feature: add optional email delay

resolves #256

docs/notifications.md

@@ -26,6 +26,7 @@ To receive notifications by email, the following command-line options, or their
 - `--notification-email-server-port` (env. `WATCHTOWER_NOTIFICATION_EMAIL_SERVER_PORT`): The port used to connect to the SMTP server to send e-mails through. Defaults to `25`.
 - `--notification-email-server-user` (env. `WATCHTOWER_NOTIFICATION_EMAIL_SERVER_USER`): The username to authenticate with the SMTP server with.
 - `--notification-email-server-password` (env. `WATCHTOWER_NOTIFICATION_EMAIL_SERVER_PASSWORD`): The password to authenticate with the SMTP server with.
+- `--notification-email-delay` (env. `WATCHTOWER_NOTIFICATION_EMAIL_DELAY`): Delay before sending notifications expressed in seconds.
 
 Example:
 
@@ -39,6 +40,7 @@ docker run -d \
   -e WATCHTOWER_NOTIFICATION_EMAIL_SERVER=smtp.gmail.com \
   -e WATCHTOWER_NOTIFICATION_EMAIL_SERVER_USER=fromaddress@gmail.com \
   -e WATCHTOWER_NOTIFICATION_EMAIL_SERVER_PASSWORD=app_password \
+  -e WATCHTOWER_NOTIFICATION_EMAIL_DELAY=2 \
   containrrr/watchtower
 ```
 

internal/flags/flags.go

@@ -128,6 +128,12 @@ func RegisterNotificationFlags(rootCmd *cobra.Command) {
 		"",
 		viper.GetString("WATCHTOWER_NOTIFICATION_EMAIL_TO"),
 		"Address to send notification emails to")
+	
+	flags.IntP(
+		"notification-email-delay",
+		"",
+		viper.GetInt("WATCHTOWER_NOTIFICATION_EMAIL_DELAY"),
+		"Delay before sending notifications, expressed in seconds")
 
 	flags.StringP(
 		"notification-email-server",

pkg/notifications/email.go

@@ -29,6 +29,7 @@ type emailTypeNotifier struct {
 	tlsSkipVerify          bool
 	entries                []*log.Entry
 	logLevels              []log.Level
+	delay                  time.Duration
 }
 
 func newEmailNotifier(c *cobra.Command, acceptedLogLevels []log.Level) t.Notifier {
@@ -41,6 +42,7 @@ func newEmailNotifier(c *cobra.Command, acceptedLogLevels []log.Level) t.Notifie
 	password, _ := flags.GetString("notification-email-server-password")
 	port, _ := flags.GetInt("notification-email-server-port")
 	tlsSkipVerify, _ := flags.GetBool("notification-email-server-tls-skip-verify")
+	delay, _ := flags.GetInt("notification-email-delay")
 
 	n := &emailTypeNotifier{
 		From:          from,
@@ -51,6 +53,7 @@ func newEmailNotifier(c *cobra.Command, acceptedLogLevels []log.Level) t.Notifie
 		Port:          port,
 		tlsSkipVerify: tlsSkipVerify,
 		logLevels:     acceptedLogLevels,
+		delay:         time.Duration(delay) * time.Second,
 	}
 
 	log.AddHook(n)
@@ -117,9 +120,15 @@ func (e *emailTypeNotifier) StartNotification() {
 }
 
 func (e *emailTypeNotifier) SendNotification() {
-	if e.entries != nil && len(e.entries) != 0 {
+	if e.entries == nil || len(e.entries) <= 0 {
-		e.sendEntries(e.entries)
+		return
 	}
+
+	if e.delay > 0 {
+		time.Sleep(e.delay)
+	}
+
+	e.sendEntries(e.entries)	
 	e.entries = nil
 }