1 files changed, 151 insertions, 0 deletions
diff --git a/mta_recv.go b/mta_recv.go
new file mode 100644
index 0000000..2b4679a
--- /dev/null
+++ b/mta_recv.go
@@ -0,0 +1,151 @@
+package main
+
+import (
+	"bufio"
+	"bytes"
+	"slices"
+	"strings"
+)
+
+type server_state_t uint
+
+const (
+	server_state_begin server_state_t = iota
+	server_state_helo
+	server_state_mail
+	server_state_rcpt
+)
+
+func handle_incoming_server_connection(reader *bufio.Reader, writer *bufio.Writer) error {
+	_, _ = writer.WriteString("220 " + config.Server_name + " " + VERSION + "\r\n")
+	_ = writer.Flush()
+	server_state := server_state_begin
+	var remote_server_name string
+	var current_mail_from string
+	var current_rcpt_to []string
+	for {
+		line, err := reader.ReadString('\n')
+		if err != nil {
+			return err
+		}
+		line = strings.TrimSuffix(line, "\n")
+		line = strings.TrimSuffix(line, "\r")
+		cmd_end := strings.IndexByte(line, ' ')
+		var param_start int
+		if cmd_end == -1 {
+			cmd_end = len(line)
+			param_start = len(line)
+		} else {
+			param_start = cmd_end + 1
+		}
+		cmd := strings.ToUpper(line[:cmd_end])
+		param := line[param_start:]
+	switch_cmd:
+		switch cmd {
+		case "HELO":
+			if param == "" { // TODO: actually validate the hostname
+				_, _ = writer.WriteString("501 Syntax: HELO hostname\r\n")
+				_ = writer.Flush()
+				break
+			}
+			remote_server_name = param
+			_ = remote_server_name // TODO
+			server_state = server_state_helo
+			_, _ = writer.WriteString("250 " + config.Server_name + "\r\n")
+			_ = writer.Flush()
+		case "MAIL":
+			switch server_state {
+			case server_state_begin:
+				_, _ = writer.WriteString("503 5.5.1 Error: send HELO/EHLO first\r\n")
+				_ = writer.Flush()
+				break switch_cmd
+			case server_state_helo:
+				break
+			case server_state_mail:
+				_, _ = writer.WriteString("503 5.5.1 Error: nested MAIL command\r\n")
+				_ = writer.Flush()
+				break switch_cmd
+			}
+			if len(param) <= len("FROM:") || strings.ToUpper(param[:len("FROM:")]) != "FROM:" {
+				_, _ = writer.WriteString("501 5.5.4 Syntax: MAIL FROM:<address>\r\n")
+				_ = writer.Flush()
+				break
+			}
+			current_mail_from = param[len("FROM:"):]
+			current_rcpt_to = []string{}
+			server_state = server_state_mail
+			_, _ = writer.WriteString("250 2.1.0 Ok\r\n")
+			_ = writer.Flush()
+			// TODO: Address validation
+		case "RCPT":
+			if server_state != server_state_mail && server_state != server_state_rcpt {
+				_, _ = writer.WriteString("503 5.5.1 Error: need MAIL command\r\n")
+				_ = writer.Flush()
+				break
+			}
+			if len(param) <= len("TO:") || strings.ToUpper(param[:len("TO:")]) != "TO:" {
+				_, _ = writer.WriteString("501 5.5.4 Syntax: RCPT TO:<address>\r\n")
+				_ = writer.Flush()
+				break
+			}
+			current_rcpt_to = append(current_rcpt_to, param[len("TO:"):])
+			server_state = server_state_rcpt
+			_, _ = writer.WriteString("250 2.1.5 Ok\r\n")
+			_ = writer.Flush()
+		case "DATA":
+			if server_state != server_state_rcpt {
+				_, _ = writer.WriteString("503 5.5.1 Error: need RCPT command\r\n")
+				_ = writer.Flush()
+				break
+			}
+			_, _ = writer.WriteString("354 End data with <CR><LF>.<CR><LF>\r\n")
+			_ = writer.Flush()
+			var current_data []byte
+			for {
+				tmp, err := reader.ReadSlice('\r')
+				if err != nil {
+					return err
+				}
+
+				// reader.ReadSlice returns an internal buffer that gets
+				// overwritten on the next reader operation. So we must
+				// make a copy; also we have to allocate data_part to
+				// the correct length because [[builtin.copy]] copies
+				// min(len(dst), len(src)) items.
+				data_part := make([]byte, len(tmp))
+				copy(data_part, tmp)
+
+				next_four, err := reader.Peek(4)
+				if err != nil {
+					return err
+				}
+				if bytes.Equal(next_four, []byte{'\n', '.', '\r', '\n'}) {
+					current_data = slices.Concat(current_data, data_part[:len(data_part)-1])
+					break
+				}
+				current_data = slices.Concat(current_data, data_part)
+			}
+			_, err := reader.Discard(4)
+			if err != nil {
+				return err
+			}
+			deliver_incoming(current_mail_from, current_rcpt_to, current_data)
+			server_state = server_state_helo
+			_, _ = writer.WriteString("250 2.0.0 Ok: Accepted\r\n")
+			_ = writer.Flush()
+		case "QUIT":
+			_, _ = writer.WriteString("221 2.0.0 Bye\r\n")
+			_ = writer.Flush()
+			return nil
+		case "RSET":
+			if server_state != server_state_begin {
+				server_state = server_state_helo
+			}
+			_, _ = writer.WriteString("250 2.0.0 Ok\r\n")
+			_ = writer.Flush()
+		default:
+			_, _ = writer.WriteString("500 5.5.2 Error: command not recognized\r\n")
+			_ = writer.Flush()
+		}
+	}
+}