Golang ReverseProxy race condition

main.go

package main

import (
	"bytes"
	"io"
	"net/http"
	"net/http/httptrace"
	"net/http/httputil"
	"net/url"
	"time"
)

func main() {
	readyCh := make(chan struct{})
	backendServer := http.NewServeMux()
	backendServer.HandleFunc("/", func(rw http.ResponseWriter, r *http.Request) {
		rw.WriteHeader(http.StatusContinue)
		hj, _ := rw.(http.Hijacker)
		conn, _, _ := hj.Hijack()
		conn.Close()
	})
	go http.ListenAndServe(":8081", backendServer)
	time.Sleep(1 * time.Second)

	handler := func(p *httputil.ReverseProxy) func(http.ResponseWriter, *http.Request) {
		return func(rw http.ResponseWriter, r *http.Request) {
			r = r.WithContext(httptrace.WithClientTrace(r.Context(), &httptrace.ClientTrace{
				Got100Continue: func() {
					// Delay the 1xx hook
					<-readyCh
				},
			}))
			p.ServeHTTP(rw, r)
			rw.Header().Set("X-Something", "Hello")
		}
	}
	// trigger trace context once, blocking first time
	go func() {
		readyCh <- struct{}{}
	}()

	target, err := url.Parse("http://localhost:8081")
	if err != nil {
		panic(err)
	}
	proxy := httputil.NewSingleHostReverseProxy(target)
	proxy.ErrorHandler = func(rw http.ResponseWriter, req *http.Request, err error) {
		readyCh <- struct{}{}
		for i := 0; i < 10000000; i++ {
			rw.Header().Set("X-Something", "Hello")
		}
	}
	proxy.Transport = &RetryTransport{T: http.DefaultTransport}

	http.HandleFunc("/", handler(proxy))
	go http.ListenAndServe(":8080", nil)
	time.Sleep(1 * time.Second)

	data := bytes.NewBufferString("Hello!")
	req, err := http.NewRequest("POST", "http://localhost:8080", data)
	if err != nil {
		panic(err)
	}
	req.Header.Set("Expect", "100-continue")
	resp, err := http.DefaultClient.Do(req)
	if err != nil {
		panic(err)
	}
	defer resp.Body.Close()
	_, err = io.ReadAll(resp.Body)
	if err != nil {
		panic(err)
	}

	time.Sleep(10 * time.Second)
	println("done")
}

type RetryTransport struct {
	T       http.RoundTripper
}

func (tr *RetryTransport) RoundTrip(req *http.Request) (res *http.Response, err error) {
	for i := 0; i < 3; i++ {
		res, err = tr.T.RoundTrip(req)
		if err == nil {
			return res, err
		}
	}
	return res, err
}