chrome webstore downloader

main.go

package main

import (
	"encoding/binary"
	"fmt"
	"io"
	"net/http"
	"net/url"
	"os"
	"regexp"
)

var webstoreRgx = regexp.MustCompile(`https://chrome.google.com/webstore/detail/([^/]+)/([^/]+)`)

func main() {
	if len(os.Args) == 1 {
		fmt.Println("webstore url missing")
		return
	}
	chromeWebstoreURL := os.Args[1] // https://chrome.google.com/webstore/detail/extension-name/extension-id
	m := webstoreRgx.FindStringSubmatch(chromeWebstoreURL)
	if len(m) != 3 {
		fmt.Println("invalid webstore url")
		return
	}
	extensionName, extensionID := m[1], m[2]
	if err := downloadExt(extensionName, extensionID); err != nil {
		fmt.Println(err)
		return
	}
}

func downloadExt(extensionName, extensionID string) error {
	downloadLink := buildDownloadLink(extensionID)
	resp, err := http.Get(downloadLink)
	if err != nil {
		return err
	}
	defer resp.Body.Close()
	parse(resp.Body)

	out, err := os.Create(extensionName + `.zip`)
	if err != nil {
		return err
	}
	defer out.Close()
	_, err = io.Copy(out, resp.Body)
	return err
}

func buildDownloadLink(extensionID string) string {
	v := url.Values{
		"response":     {"redirect"},
		"prodversion":  {"49.0"},
		"acceptformat": {"crx3"},
		"x":            {"id=" + extensionID + "&installsource=ondemand&uc"},
	}
	return "https://clients2.google.com/service/update2/crx?" + v.Encode()
}

func parse(reader io.Reader) {
	var magic, version, headerLength uint32
	_ = binary.Read(reader, binary.BigEndian, &magic)
	_ = binary.Read(reader, binary.BigEndian, &version)
	_ = binary.Read(reader, binary.LittleEndian, &headerLength)
	if magic != 0x43723234 { // Cr24
		return // Magic is broken
	}
	buf := make([]byte, headerLength)
	_, _ = reader.Read(buf)
	return
}