main.go

package main

import (
	"bufio"
	"bytes"
	"net"
	"net/http"
	"net/http/httptest"
	"net/url"
)

func main() {
	srv := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, r *http.Request) {
		go r.Body.Read(make([]byte, 1024))
		go r.Body.Close()
	}))

	u, err := url.Parse(srv.URL)
	if err != nil {
		panic(err)
	}

	for {
		conn, err := net.Dial("tcp", u.Host)
		if err != nil {
			panic(err)
		}

		req, err := http.NewRequest("POST", srv.URL, bytes.NewBufferString("test body"))
		if err != nil {
			panic(err)
		}
		req.Header.Set("Expect", "100-continue") // commenting this out avoids the race

		if err := req.Write(conn); err != nil {
			continue
		}

		_, err = http.ReadResponse(bufio.NewReader(conn), req)
		if err != nil {
			continue
		}

		conn.Close()
	}
}