fix: remove last 0 byte for mysql_old_password when password is empty

AUTHORS

@@ -85,6 +85,7 @@ Soroush Pour <me at soroushjp.com>
 Stan Putrya <root.vagner at gmail.com>
 Stanley Gunawan <gunawan.stanley at gmail.com>
 Steven Hartland <steven.hartland at multiplay.co.uk>
+Tan Jinhua <312841925 at qq.com>
 Thomas Wodarek <wodarekwebpage at gmail.com>
 Tim Ruffles <timruffles at gmail.com>
 Tom Jenkinson <tom at tjenkinson.me>

auth.go

@@ -136,10 +136,6 @@ func pwHash(password []byte) (result [2]uint32) {
 
 // Hash password using insecure pre 4.1 method
 func scrambleOldPassword(scramble []byte, password string) []byte {
-	if len(password) == 0 {
-		return nil
-	}
-
 	scramble = scramble[:8]
 
 	hashPw := pwHash([]byte(password))
@@ -247,6 +243,9 @@ func (mc *mysqlConn) auth(authData []byte, plugin string) ([]byte, error) {
 		if !mc.cfg.AllowOldPasswords {
 			return nil, ErrOldPassword
 		}
+		if len(mc.cfg.Passwd) == 0 {
+			return nil, nil
+		}
 		// Note: there are edge cases where this should work but doesn't;
 		// this is currently "wontfix":
 		// https://github.com/go-sql-driver/mysql/issues/184

auth_test.go

@@ -1157,7 +1157,7 @@ func TestAuthSwitchOldPasswordEmpty(t *testing.T) {
 		t.Errorf("got error: %v", err)
 	}
 
-	expectedReply := []byte{1, 0, 0, 3, 0}
+	expectedReply := []byte{0, 0, 0, 3}
 	if !bytes.Equal(conn.written, expectedReply) {
 		t.Errorf("got unexpected data: %v", conn.written)
 	}
@@ -1184,7 +1184,7 @@ func TestOldAuthSwitchPasswordEmpty(t *testing.T) {
 		t.Errorf("got error: %v", err)
 	}
 
-	expectedReply := []byte{1, 0, 0, 3, 0}
+	expectedReply := []byte{0, 0, 0, 3}
 	if !bytes.Equal(conn.written, expectedReply) {
 		t.Errorf("got unexpected data: %v", conn.written)
 	}