From d831e7db9a12461057c2d0edb0e868b02d925035 Mon Sep 17 00:00:00 2001
From: Yinka Adedire <yinkakun@gmail.com>
Date: Fri, 22 Nov 2024 17:00:09 +0100
Subject: [PATCH] Add exponential backoff retry for email sending and file
 downloads

---
 go.mod  |  3 ++-
 go.sum  |  2 ++
 main.go | 21 ++++++++++++++++-----
 3 files changed, 20 insertions(+), 6 deletions(-)

diff --git a/go.mod b/go.mod
index 9ccd5b0..3c71735 100644
--- a/go.mod
+++ b/go.mod
@@ -1,8 +1,9 @@
 module book-to-kindle-bot
 
-go 1.22.4
+go 1.23.3
 
 require (
+	github.com/cenkalti/backoff/v4 v4.3.0
 	github.com/go-telegram-bot-api/telegram-bot-api/v5 v5.5.1
 	github.com/joho/godotenv v1.5.1
 	gopkg.in/gomail.v2 v2.0.0-20160411212932-81ebce5c23df
diff --git a/go.sum b/go.sum
index 37e3df9..83488bf 100644
--- a/go.sum
+++ b/go.sum
@@ -1,3 +1,5 @@
+github.com/cenkalti/backoff/v4 v4.3.0 h1:MyRJ/UdXutAwSAT+s3wNd7MfTIcy71VQueUuFK343L8=
+github.com/cenkalti/backoff/v4 v4.3.0/go.mod h1:Y3VNntkOUPxTVeUxJ/G5vcM//AlwfmyYozVcomhLiZE=
 github.com/dustin/go-humanize v1.0.1 h1:GzkhY7T5VNhEkwH0PVJgjz+fX1rhBrR7pRT3mDkpeCY=
 github.com/dustin/go-humanize v1.0.1/go.mod h1:Mu1zIs6XwVuF/gI1OepvI0qD18qycQx+mFykh5fBlto=
 github.com/go-telegram-bot-api/telegram-bot-api/v5 v5.5.1 h1:wG8n/XJQ07TmjbITcGiUaOtXxdrINDz1b0J1w0SzqDc=
diff --git a/main.go b/main.go
index a6c127c..1f48678 100644
--- a/main.go
+++ b/main.go
@@ -13,6 +13,7 @@ import (
 	"strings"
 	"time"
 
+	"github.com/cenkalti/backoff/v4"
 	tgbotapi "github.com/go-telegram-bot-api/telegram-bot-api/v5"
 	"github.com/joho/godotenv"
 	"gopkg.in/gomail.v2"
@@ -20,6 +21,7 @@ import (
 )
 
 type BotConfig struct {
+	MaxWorkers      int
 	DbPath          string
 	BotEmail        string
 	TelegramToken   string
@@ -148,9 +150,7 @@ func (b *BookToKindleBot) Start(ctx context.Context) error {
 	updateConfig := tgbotapi.NewUpdate(0)
 	updateConfig.Timeout = int(time.Second * 60)
 	updates := b.telegramBotApi.GetUpdatesChan(updateConfig)
-
-	maxWorkers := 10
-	workerPool := make(chan struct{}, maxWorkers)
+	workerPool := make(chan struct{}, b.config.MaxWorkers)
 
 	for {
 		select {
@@ -266,7 +266,11 @@ func (b *BookToKindleBot) sendEmail(kindleEmail string, fileBytes []byte, fileNa
 
 	d := gomail.NewDialer("email-smtp.us-east-1.amazonaws.com", 587, os.Getenv("AWS_SES_SMTP_USERNAME"), os.Getenv("AWS_SES_SMTP_PASSWORD"))
 
-	if err := d.DialAndSend(m); err != nil {
+	err := backoff.Retry(func() error {
+		return d.DialAndSend(m)
+	}, backoff.NewExponentialBackOff())
+
+	if err != nil {
 		return fmt.Errorf("error sending email: %w", err)
 	}
 
@@ -371,7 +375,13 @@ func (b *BookToKindleBot) downloadTelegramFile(fileId string) ([]byte, error) {
 		return nil, fmt.Errorf("error getting file URL: %w", err)
 	}
 
-	resp, err := b.httpClient.Get(fileUrl)
+	var resp *http.Response
+	err = backoff.Retry(func() error {
+		var err error
+		resp, err = b.httpClient.Get(fileUrl)
+		return err
+	}, backoff.NewExponentialBackOff())
+
 	if err != nil {
 		return nil, fmt.Errorf("error downloading file: %w", err)
 	}
@@ -401,6 +411,7 @@ func main() {
 	bookToKindleBot, err := NewBookToKindleBot(BotConfig{
 		DownloadTimeout: 30 * time.Second,
 		MaxFileSize:     20 * 1024 * 1024,
+		MaxWorkers:      10,
 		DbPath:          os.Getenv("DB_PATH"),
 		BotEmail:        os.Getenv("BOT_EMAIL"),
 		TelegramToken:   os.Getenv("TELEGRAM_BOT_TOKEN"),