting status code >= 400 from serever, retry handler throws an error which closes the open connection

Reproducible By

you will see here

New connection
connection closed
New connection
Hello World

const fastify = require("fastify");
const { RetryAgent, Agent } = require("undici");

const app = fastify();

let tries = 0;

app.get("/", (req, res) => {
  if (++tries % 2 !== 0) {
    return res.status(500).send("Test error");
  }

  res.send("Hello World");
});

app.server.on("connection", (socket) => {
  console.log("New connection");

  socket.on("close", () => {
    console.log("connection closed");
  });
});

async function start() {
  await app.listen({ port: 3000 });

  const a = new RetryAgent(new Agent({ connections: 1 }));
  const res = await a.request({
    origin: "http://localhost:3000",
    path: "/",
    method: "GET",
  });
  console.log(await res.body.text());
}
start();

Expected Behavior

It will pass the error but will leave the connection open if its non-connection related error

Additional context

By destroying the socket it also prevents the user get the error body from response

[mcollina]

Thanks for reporting! Would you like to send a Pull Request to address this issue? Remember to add unit tests.

[fatal10110]

Thanks for reporting! Would you like to send a Pull Request to address this issue? Remember to add unit tests.

I prepared some code, hopefully it is the correct fix, will send the PR soon