main.go

package main

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

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

// Retrieve a token, saves the token, then returns the generated client.
func getClient(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.
	tokFile := "token.json"
	tok, err := tokenFromFile(tokFile)
	if err != nil {
		tok = getTokenFromWeb(config)
		saveToken(tokFile, tok)
	}
	return config.Client(context.Background(), tok)
}

// Request a token from the web, then returns the retrieved token.
func getTokenFromWeb(config *oauth2.Config) *oauth2.Token {
	authURL := config.AuthCodeURL("state-token", oauth2.AccessTypeOffline)
	fmt.Printf("Go to the following link 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(context.TODO(), 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()
	json.NewEncoder(f).Encode(token)
}

func main() {
	b, err := ioutil.ReadFile("credentials.json")
	if err != nil {
		log.Fatalf("Unable to read client secret file: %v", err)
	}

	// If modifying these scopes, delete your previously saved token.json.
	config, err := google.ConfigFromJSON(b, gmail.GmailReadonlyScope)
	if err != nil {
		log.Fatalf("Unable to parse client secret file to config: %v", err)
	}

	client := getClient(config)

	srv, err := gmail.NewService(context.TODO(), option.WithHTTPClient(client))
	if err != nil {
		log.Fatalf("Unable to retrieve Gmail client: %v", err)
	}

	user := "me"

	tt, err := srv.Users.Threads.List(user).MaxResults(10).Do()
	if err != nil {
		log.Fatalf("Unable to retrieve threads: %v", err)
	}

	if len(tt.Threads) == 0 {
		fmt.Println("No threads found.")
		return
	}

	wg := &sync.WaitGroup{}
	wg.Add(len(tt.Threads))

	fmt.Println("Threads:")
	for _, t := range tt.Threads {
		go func(t *gmail.Thread) {
			defer wg.Done()

			start := time.Now()
			defer func() {
				fmt.Printf("Took %s\n", time.Since(start).String())
			}()

			thread, err := srv.Users.Threads.Get(user, t.Id).Do()
			if err != nil {
				log.Fatalf("Unable to fetch thread: %s: %v", t.Id, err)
			}

			if len(thread.Messages) == 0 {
				return
			}

			t.Messages = thread.Messages
		}(t)
	}

	wg.Wait()

	for _, t := range tt.Threads {
		if len(t.Messages) == 0 {
			continue
		}

		m := t.Messages[0]

		var subject gmail.MessagePartHeader
		for _, h := range m.Payload.Headers {
			if h.Name == "Subject" {
				subject = *h
				break
			}
		}

		fmt.Printf("- %s: %.25s\n", subject.Value, t.Snippet)
	}
}