Gmail auth implementation for SMTP (#1260)

* init

* fix typo in TokenStore constructor

* simplify naming

* simplify RefreshToken signature

* fix lock usage and remove unnecessary code

* change octal to hex

* remove unnecessary byte reading

internal/mail/gmail/gmailauth.go

@@ -0,0 +1,146 @@
+// Copyright (C) 2019 Storj Labs, Inc.
+// See LICENSE for copying information
+
+package gmail
+
+import (
+	"encoding/json"
+	"fmt"
+	"net/http"
+	"net/smtp"
+	"net/url"
+	"strings"
+	"sync"
+	"time"
+
+	"github.com/zeebo/errs"
+)
+
+// Auth is XOAUTH2 implementation of smtp.Auth interface for gmail
+type Auth struct {
+	UserEmail string
+
+	Storage *TokenStore
+}
+
+// Start returns proto and auth credentials for first auth msg
+func (auth *Auth) Start(server *smtp.ServerInfo) (proto string, toServer []byte, err error) {
+	token, err := auth.Storage.Token()
+	if err != nil {
+		return "", nil, err
+	}
+
+	format := fmt.Sprintf("user=%s\x01auth=%s %s\x01\x01", auth.UserEmail, token.Type, token.AccessToken)
+	return "XOAUTH2", []byte(format), nil
+}
+
+// Next sends empty response to solve SASL challenge if response code is 334
+func (auth *Auth) Next(fromServer []byte, more bool) (toServer []byte, err error) {
+	if more {
+		return make([]byte, 0), nil
+	}
+
+	return nil, nil
+}
+
+// Token represents OAUTH2 token
+type Token struct {
+	AccessToken  string    `json:"access_token"`
+	RefreshToken string    `json:"refresh_token"`
+	Type         string    `json:"token_type"`
+	Expiry       time.Time `json:"expiry"`
+}
+
+// Credentials represents OAUTH2 credentials
+type Credentials struct {
+	ClientID     string `json:"client_id"`
+	ClientSecret string `json:"client_secret"`
+	TokenURI     string `json:"token_uri"`
+}
+
+// TokenStore is a thread safe storage for OAUTH2 token and credentials
+type TokenStore struct {
+	mu    sync.Mutex
+	token Token
+	creds Credentials
+}
+
+// NewTokenStore creates new instance of token storage
+func NewTokenStore(creds Credentials, token Token) *TokenStore {
+	return &TokenStore{
+		token: token,
+		creds: creds,
+	}
+}
+
+// Token retrieves token in a thread safe way and refreshes it if needed
+func (s *TokenStore) Token() (*Token, error) {
+	s.mu.Lock()
+	defer s.mu.Unlock()
+
+	token := new(Token)
+	if s.token.Expiry.Before(time.Now()) {
+		var err error
+		token, err = RefreshToken(s.creds, s.token.RefreshToken)
+		if err != nil {
+			return nil, err
+		}
+		s.token = *token
+	}
+
+	*token = s.token
+	return token, nil
+}
+
+// RefreshToken is a helper method that refreshes token with given credentials and OUATH2 refresh token
+func RefreshToken(creds Credentials, refreshToken string) (*Token, error) {
+	values := url.Values{
+		"grant_type":    {"refresh_token"},
+		"refresh_token": {refreshToken},
+	}
+
+	req, err := http.NewRequest("POST", creds.TokenURI, strings.NewReader(values.Encode()))
+	if err != nil {
+		return nil, err
+	}
+
+	req.Header.Set("Content-Type", "application/x-www-form-urlencoded")
+	req.SetBasicAuth(url.QueryEscape(creds.ClientID), url.QueryEscape(creds.ClientSecret))
+
+	client := http.Client{}
+
+	resp, err := client.Do(req)
+	if err != nil {
+		return nil, err
+	}
+	defer func() {
+		err = errs.Combine(resp.Body.Close())
+	}()
+
+	// handle google expires_in field value
+	var t struct {
+		AccessToken  string        `json:"access_token"`
+		RefreshToken string        `json:"refresh_token"`
+		Type         string        `json:"token_type"`
+		Expires      time.Duration `json:"expires_in"`
+	}
+	err = json.NewDecoder(resp.Body).Decode(&t)
+	if err != nil {
+		return nil, err
+	}
+
+	if t.AccessToken == "" {
+		return nil, errs.New("no access token were granted")
+	}
+
+	if t.RefreshToken == "" {
+		t.RefreshToken = refreshToken
+	}
+
+	return &Token{
+		AccessToken:  t.AccessToken,
+		RefreshToken: t.RefreshToken,
+		Type:         t.Type,
+		Expiry:       time.Now().Add(t.Expires * time.Second),
+	}, nil
+}