feat: reopen SMTP connection if closed

pkg/mail/mail.go

@@ -19,6 +19,9 @@ package mail
 import (
 	"context"
 	"crypto/tls"
+	"errors"
+	"fmt"
+	"strings"
 	"time"
 
 	"code.vikunja.io/api/pkg/config"
@@ -46,7 +49,6 @@ func getClient() (*mail.Client, error) {
 	if config.MailerForceSSL.GetBool() {
 		tlsPolicy = mail.TLSMandatory
 	}
-
 	opts := []mail.Option{
 		mail.WithTLSPortPolicy(tlsPolicy),
 		mail.WithTLSConfig(&tls.Config{
@@ -82,6 +84,38 @@ func getClient() (*mail.Client, error) {
 	)
 }
 
+func resilientSend(client *mail.Client, message *mail.Msg) error {
+	const maxRetries = 3
+	retries := 0
+
+	for retries < maxRetries {
+		log.Debug("Sending mail")
+		err := client.Send(message)
+		if err == nil {
+			log.Debug("Mail sent")
+			return nil
+		}
+
+		var sendError *mail.SendError
+		_ = errors.As(err, &sendError)
+
+		if sendError.Reason != mail.ErrConnCheck {
+			// only ErrConnCheck is retried, other exceptions are thrown
+			return err
+		}
+
+		log.Warning("Connection was closed, reconnecting before retrying")
+		err = client.DialWithContext(context.Background())
+		if err != nil {
+			return err
+		}
+		log.Debug("Connection re-established")
+		retries++
+	}
+
+	return errors.New(fmt.Sprintf("could not send mail after %d retries", retries))
+}
+
 // StartMailDaemon starts the mail daemon
 func StartMailDaemon() {
 	Queue = make(chan *mail.Msg, config.MailerQueuelength.GetInt())
@@ -110,25 +144,32 @@ func StartMailDaemon() {
 					return
 				}
 				if !open {
+					log.Debug("Opening connection to mail server")
 					err = c.DialWithContext(context.Background())
 					if err != nil {
 						log.Errorf("Error during connect to smtp server: %s", err)
 						break
 					}
+					log.Debug("Connected to mail server")
 					open = true
 				}
-				err = c.Send(m)
+				err = resilientSend(c, m)
 				if err != nil {
-					log.Errorf("Error when sending mail: %s", err)
+					log.Errorf("Error sending mail: %s", err)
 					break
 				}
 				// Close the connection to the SMTP server if no email was sent in
 				// the last 30 seconds.
 			case <-time.After(config.MailerQueueTimeout.GetDuration() * time.Second):
 				if open {
+					log.Debug("Closing connection to mail server due to queue timeout")
 					open = false
 					err = c.Close()
 					if err != nil {
+						if strings.HasPrefix(err.Error(), "not connected to SMTP server") {
+							log.Warning("Connection to mail server was already closed")
+							break
+						}
 						log.Errorf("Error closing the mail server connection: %s\n", err)
 						break
 					}