memory leak with html render function

I'm facing issue where even trying just 200 requests cause program to eat up 6Gb of memory for container and eventually be killed by OOM.
Idea is I'm extracting all text nodes present in html and then processing them to extarct their names, html of that tag, and text. So for generating html of perticular tags I'm using Render function from golang.org/x/net/html. In which i provide strings.Builder as io.Writer to write generated html. But for some reason builder eats up too much memory.

package main

import (
	"encoding/csv"
	"io"
	"log"
	"net/http"
	"strings"
	"golang.org/x/net/html"
)

func main() {
	mux := http.NewServeMux()
	mux.HandleFunc("/data", GetData)
	if err := http.ListenAndServe(":8001", mux); err != nil {
		log.Println(err)
	}
}

type TagInfo struct {
	Tag  string
	Name string
	Text string
}

// http.handler
func GetData(w http.ResponseWriter, r *http.Request) {
	u := r.URL.Query().Get("url")
	doc, err := GetDoc(u)
	if err != nil {
		log.Println(err)
		w.WriteHeader(500)
		return
	}
	var buf strings.Builder
	data := Extract(doc, &buf)
	csvw := csv.NewWriter(io.Discard)
	for _, d := range data {
		csvw.Write([]string{d.Name, d.Tag, d.Text})
	}
}

// fires request and get text/html
func GetDoc(u string) (*html.Node, error) {
	res, err := http.Get(u)
	if err != nil {
		return nil, err
	}
	defer res.Body.Close()
	return html.Parse(res.Body)
}

func Extract(doc *html.Node, buf *strings.Builder) []TagInfo {
	var (
		tags = make([]TagInfo, 0, 100)
		f    func(*html.Node)
	)

	f = func(n *html.Node) {
		if n.Type == html.TextNode {
			text := strings.TrimSpace(n.Data)
			if text != "" {
				parent := n.Parent
				tag := Render(parent, buf)
				tagInfo := TagInfo{
					Tag:  tag,
					Name: parent.Data,
					Text: n.Data,
				}
				tags = append(tags, tagInfo)
			}
		}
		for child := n.FirstChild; child != nil; child = child.NextSibling {
			f(child)
		}
	}
	f(doc)
	return tags
}

// Render the html around the tag
// if node is text then pass the
// parent node paramter in function
func Render(n *html.Node, buf *strings.Builder) string {
	defer buf.Reset()
	if err := html.Render(buf, n); err != nil {
		log.Println(err)
		return ""
	}
	return buf.String()
}

if yu want particular url list here it is. i fired around 60 request at a time.

i tried bytes.Buffer and sync.Pool using bytes.Buffer but both have same issue. using pprof i noticed that strings.Builder's WriteString method is causing huge memory use.

So the basic issue here was accepting any content-type which is not acceptable in terms of scraping most of the site are required to send text/html.

The problem was even if the url sends any content which is not represents the html data golang.org/x/net/html still accept it without throwing error.

Let's take a example where application/pdf is returned and then the body will contain binary data of pdf which html.Parse parse and doesn't return any error it's weird behaviour thinking library made for scraping/crawling acceping binary data.

The solution is: Check the response header and then proceed if only data is html other wise the there will be ambiguity or higher memory usage(may be lower) but we can't predict what will happen.