crx.go

// Copyright (c) 2017 Opsmate, Inc.
//
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// Copyright (c) 2012 The Chromium Authors. All rights reserved.
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// modification, are permitted provided that the following conditions are
// met:
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// * Redistributions of source code must retain the above copyright
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package main

import (
	"archive/zip"
	"bytes"
	"encoding/binary"
	"fmt"
	"io/ioutil"
	"net/http"
	"net/url"
)

// buildCRXURL returns a URL from which the latest version of a CRX can be fetched.
func buildCrxURL(appid string) string {
	args := url.Values(make(map[string][]string))
	args.Add("response", "redirect")
	args.Add("x", "id="+appid+"&v=&uc&acceptformat=crx3")

	return (&url.URL{
		Scheme:   "https",
		Host:     "clients2.google.com",
		Path:     "/service/update2/crx",
		RawQuery: args.Encode(),
	}).String()
}

// crxHeader reflects the binary header of a CRX file.
type crxHeader struct {
	Magic       [4]byte
	Version     uint32
	HeaderLen   uint32
}

// zipReader is a small wrapper around a []byte which implements ReadAt.
type zipReader []byte

func (z zipReader) ReadAt(p []byte, pos int64) (int, error) {
	if int(pos) < 0 {
		return 0, nil
	}
	return copy(p, []byte(z)[int(pos):]), nil
}

func fetchCRX(appid string) (*zip.Reader, error) {
	resp, err := http.Get(buildCrxURL(appid))
	if err != nil {
		return nil, fmt.Errorf("Failed to get CRX: %s", err)
	}
	defer resp.Body.Close()

	// zip needs to seek around, so we read the whole reply into memory.
	crxBytes, err := ioutil.ReadAll(resp.Body)
	if err != nil {
		return nil, fmt.Errorf("Failed to download CRX: %s", err)
	}
	crx := bytes.NewBuffer(crxBytes)

	var header crxHeader
	if err := binary.Read(crx, binary.LittleEndian, &header); err != nil {
		return nil, fmt.Errorf("Failed to parse CRX header: %s", err)
	}

	if !bytes.Equal(header.Magic[:], []byte("Cr24")) || int(header.HeaderLen) < 0 {
		return nil, fmt.Errorf("Downloaded file doesn't look like a CRX")
	}

	protoHeader := crx.Next(int(header.HeaderLen))
	if len(protoHeader) != int(header.HeaderLen) {
		return nil, fmt.Errorf("Downloaded file doesn't look like a CRX")
	}

	zipBytes := crx.Bytes()
	zipReader := zipReader(zipBytes)

	z, err := zip.NewReader(zipReader, int64(len(zipBytes)))
	if err != nil {
		return nil, fmt.Errorf("Failed to parse ZIP file: %s", err)
	}

	return z, nil
}