executor: forbid user to drop important system table

executor/ddl.go

@@ -156,6 +156,13 @@ func (e *DDLExec) executeCreateIndex(s *ast.CreateIndexStmt) error {
 
 func (e *DDLExec) executeDropDatabase(s *ast.DropDatabaseStmt) error {
 	dbName := model.NewCIStr(s.Name)
+
+	// Protect important system table from been dropped by a mistake.
+	// I can hardly find a case that a user really need to do this.
+	if dbName.L == "mysql" {
+		return errors.New("Drop 'mysql' database is forbidden")
+	}
+
 	err := domain.GetDomain(e.ctx).DDL().DropSchema(e.ctx, dbName)
 	if infoschema.ErrDatabaseNotExists.Equal(err) {
 		if s.IfExists {
@@ -179,6 +186,24 @@ func (e *DDLExec) executeDropDatabase(s *ast.DropDatabaseStmt) error {
 	return errors.Trace(err)
 }
 
+// If one drop those tables by mistake, it's difficult to recover.
+// In the worst case, the whole TiDB cluster fails to bootstrap, so we prevent user from dropping them.
+var systemTables = map[string]struct{}{
+	"tidb":                 {},
+	"gc_delete_range":      {},
+	"gc_delete_range_done": {},
+}
+
+func isSystemTable(schema, table string) bool {
+	if schema != "mysql" {
+		return false
+	}
+	if _, ok := systemTables[table]; ok {
+		return true
+	}
+	return false
+}
+
 func (e *DDLExec) executeDropTable(s *ast.DropTableStmt) error {
 	var notExistTables []string
 	for _, tn := range s.Tables {
@@ -198,6 +223,12 @@ func (e *DDLExec) executeDropTable(s *ast.DropTableStmt) error {
 			return errors.Trace(err)
 		}
 
+		// Protect important system table from been dropped by a mistake.
+		// I can hardly find a case that a user really need to do this.
+		if isSystemTable(tn.Schema.L, tn.Name.L) {
+			return errors.Errorf("Drop tidb system table '%s.%s' is forbidden", tn.Schema.L, tn.Name.L)
+		}
+
 		if config.CheckTableBeforeDrop {
 			log.Warnf("admin check table `%s`.`%s` before drop.", fullti.Schema.O, fullti.Name.O)
 			sql := fmt.Sprintf("admin check table `%s`.`%s`", fullti.Schema.O, fullti.Name.O)

executor/ddl_test.go

@@ -128,6 +128,9 @@ func (s *testSuite) TestCreateDropDatabase(c *C) {
 	c.Assert(err.Error(), Equals, plan.ErrNoDB.Error())
 	_, err = tk.Exec("select * from t;")
 	c.Assert(err.Error(), Equals, plan.ErrNoDB.Error())
+
+	_, err = tk.Exec("drop database mysql")
+	c.Assert(err, NotNil)
 }
 
 func (s *testSuite) TestCreateDropTable(c *C) {
@@ -137,6 +140,9 @@ func (s *testSuite) TestCreateDropTable(c *C) {
 	tk.MustExec("drop table if exists drop_test")
 	tk.MustExec("create table drop_test (a int)")
 	tk.MustExec("drop table drop_test")
+
+	_, err := tk.Exec("drop table mysql.gc_delete_range")
+	c.Assert(err, NotNil)
 }
 
 func (s *testSuite) TestCreateDropIndex(c *C) {