gmail_auth.go

package main

import (
	"context"
	"encoding/json"
	"fmt"
	"io/ioutil"
	"log"
	"net/http"
	"os"
	"path"

	"golang.org/x/oauth2"
	"golang.org/x/oauth2/google"
	"google.golang.org/api/gmail/v1"
)

func buildGmailService(ctx context.Context) (*gmail.Service, error) {
	configDir := MustGetenv(envVarConfigDir)
	credentialsFilename := path.Join(configDir, "credentials.json")
	b, err := ioutil.ReadFile(credentialsFilename)
	if err != nil {
		return nil, fmt.Errorf("unable to read client credentials file (credentials.json): %w", err)
	}
	config, err := google.ConfigFromJSON(b, gmail.GmailModifyScope, gmail.GmailSendScope) // If modifying these scopes, delete any previously saved token.json.
	if err != nil {
		return nil, fmt.Errorf("unable to parse client secret file to config: %w", err)
	}
	client := getClient(ctx, config)
	srv, err := gmail.New(client)
	if err != nil {
		return nil, fmt.Errorf("unable to build Gmail client: %w", err)
	}
	return srv, nil
}

// Retrieve a token, saves the token, then returns the generated client.
func getClient(ctx context.Context, config *oauth2.Config) *http.Client {
	// The file token.json stores the user's access and refresh tokens, and is
	// created automatically when the authorization flow completes for the first time.
	configDir := MustGetenv(envVarConfigDir)
	tokFile := path.Join(configDir, "token.json")
	tok, err := tokenFromFile(tokFile)
	if err != nil {
		tok = getTokenFromWeb(ctx, config)
		saveToken(tokFile, tok)
	}
	return config.Client(ctx, tok)
}

// Request a token from the web, then returns the retrieved token.
func getTokenFromWeb(ctx context.Context, config *oauth2.Config) *oauth2.Token {
	authURL := config.AuthCodeURL("state-token", oauth2.AccessTypeOffline)
	fmt.Printf("Go to the following URL in your browser then type the "+
		"authorization code: \n%v\n", authURL)
	var authCode string
	if _, err := fmt.Scan(&authCode); err != nil {
		log.Fatalf("Unable to read authorization code: %v", err)
	}
	tok, err := config.Exchange(ctx, authCode)
	if err != nil {
		log.Fatalf("Unable to retrieve token from web: %v", err)
	}
	return tok
}

// Retrieves a token from a local file.
func tokenFromFile(file string) (*oauth2.Token, error) {
	f, err := os.Open(file)
	if err != nil {
		return nil, err
	}
	defer f.Close()
	tok := &oauth2.Token{}
	err = json.NewDecoder(f).Decode(tok)
	return tok, err
}

// Saves a token to a file path.
func saveToken(path string, token *oauth2.Token) {
	fmt.Printf("Saving credential file to: %s\n", path)
	f, err := os.OpenFile(path, os.O_RDWR|os.O_CREATE|os.O_TRUNC, 0600)
	if err != nil {
		log.Fatalf("Unable to cache oauth token: %v", err)
	}
	defer f.Close()
	if err := json.NewEncoder(f).Encode(token); err != nil {
		log.Fatalf("Unable to encode oauth token: %v", err)
	}
}