contact/email.go

package contact

import (
	"bytes"
	"context"
	"crypto/sha256"
	"crypto/tls"
	"encoding/base64"
	"encoding/json"
	"fmt"
	"io"
	"log"
	"net/http"
	"net/mail"
	"net/smtp"
	"net/url"
	"os"
	"strings"
	"time"

	"github.com/bytbox/go-pop3"
	"github.com/emersion/go-imap"
	"github.com/emersion/go-imap/client"
	"github.com/emersion/go-sasl"
	"github.com/google/uuid"
	"golang.org/x/oauth2/authhandler"
	"golang.org/x/oauth2/google"
)

type Emailer struct {
	From     string
	SMTP     string
	POP3     string
	IMAP     string
	Password string
	Limit    int
	OAuth    string
	Inbox    string
}

func NewEmailer() *Emailer {
	envOr := func(key, def string) string {
		if v, ok := os.LookupEnv(strings.ToUpper(key)); ok {
			return v
		}
		return def
	}
	return &Emailer{
		From:     envOr("FROM", "breellocaldev@gmail.com"),
		SMTP:     envOr("SMTP", "smtp.gmail.com:465"),
		Password: envOr("PASSWORD", "lhnjijrvqaesiufp"),
		OAuth:    envOr("OAUTH", ""),
		Inbox:    envOr("INBOX", "INBOX"),
	}
}

func (e *Emailer) Read() (chan *mail.Message, error) {
	return e.ReadIMAP()
}

func (e *Emailer) ReadIMAP() (chan *mail.Message, error) {
	if e.OAuth != "" {
		return e.oauthThenReadIMAP()
	}
	return e.readIMAP(e.From, e.Password)
}

func (e *Emailer) oauthThenReadIMAP() (chan *mail.Message, error) {
	configJSON, err := e.configJSON()
	if err != nil {
		return nil, err
	}

	if token, _ := os.ReadFile(e.OAuth + ".token"); len(token) == 0 {
		state := uuid.NewString()

		verifier := uuid.NewString()
		s256 := sha256.Sum256([]byte(verifier))
		challenge := base64.RawURLEncoding.EncodeToString(s256[:])
		pkceParams := authhandler.PKCEParams{Challenge: challenge, ChallengeMethod: "S256", Verifier: verifier}
		config, err := google.ConfigFromJSON([]byte(configJSON), "https://mail.google.com/")
		if err != nil {
			return nil, err
		}
		credentialsParams := google.CredentialsParams{Scopes: config.Scopes, State: state, AuthHandler: func(authCodeURL string) (string, string, error) {
			fmt.Println(authCodeURL + "&access_type=offline&prompt=consent")
			var callback string
			_, err := fmt.Scan(&callback)
			u, _ := url.Parse(callback)
			fmt.Println()
			return u.Query().Get("code"), state, err
		}, PKCE: &pkceParams}
		credentials, err := google.CredentialsFromJSONWithParams(context.Background(), []byte(configJSON), credentialsParams)
		if err != nil {
			return nil, err
		}

		token, err := credentials.TokenSource.Token()
		if err != nil {
			return nil, err
		}
		tokenFileJSON, _ := json.Marshal(token)
		os.WriteFile(e.OAuth+".token", tokenFileJSON, os.ModePerm)
		log.Printf("%s", tokenFileJSON)
	}
	var tokenStruct struct {
		AccessToken  string    `json:"access_token"`
		RefreshToken string    `json:"refresh_token"`
		Expiry       time.Time `json:"expiry"`
		ExpiresIn    int       `json:"expires_in"`
	}
	token, _ := os.ReadFile(e.OAuth + ".token")
	json.Unmarshal(token, &tokenStruct)
	if time.Until(tokenStruct.Expiry) < time.Minute*5 {
		var appStruct struct {
			Installed struct {
				ClientID     string `json:"client_id"`
				ClientSecret string `json:"client_secret"`
				GrantType    string `json:"grant_type"`
				RefreshToken string `json:"refresh_token"`
			} `json:"installed"`
		}
		json.Unmarshal([]byte(configJSON), &appStruct)
		appStruct.Installed.GrantType = "refresh_token"
		appStruct.Installed.RefreshToken = tokenStruct.RefreshToken
		b, _ := json.Marshal(appStruct.Installed)
		resp, err := http.Post(`https://oauth2.googleapis.com/token`, `application/json`, bytes.NewReader(b))
		if err != nil {
			return nil, err
		}
		defer resp.Body.Close()
		b, _ = io.ReadAll(resp.Body)
		if resp.StatusCode != http.StatusOK {
			return nil, fmt.Errorf("(%d) %s", resp.StatusCode, b)
		}
		tokenStruct.Expiry = time.Time{}
		json.Unmarshal(b, &tokenStruct)
		tokenStruct.Expiry = time.Now().Add(time.Duration(tokenStruct.ExpiresIn) * time.Second)
		tokenStruct.RefreshToken = appStruct.Installed.RefreshToken
		b, _ = json.Marshal(tokenStruct)
		os.WriteFile(e.OAuth+".token", b, os.ModePerm)
	}
	log.Println("OAUTH", e.From, time.Until(tokenStruct.Expiry), tokenStruct.AccessToken)
	return e.readIMAP(e.From, tokenStruct.AccessToken)
}

func (e *Emailer) configJSON() (string, error) {
	b, err := os.ReadFile(e.OAuth)
	return string(b), err
}

func (e *Emailer) readIMAP(authU, authP string) (chan *mail.Message, error) {
	c, err := client.DialTLS(e.IMAP, nil)
	if err != nil {
		return nil, err
	}
	if e.OAuth == "" {
		if err := c.Login(authU, authP); err != nil {
			return nil, err
		}
	} else {
		if err := c.Authenticate(sasl.NewOAuthBearerClient(&sasl.OAuthBearerOptions{
			Username: authU,
			Token:    authP,
			Host:     "",
			Port:     0,
		})); err != nil {
			return nil, err
		}
	}
	mbox, err := c.Select(e.INBOX, true) //readonly
	if err != nil {
		return nil, err
	}
	from := uint32(1)
	to := mbox.Messages
	if e.Limit > 0 && to > uint32(e.Limit) {
		from = to - uint32(e.Limit-1)
	}
	seqset := new(imap.SeqSet)
	seqset.AddRange(from, to)
	emails := make(chan *mail.Message)
	messages := make(chan *imap.Message)
	go func() {
		defer c.Logout()
		section := &imap.BodySectionName{}
		if err := c.Fetch(seqset, []imap.FetchItem{imap.FetchEnvelope, section.FetchItem()}, messages); err != nil {
			log.Println("error fetching:", err)
		}
	}()
	go func() {
		defer close(emails)
		for msg := range messages {
			for _, v := range msg.Body {
				m, err := mail.ReadMessage(v)
				if err != nil {
					log.Println(err)
				} else {
					emails <- m
				}
			}
		}
	}()
	return emails, nil
}

func (e *Emailer) ReadPOP3() (chan *mail.Message, error) {
	limit := e.Limit
	if limit < 1 {
		limit = 1000
	}
	emails := make(chan *mail.Message, limit)
	defer close(emails)

	log.Printf("pop3.DialTLS(%s)", e.POP3)
	c, err := pop3.DialTLS(e.POP3)
	if err != nil {
		return nil, err
	}
	defer c.Rset()

	log.Printf("c.Auth(%s, xyz)", e.From)
	if err := c.Auth(e.From, e.Password); err != nil {
		return nil, err
	}

	log.Printf("c.ListAll()")
	ids, _, err := c.ListAll()
	if err != nil {
		return nil, err
	}
	log.Printf("/c.ListAll() = %v", ids)

	for i := len(ids) - 1; i >= 0; i-- {
		id := ids[i]
		log.Printf("c.Retr(%v)", id)
		raw, err := c.Retr(id)
		if err != nil {
			return nil, err
		}
		msg, err := mail.ReadMessage(strings.NewReader(raw))
		if err != nil {
			return nil, err
		}
		select {
		case emails <- msg:
		default:
			return emails, nil
		}
	}
	return emails, nil
}

func (e *Emailer) Send(to, subj, msg string) error {
	headers := e.headers(to, e.From, subj)
	body := e.body(headers, msg)
	smtp, err := e.smtp()
	if err != nil {
		return err
	}
	defer smtp.Quit()
	if err := smtp.Mail(e.From); err != nil {
		return err
	}
	if err := smtp.Rcpt(to); err != nil {
		return err
	}
	w, err := smtp.Data()
	if err != nil {
		return err
	}
	if _, err := w.Write([]byte(body)); err != nil {
		return err
	}
	return w.Close()
}

func (e *Emailer) headers(to, from, subj string) map[string]string {
	return map[string]string{
		"To":      to,
		"From":    from,
		"Subject": subj,
	}
}

func (e *Emailer) body(headers map[string]string, body string) string {
	b := ""
	for k, v := range headers {
		b += fmt.Sprintf("%s: %s\r\n", k, v)
	}
	return b + "\r\n" + body
}

func (e *Emailer) smtp() (*smtp.Client, error) {
	host := strings.Split(e.SMTP, ":")[0]
	auth := smtp.PlainAuth("", e.From, e.Password, host)
	tlsconfig := &tls.Config{ServerName: host}
	conn, err := tls.Dial("tcp", e.SMTP, tlsconfig)
	if err != nil {
		return nil, fmt.Errorf("cannot dial %v: %v", e.SMTP, err)
	}
	smtp, err := smtp.NewClient(conn, host)
	if err != nil {
		return nil, fmt.Errorf("cannot new client %v: %v", host, err)
	}
	if err := smtp.Auth(auth); err != nil {
		return nil, fmt.Errorf("cannot auth client: %v", err)
	}
	return smtp, nil
}