test: Update fuzz tests to use the built-in infrastructure

Go 1.18 supports fuzzing natively, so this patch migrates our fuzzing
tests to make use of it.

internal/smtpsrv/fuzz_test.go

@@ -0,0 +1,96 @@
+// Fuzz testing for package smtpsrv.  Based on server_test.
+package smtpsrv
+
+import (
+	"bufio"
+	"bytes"
+	"crypto/tls"
+	"fmt"
+	"net"
+	"net/textproto"
+	"strings"
+	"testing"
+)
+
+func fuzzConnection(t *testing.T, modeI int, data []byte) {
+	var mode SocketMode
+	addr := ""
+	switch modeI {
+	case 0:
+		mode = ModeSMTP
+		addr = smtpAddr
+	case 1:
+		mode = ModeSubmission
+		addr = submissionAddr
+	case 2:
+		mode = ModeSubmissionTLS
+		addr = submissionTLSAddr
+	default:
+		mode = ModeSMTP
+		addr = smtpAddr
+	}
+
+	var err error
+	var conn net.Conn
+	if mode.TLS {
+		conn, err = tls.Dial("tcp", addr, tlsConfig)
+	} else {
+		conn, err = net.Dial("tcp", addr)
+	}
+	if err != nil {
+		panic(fmt.Errorf("failed to dial: %v", err))
+	}
+	defer conn.Close()
+
+	tconn := textproto.NewConn(conn)
+	defer tconn.Close()
+
+	scanner := bufio.NewScanner(bytes.NewBuffer(data))
+	for scanner.Scan() {
+		line := scanner.Text()
+		cmd := strings.TrimSpace(strings.ToUpper(line))
+
+		// Skip STARTTLS if it happens on a non-TLS connection - the jump is
+		// not going to happen via fuzzer, it will just cause a timeout (which
+		// is considered a crash).
+		if cmd == "STARTTLS" && !mode.TLS {
+			continue
+		}
+
+		if err = tconn.PrintfLine(line); err != nil {
+			break
+		}
+
+		if _, _, err = tconn.ReadResponse(-1); err != nil {
+			break
+		}
+
+		if cmd == "DATA" {
+			// We just sent DATA and got a response; send the contents.
+			err = exchangeData(scanner, tconn)
+			if err != nil {
+				break
+			}
+		}
+	}
+}
+
+func FuzzConnection(f *testing.F) {
+	f.Fuzz(fuzzConnection)
+}
+
+func exchangeData(scanner *bufio.Scanner, tconn *textproto.Conn) error {
+	for scanner.Scan() {
+		line := scanner.Text()
+		if err := tconn.PrintfLine(line); err != nil {
+			return err
+		}
+		if line == "." {
+			break
+		}
+	}
+
+	// Read the "." response.
+	_, _, err := tconn.ReadResponse(-1)
+	return err
+}