Go download stream response

main.go

// https://rafallorenz.com/go/go-http-stream-download/
package main

import (
	"errors"
	"io"
	"io/ioutil"
	"net/http"
	"time"
)

type stream struct {
	chunk     []byte
	i         int64                     // current reading index
	fetch     func(offset int64) []byte // fetch callback
	totalSize func() int64              // total size callback
}

func (s *stream) Read(b []byte) (n int, err error) {
	if len(s.chunk) == 0 {
		return 0, io.EOF
	}

	// simply copy our chunk
	n = copy(b, s.chunk)
	s.i += int64(n)

	return
}

func (s *stream) Seek(offset int64, whence int) (int64, error) {
	var abs int64
	switch whence {
	case io.SeekStart:
		abs = offset
	case io.SeekCurrent:
		abs = s.i + offset
	case io.SeekEnd:
		abs = s.totalSize() + offset
	default:
		return 0, errors.New("stream..Reader.Seek: invalid whence")
	}
	if abs < 0 {
		return 0, errors.New("stream..Reader.Seek: negative position")
	}

	s.i = abs
	s.chunk = s.fetch(abs) // fetch chunk and set to buffer

	return abs, nil
}

func downloadHandler(w http.ResponseWriter, r *http.Request) {
	resp, err := http.Get("https://golangcode.com/logo.svg")
	if err != nil {
		http.Error(w, "could not write response", http.StatusInternalServerError)
		return
	}
	defer resp.Body.Close()

	body, err := ioutil.ReadAll(resp.Body)
	if err != nil {
		http.Error(w, "could not read body", http.StatusInternalServerError)
		return
	}

	s := stream{
		fetch: func(offset int64) []byte {
			// for this example we will just get our chunk from buffer we already have
			// in real life you probably would fetch it from some other place
			// or generate it
			return body[offset:]
		},
		totalSize: func() int64 {
			// total size for ServeContent to parse ranges
			// TODO: handle unknown total size
			return int64(len(body))
		},
	}

	// we set the type as we know it and ServeContent doesnt have to guess it
	// we as well do not want it to return whole content at once if it can not guess the type
	w.Header().Set("Content-Type", "image/svg+xml")
	http.ServeContent(w, r, "logo.svg", time.Now(), &s)
}

func main() {
	mux := http.NewServeMux()
	mux.HandleFunc("/", downloadHandler)

	http.ListenAndServe(":3000", mux)
}