Concurrent HTTP file downloader in Golang. https://www.reddit.com/r/golang/comments/9ttjb9/how_to_download_single_file_concurrently/

concurrent_file_downloader.go

package phttp

import (
	"context"
	"errors"
	"fmt"
	"io"
	"net/http"

	"golang.org/x/net/context/ctxhttp"
	"golang.org/x/sync/errgroup"
)

func PGet(ctx context.Context, client *http.Client, w io.WriterAt, n int, url string) error {
	resp, err := ctxhttp.Head(ctx, client, url)
	if err != nil {
		return err
	}
	resp.Body.Close()
	if resp.StatusCode != http.StatusOK {
		return fmt.Errorf("server responded with %d status code", resp.StatusCode)
	}
	if resp.Header.Get("Accept-Ranges") != "bytes" {
		return errors.New("server does not support range requests")
	}
	if resp.ContentLength < 0 {
		return errors.New("server sent invalid Content-Length header")
	}
	sectionLen := resp.ContentLength / int64(n)
	wg, ctx := errgroup.WithContext(ctx)
	for off := int64(0); off < resp.ContentLength; off += sectionLen {
		off := off
		lim := off + sectionLen
		if lim >= resp.ContentLength {
			lim = resp.ContentLength
		}
		wg.Go(func() error { return getPart(ctx, client, w, url, off, lim) })
	}
	return wg.Wait()
}

func getPart(ctx context.Context, client *http.Client, w io.WriterAt, url string, off, lim int64) error {
	req, err := http.NewRequest("GET", url, nil)
	if err != nil {
		return err
	}
	req.Header.Set("Range", fmt.Sprintf("bytes=%d-%d", off, lim))
	resp, err := ctxhttp.Do(ctx, client, req)
	if err != nil {
		return err
	}
	defer resp.Body.Close()
	if resp.StatusCode != http.StatusPartialContent {
		return fmt.Errorf("server responded with %d status code, expected %d", resp.StatusCode, http.StatusPartialContent)
	}
	_, err = io.Copy(newSectionWriter(w, off), resp.Body)
	return err
}

func newSectionWriter(w io.WriterAt, off int64) *sectionWriter {
	return &sectionWriter{w, off}
}

type sectionWriter struct {
	w   io.WriterAt
	off int64
}

func (w *sectionWriter) Write(p []byte) (n int, err error) {
	n, err = w.w.WriteAt(p, w.off)
	w.off += int64(n)
	return
}