get oauth2-token and read email labels

.gitignore

@@ -1 +1,2 @@
 **/credentials.json
+**/token.json

main.go

@@ -0,0 +1,96 @@
+package main
+
+import (
+	"context"
+	"encoding/json"
+	"fmt"
+	"log"
+	"net/http"
+	"os"
+
+	"golang.org/x/oauth2"
+	"golang.org/x/oauth2/google"
+	"google.golang.org/api/gmail/v1"
+	"google.golang.org/api/option"
+)
+
+func getClient(config *oauth2.Config) *http.Client {
+	token_file := "token.json"
+	token, err := tokenFromFile(token_file)
+	if err != nil {
+		token = getTokenFromWeb(config)
+		saveToken(token_file, token)
+	}
+	return config.Client(context.Background(), token)
+}
+
+func getTokenFromWeb(config *oauth2.Config) *oauth2.Token {
+	auth_url := config.AuthCodeURL("state-token", oauth2.AccessTypeOffline)
+	fmt.Printf("Go to the following link in your browser, then type the " +
+		"authorization code: \n%v\n", auth_url)
+	
+	var auth_code string
+	if _, err := fmt.Scan(&auth_code) ; err != nil {
+		log.Fatalf("Unable to read authorization code: %v", err)
+	}
+
+	token, err := config.Exchange(context.TODO(), auth_code)
+	if err != nil {
+		log.Fatalf("Unable to retrieve token from web: %v", err)
+	}
+	return token
+}
+
+func tokenFromFile(file string) (*oauth2.Token, error) {
+	f, err := os.Open(file)
+	if err != nil {
+		return nil, err
+	}
+	defer f.Close()
+	token := &oauth2.Token{}
+	err = json.NewDecoder(f).Decode(token)
+	return token, err
+}
+
+func saveToken(path string, token *oauth2.Token) {
+	fmt.Printf("Saving credential file to: %s\n", path)
+	file, 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 file.Close()
+	json.NewEncoder(file).Encode(token)
+}
+
+func main() {
+	ctx := context.Background()
+	creds_b, err := os.ReadFile("credentials.json")
+	if err != nil {
+		log.Fatalf("Unable to read client secret file: %v", err)
+	}
+
+	config, err := google.ConfigFromJSON(creds_b, gmail.GmailReadonlyScope)
+	if err != nil {
+		log.Fatalf("Unable to parse client secret file to config: %v", err)
+	}
+	client := getClient(config)
+
+	service, err := gmail.NewService(ctx, option.WithHTTPClient(client))
+	if err != nil {
+		log.Fatalf("Unable to retrieve Gmail client: %v", err)
+	}
+
+	user := "me"
+	r, err := service.Users.Labels.List(user).Do()
+	if err != nil {
+		log.Fatalf("Unable to retrieve labels: %v", err)
+	}
+	if len(r.Labels) == 0 {
+		fmt.Println("No labels found")
+		return
+	}
+	fmt.Println("Labels:")
+	for _, label := range r.Labels {
+		fmt.Printf("- %s\n", label.Name)
+	}
+}

token.json.example

@@ -0,0 +1,6 @@
+{
+  "access_token": "ACCESS_TOKEN",
+  "token_type": "Bearer",
+  "refresh_token": "REFRESH_TOKEN",
+  "expiry": "EXPIRY_TIMESTAMP"
+}