M.go

package main

import (
        "io"
        "net/http"
        "net/http/httptest"
        "time"
)

func main() {
        ts := httptest.NewServer(http.HandlerFunc(func(w http.ResponseWriter, req *http.Request) {
                defer func() { _ = req.Body.Close() }() // Best effort.

                conn, _, err := w.(http.Hijacker).Hijack()
                if err != nil {
                        println("Error hijack:", err.Error())
                        return
                }
                _, _ = conn.Write([]byte("HTTP/1.1 200 OK\r\n\r\n")) // Best effort.

                for buf, n := make([]byte, 1024), 0; err == nil; n, err = req.Body.Read(buf) {
                        println("Handler -->", string(buf[:n]))
                        _, _ = conn.Write([]byte("ack\n")) // Best effort.
                }
                if err != nil && err != io.EOF {
                        println("Unexpected error:", err.Error())
                }
        }))
        defer ts.Close()

        r, w := io.Pipe()
        end := make(chan struct{})
        go func() {
                defer close(end)
                defer func() { _ = w.Close() }() // Best effort.
                for i := 0; i < 3; i++ {
                        println("Request body producer")
                        if _, err := w.Write([]byte("hello\n")); err != nil {
                                if err != io.EOF {
                                        println("Unexpected error:", err.Error())
                                }
                                break
                        }
                        time.Sleep(1e9)
                }
        }()

        req, err := http.NewRequest(http.MethodPost, ts.URL, r)
        if err != nil {
                panic(err)
        }
        resp, err := (&http.Client{}).Do(req)
        if err != nil {
                panic(err)
        }
        println("http resp:", resp.StatusCode)
        <-end
}