How to save cookies as a file in Golang

Go’s cookiejar doesn’t have any function to persistence cookies, so you need to do something to save cookies to a local file. To make it easy, you can use juju/persistent-cookiejar instead.

juju / persistent-cookiejar

cookiejar is a fork of net/http/cookiejar that allows serialisation of the stored cookies

Usage

You can use it as same as net/http/cookiejar, and can save cookies by Save.

jar, _ := cookiejar.New(nil)
http.DefaultClient.Jar = jar

// Some tasks

jar.Save()

Where to save is decided by an option you put into New; it is set to $GOCOOKIES or $HOME/.go-cookies by default. If the file doesn't exist, it returns no errors and creates it when saving.

jar, _ := cookiejar.New(&cookiejar.Options{Filename: "path/to/cookie"})

Try to use

This is a sample program to log in to AtCoder. It outputs Already logged in! if you've already logged in, or asks your username and password if not yet.

code

Some error handlings were omitted.

package main

import (
    "fmt"
    "log"
    "net/http"
    "net/url"
    "os"
    "strings"

    "github.com/PuerkitoBio/goquery"
    "github.com/juju/persistent-cookiejar"
    "golang.org/x/term"
)

func main() {
    jar, _ := cookiejar.New(nil)
    http.DefaultClient.Jar = jar
    defer jar.Save()

    // If the file exists, check whether logged in
    _, err := os.Stat(cookiejar.DefaultCookieFile())
    if err == nil {
        doc, _ := getDocument("https://atcoder.jp/home")
        navbarRight := doc.Find("div#navbar-collapse > ul.navbar-right")
        if navbarRight.Children().Length() == 2 {
            fmt.Println("Already logged in!")
            return
        }
    }

    var username string
    fmt.Print("Username: ")
    fmt.Scan(&username)

    fmt.Print("Password: ")
    // Don't press ctrl+c while Go reads password because your terminal won't display any inputs after that.
    // To resolve it, see gist.github.com/montanaflynn/5ae3eeae7212b0ba232f46e88f1ab67f
    bypePassword, _ := term.ReadPassword(int(os.Stdin.Fd()))

    loginUrl := "https://atcoder.jp/login"
    doc, _ := getDocument(loginUrl)
    token, found := doc.Find(`form input[type="hidden"]`).Attr("value")
    if !found {
        log.Fatal("error: cannot find CSRF token")
    }

    values := url.Values{
        "username":   {username},
        "password":   {string(bypePassword)},
        "csrf_token": {token},
    }
    req, _ := http.NewRequest("POST", loginUrl, strings.NewReader(values.Encode()))
    req.Header.Set("Content-Type", "application/x-www-form-urlencoded")

    resp, err := http.DefaultClient.Do(req)
    if err != nil {
        log.Fatal(err)
    }
    defer resp.Body.Close()

    if resp.Request.URL.String() == loginUrl {
        log.Fatal("Failed to login. Check your username/password")
    }

    fmt.Println("Successfully logged in!")
}

func getDocument(url string) (*goquery.Document, error) {
    resp, err := http.Get(url)
    if err != nil {
        log.Fatal(err)
    }
    defer resp.Body.Close()

    return goquery.NewDocumentFromReader(resp.Body)
}

Now you can find the cookie as a file.

$ cat ~/.go-cookies
[{"Name":"REVEL_SESSION","Value":"xxxxx...","Domain":"atcoder.jp","Path":"/","Secure":true,"HttpOnly":true,"Persistent":true,"HostOnly":true,"Expires":"2022-09-13T18:19:25.9840269+09:00","Creation":"2022-03-17T18:19:15.4703732+09:00","LastAccess":"2022-03-17T18:19:25.9840269+09:00","Updated":"2022-03-17T18:19:25.9840269+09:00","CanonicalHost":"atcoder.jp"}]

If you execute it again when the file exists,

$ go run main.go
Already logged in!

it won't ask you username/password.