Improved performance by manually handling mail server communication instead of using smtp.SendMail() and thus opening a new connection for each mail

mail.go

@@ -3,12 +3,13 @@
 package main
 
 import (
+	"crypto/tls"
 	"encoding/base64"
 	"fmt"
 	"net/mail"
 	"net/smtp"
 	"slices"
-	"strings"
+	"time"
 )
 
 const MAIL_LINE_SEP = "\r\n"
@@ -22,13 +23,7 @@ func (c MailContent) serializeValidMail(from string, to string) []byte {
 	// We'll send base64 encoded Subject & Body, because we Dschörmäns have umlauts
 	// and I'm too lazy to encode ä into =E4 and so on
 	subjectEncoded := base64.StdEncoding.EncodeToString([]byte(c.Subject))
-	bodyEncoded := base64.StdEncoding.EncodeToString(
+	bodyEncoded := base64.StdEncoding.EncodeToString([]byte(c.Body))
-		[]byte( // ensure that all lines end with CRLF
-			strings.ReplaceAll(
-				strings.ReplaceAll(c.Body, "\n", MAIL_LINE_SEP), "\r\r", "\r",
-			),
-		),
-	)
 	data := []byte(fmt.Sprintf(
 		"Content-Type: text/plain; charset=\"utf-8\"\r\nContent-Transfer-Encoding: base64\r\nFrom: %v%vTo: %v%vSubject: =?utf-8?b?%v?=%v%v%v",
 		from, MAIL_LINE_SEP,
@@ -68,6 +63,7 @@ func (r Recipient) filterAndSendNotices(notices []WidNotice, template MailTempla
 	logger.debug("Generating and sending mails to " + r.Address + " ...")
 	cacheHits := 0
 	cacheMisses := 0
+	mails := [][]byte{}
 	for _, n := range filteredNotices {
 		var data []byte
 		cacheResult := (*cache)[n.Uuid]
@@ -86,20 +82,78 @@ func (r Recipient) filterAndSendNotices(notices []WidNotice, template MailTempla
 			// add to cache
 			(*cache)[n.Uuid] = data
 		}
-		err := smtp.SendMail(
+		mails = append(mails, data)
-			fmt.Sprintf("%v:%v", smtpConfig.ServerHost, smtpConfig.ServerPort),
+	}
-			auth,
+	logger.debug(fmt.Sprintf("%v mail cache hits, %v misses", cacheHits, cacheMisses))
-			smtpConfig.From,
+	err := sendMails(
-			[]string{r.Address},
+		smtpConfig,
-			data,
+		auth,
-		)
+		r.Address,
+		mails,
+	)
+	if err != nil {
+		return err
+	}
+	logger.debug("Successfully sent all mails to " + r.Address)
+	return nil
+}
+
+func sendMails(smtpConf SmtpSettings, auth smtp.Auth, to string, data [][]byte) error {
+	addr := fmt.Sprintf("%v:%v", smtpConf.ServerHost, smtpConf.ServerPort)
+	logger.debug("Connecting to mail server at " + addr + " ...")
+	connection, err := smtp.Dial(addr)
+	if err != nil {
+		return err
+	}
+	defer connection.Close()
+	// can leave out connection.Hello
+	hasTlsExt, _ := connection.Extension("starttls")
+	if hasTlsExt {
+		err = connection.StartTLS(&tls.Config{ServerName: smtpConf.ServerHost})
 		if err != nil {
 			return err
 		}
+		logger.debug("Mail Server supports TLS")
+	} else {
+		logger.debug("Mail Server doesn't support TLS")
 	}
-	logger.debug(fmt.Sprintf("%v mail cache hits, %v misses", cacheHits, cacheMisses))
+	logger.debug("Authenticating to mail server ...")
-	logger.debug("Successfully sent all mails to " + r.Address)
+	err = connection.Auth(auth)
-	return nil
+	if err != nil {
+		return err
+	}
+	if logger.LogLevel >= 3 {
+		fmt.Printf("DEBUG %v Sending mails to server ", time.Now().Format("2006/01/02 15:04:05.000000"))
+	}
+	for _, d := range data {
+		err = connection.Mail(smtpConf.From)
+		if err != nil {
+			return err
+		}
+		err = connection.Rcpt(to)
+		if err != nil {
+			return err
+		}
+		writer, err := connection.Data()
+		if err != nil {
+			return err
+		}
+		_, err = writer.Write(d)
+		if err != nil {
+			return err
+		}
+		err = writer.Close()
+		if err != nil {
+			return err
+		}
+		if logger.LogLevel >= 3 {
+			print(".")
+		}
+	}
+	if logger.LogLevel >= 3 {
+		print("\n")
+	}
+	return connection.Quit()
 }
 
 func mailAddressIsValid(address string) bool {