chore: refactor smtp/handler if-else chain (#504)

* chore: convert smtp/handler if-else chain to switch-case

Signed-off-by: James Hillyerd <james@hillyerd.com>

* chore: extract long case into parseMailCmd func

Signed-off-by: James Hillyerd <james@hillyerd.com>

* chore: remove extraneous braces in cases

Signed-off-by: James Hillyerd <james@hillyerd.com>

---------

Signed-off-by: James Hillyerd <james@hillyerd.com>

pkg/server/smtp/handler.go

@@ -330,7 +330,8 @@ func (s *Session) passwordHandler() {
 // READY state -> waiting for MAIL
 // AUTH can change
 func (s *Session) readyHandler(cmd string, arg string) {
-	if cmd == "STARTTLS" {
+	switch cmd {
+	case "STARTTLS":
 		if !s.Server.config.TLSEnabled {
 			// Invalid command since TLS unconfigured.
 			s.logger.Debug().Msgf("454 TLS unavailable on the server")
@@ -353,12 +354,12 @@ func (s *Session) readyHandler(cmd string, arg string) {
 		s.tlsState = new(tls.ConnectionState)
 		*s.tlsState = tlsConn.ConnectionState()
 		s.enterState(GREET)
-	} else if cmd == "AUTH" {
+
+	case "AUTH":
 		args := strings.SplitN(arg, " ", 3)
 		authMethod := args[0]
 		switch authMethod {
 		case "PLAIN":
-			{
 			if len(args) != 2 {
 				s.send("500 Bad auth arguments")
 				s.logger.Warn().Msgf("Bad auth attempt: %q", arg)
@@ -367,20 +368,33 @@ func (s *Session) readyHandler(cmd string, arg string) {
 			s.logger.Info().Msgf("Accepting credentials: %q", args[1])
 			s.send("235 2.7.0 Authentication successful")
 			return
-			}
+
 		case "LOGIN":
-			{
 			s.send(fmt.Sprintf("334 %v", usernameChallenge))
 			s.enterState(LOGIN)
 			return
-			}
+
 		default:
-			{
 			s.send(fmt.Sprintf("500 Unsupported AUTH method: %v", authMethod))
 			return
 		}
+
+	case "MAIL":
+		s.parseMailFromCmd(arg)
+
+	case "EHLO":
+		// Reset session
+		s.logger.Debug().Msgf("Resetting session state on EHLO request")
+		s.reset()
+		s.send("250 Session reset")
+
+	default:
+		s.ooSeq(cmd)
 	}
-	} else if cmd == "MAIL" {
+}
+
+// Parses `MAIL FROM` command.
+func (s *Session) parseMailFromCmd(arg string) {
 	// Capture group 1: from address. 2: optional params.
 	m := fromRegex.FindStringSubmatch(arg)
 	if m == nil {
@@ -390,9 +404,10 @@ func (s *Session) readyHandler(cmd string, arg string) {
 	}
 	from := m[1]
 	s.logger.Debug().Msgf("Mail sender is %v", from)
+
+	// Parse from address.
 	localpart, domain, err := policy.ParseEmailAddress(from)
 	s.logger.Debug().Msgf("Origin domain is %v", domain)
-
 	if from != "" && err != nil {
 		s.send("501 Bad sender address syntax")
 		s.logger.Warn().Msgf("Bad address as MAIL arg: %q, %s", from, err)
@@ -402,15 +417,18 @@ func (s *Session) readyHandler(cmd string, arg string) {
 		from = "unspecified"
 	}
 
-		// This is where the client may put BODY=8BITMIME, but we already
+	// Parse ESMTP parameters.
-		// read the DATA as bytes, so it does not effect our processing.
 	if m[2] != "" {
+		// Here the client may put BODY=8BITMIME, but Inbucket already
+		// reads the DATA as bytes, so it does not effect mail processing.
 		args, ok := s.parseArgs(m[2])
 		if !ok {
 			s.send("501 Unable to parse MAIL ESMTP parameters")
 			s.logger.Warn().Msgf("Bad MAIL argument: %q", arg)
 			return
 		}
+
+		// Reject oversized messages.
 		if args["SIZE"] != "" {
 			size, err := strconv.ParseInt(args["SIZE"], 10, 32)
 			if err != nil {
@@ -429,9 +447,13 @@ func (s *Session) readyHandler(cmd string, arg string) {
 	// Process through extensions.
 	extResult := s.extHost.Events.BeforeMailAccepted.Emit(
 		&event.AddressParts{Local: localpart, Domain: domain})
+	if extResult != nil && !*extResult {
+		s.send("550 Mail denied by policy")
+		s.logger.Warn().Msgf("Extension denied mail from <%v>", from)
+		return
+	}
 
-		if extResult == nil || *extResult {
+	// Sender was permitted by an extension, or no extension rejected it.
-			// Permitted by extension, or none had an opinion.
 	origin, err := s.addrPolicy.ParseOrigin(from)
 	if err != nil {
 		s.send("501 Bad origin address syntax")
@@ -445,22 +467,10 @@ func (s *Session) readyHandler(cmd string, arg string) {
 		return
 	}
 
+	// Ok to transition to MAIL state.
 	s.logger.Info().Msgf("Mail from: %v", from)
 	s.send(fmt.Sprintf("250 Roger, accepting mail from <%v>", from))
 	s.enterState(MAIL)
-		} else {
-			s.send("550 Mail denied by policy")
-			s.logger.Warn().Msgf("Extension denied mail from <%v>", from)
-			return
-		}
-	} else if cmd == "EHLO" {
-		// Reset session
-		s.logger.Debug().Msgf("Resetting session state on EHLO request")
-		s.reset()
-		s.send("250 Session reset")
-	} else {
-		s.ooSeq(cmd)
-	}
 }
 
 // MAIL state -> waiting for RCPTs followed by DATA