Move compilation of allowed_sender to ConfigLoad()

This has several benefits:
- Configuration errors are caught at startup rather than upon a connection
- senderChecker() has less work to do for each connection

config.go

@@ -3,6 +3,7 @@ package main
 import (
 	"flag"
 	"net"
+	"regexp"
 
 	"github.com/vharitonsky/iniflags"
 	"github.com/sirupsen/logrus"
@@ -25,7 +26,8 @@ var (
 	localForceTLS     = flag.Bool("local_forcetls", false, "Force STARTTLS (needs local_cert and local_key)")
 	allowedNetsStr    = flag.String("allowed_nets", "127.0.0.0/8 ::1/128", "Networks allowed to send mails")
 	allowedNets       = []*net.IPNet{}
-	allowedSender     = flag.String("allowed_sender", "", "Regular expression for valid FROM EMail addresses")
+	allowedSenderStr  = flag.String("allowed_sender", "", "Regular expression for valid FROM EMail addresses")
+	allowedSender     *regexp.Regexp
 	allowedRecipients = flag.String("allowed_recipients", "", "Regular expression for valid TO EMail addresses")
 	allowedUsers      = flag.String("allowed_users", "", "Path to file with valid users/passwords")
 	remoteHost        = flag.String("remote_host", "", "Outgoing SMTP server")
@@ -59,6 +61,20 @@ func setupAllowedNetworks() {
 	}
 }
 
+func setupAllowedSender() {
+	if (*allowedSenderStr == "") {
+		return
+	}
+
+	var err error
+	allowedSender, err = regexp.Compile(*allowedSenderStr)
+	if err != nil {
+		log.WithField("allowed_sender", *allowedSenderStr).
+			WithError(err).
+			Fatal("allowed_sender pattern invalid")
+	}
+}
+
 func ConfigLoad() {
 	iniflags.Parse()
 
@@ -70,4 +86,5 @@ func ConfigLoad() {
 	}
 
 	setupAllowedNetworks()
+	setupAllowedSender()
 }

main.go

@@ -103,26 +103,20 @@ func senderChecker(peer smtpd.Peer, addr string) error {
 		}
 	}
 
-	if *allowedSender == "" {
+	if allowedSender == nil {
+		// Any sender is permitted
 		return nil
 	}
 
-	re, err := regexp.Compile(*allowedSender)
-	if err != nil {
-		log.WithFields(logrus.Fields{
-			"allowed_sender": *allowedSender,
-		}).WithError(err).Warn("allowed_sender pattern invalid")
-		return smtpd.Error{Code: 451, Message: "Bad sender address"}
-	}
-
-	if re.MatchString(addr) {
+	if allowedSender.MatchString(addr) {
+		// Permitted by regex
 		return nil
 	}
 
 	log.WithFields(logrus.Fields{
 		"sender_address": addr,
 		"peer": peer.Addr,
-	}).Warn("Sender address not allowed by allowed_sender pattern")
+	}).Warn("sender address not allowed by allowed_sender pattern")
 	return smtpd.Error{Code: 451, Message: "Bad sender address"}
 }