ddl: disallow dropping auto_increment column attribute (#12107)

ddl/db_integration_test.go

@@ -893,7 +893,7 @@ func (s *testIntegrationSuite) TestChangingDBCharset(c *C) {
 	verifyDBCharsetAndCollate("alterdb2", "utf8mb4", "utf8mb4_general_ci")
 }
 
-func (s *testIntegrationSuite) TestDropAutoIncrementIndex(c *C) {
+func (s *testIntegrationSuite) TestDropAutoIncrement(c *C) {
 	tk := testkit.NewTestKit(c, s.store)
 	tk.MustExec("create database if not exists test")
 	tk.MustExec("use test")
@@ -914,4 +914,12 @@ func (s *testIntegrationSuite) TestDropAutoIncrementIndex(c *C) {
 	tk.MustExec("create table t1 (a int(11) not null auto_increment, b int(11), c bigint, unique key (a, b, c))")
 	dropIndexSQL = "alter table t1 drop index a"
 	assertErrMsg(dropIndexSQL)
+
+	tk.MustExec("drop table if exists t1")
+	tk.MustExec("create table t1 (a int auto_increment key)")
+	_, err := tk.Exec("alter table t modify column a int")
+	c.Assert(err, NotNil)
+	tk.MustExec("set @@tidb_allow_remove_auto_inc = on")
+	tk.MustExec("alter table t1 modify column a int")
+	tk.MustExec("set @@tidb_allow_remove_auto_inc = off")
 }

ddl/db_test.go

@@ -1885,7 +1885,9 @@ func (s *testDBSuite) TestAlterColumn(c *C) {
 	result = s.tk.MustQuery("show create table mc")
 	createSQL = result.Rows()[0][1]
 	expected = "CREATE TABLE `mc` (\n  `a` bigint(20) NOT NULL AUTO_INCREMENT,\n  `b` int(11) DEFAULT NULL,\n  PRIMARY KEY (`a`)\n) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_bin"
+	s.mustExec(c, "set @@tidb_allow_remove_auto_inc = on")
 	s.mustExec(c, "alter table mc modify column a bigint") // Drops auto_increment
+	s.mustExec(c, "set @@tidb_allow_remove_auto_inc = off")
 	result = s.tk.MustQuery("show create table mc")
 	createSQL = result.Rows()[0][1]
 	expected = "CREATE TABLE `mc` (\n  `a` bigint(20) NOT NULL,\n  `b` int(11) DEFAULT NULL,\n  PRIMARY KEY (`a`)\n) ENGINE=InnoDB DEFAULT CHARSET=utf8mb4 COLLATE=utf8mb4_bin"

ddl/ddl_api.go

@@ -2091,6 +2091,10 @@ func (d *ddl) getModifiableColumnJob(ctx sessionctx.Context, ident ast.Ident, or
 	if !mysql.HasAutoIncrementFlag(col.Flag) && mysql.HasAutoIncrementFlag(newCol.Flag) {
 		return nil, errUnsupportedModifyColumn.GenWithStackByArgs("set auto_increment")
 	}
+	// Disallow modifying column from auto_increment to not auto_increment if the session variable `AllowRemoveAutoInc` is false.
+	if !ctx.GetSessionVars().AllowRemoveAutoInc && mysql.HasAutoIncrementFlag(col.Flag) && !mysql.HasAutoIncrementFlag(newCol.Flag) {
+		return nil, errUnsupportedModifyColumn.GenWithStackByArgs("to remove auto_increment without @@tidb_allow_remove_auto_inc enabled")
+	}
 
 	// We don't support modifying the type definitions from 'null' to 'not null' now.
 	if !mysql.HasNotNullFlag(col.Flag) && mysql.HasNotNullFlag(newCol.Flag) {

sessionctx/variable/session.go

@@ -336,6 +336,9 @@ type SessionVars struct {
 
 	// ConnectionInfo indicates current connection info used by current session, only be lazy assigned by plugin.
 	ConnectionInfo *ConnectionInfo
+
+	// AllowRemoveAutoInc indicates whether a user can drop the auto_increment column attribute or not.
+	AllowRemoveAutoInc bool
 }
 
 // ConnectionInfo present connection used by audit.
@@ -381,6 +384,7 @@ func NewSessionVars() *SessionVars {
 		WaitSplitRegionFinish:     DefTiDBWaitSplitRegionFinish,
 		WaitSplitRegionTimeout:    DefWaitSplitRegionTimeout,
 		CommandValue:              uint32(mysql.ComSleep),
+		AllowRemoveAutoInc:        DefTiDBAllowRemoveAutoInc,
 	}
 	vars.Concurrency = Concurrency{
 		IndexLookupConcurrency:     DefIndexLookupConcurrency,
@@ -675,6 +679,8 @@ func (s *SessionVars) SetSystemVar(name string, val string) error {
 		s.WaitSplitRegionFinish = TiDBOptOn(val)
 	case TiDBWaitSplitRegionTimeout:
 		s.WaitSplitRegionTimeout = uint64(tidbOptPositiveInt32(val, DefWaitSplitRegionTimeout))
+	case TiDBAllowRemoveAutoInc:
+		s.AllowRemoveAutoInc = TiDBOptOn(val)
 	}
 	s.systems[name] = val
 	return nil

sessionctx/variable/sysvar.go

@@ -680,6 +680,7 @@ var defaultSysVars = []*SysVar{
 	{ScopeGlobal, TiDBScatterRegion, BoolToIntStr(DefTiDBScatterRegion)},
 	{ScopeSession, TiDBWaitSplitRegionFinish, BoolToIntStr(DefTiDBWaitSplitRegionFinish)},
 	{ScopeSession, TiDBWaitSplitRegionTimeout, strconv.Itoa(DefWaitSplitRegionTimeout)},
+	{ScopeSession, TiDBAllowRemoveAutoInc, BoolToIntStr(DefTiDBAllowRemoveAutoInc)},
 }
 
 // SynonymsSysVariables is synonyms of system variables.

sessionctx/variable/tidb_vars.go

@@ -121,6 +121,9 @@ const (
 
 	// TiDBCheckMb4ValueInUTF8 is used to control whether to enable the check wrong utf8 value.
 	TiDBCheckMb4ValueInUTF8 = "tidb_check_mb4_value_in_utf8"
+
+	// TiDBAllowRemoveAutoInc indicates whether a user can drop the auto_increment column attribute or not.
+	TiDBAllowRemoveAutoInc = "tidb_allow_remove_auto_inc"
 )
 
 // TiDB system variable names that both in session and global scope.
@@ -270,6 +273,7 @@ const (
 	DefTiDBScatterRegion             = false
 	DefTiDBWaitSplitRegionFinish     = true
 	DefWaitSplitRegionTimeout        = 300 // 300s
+	DefTiDBAllowRemoveAutoInc        = false
 )
 
 // Process global variables.

sessionctx/variable/varsutil.go

@@ -390,6 +390,14 @@ func ValidateSetSystemVar(vars *SessionVars, name string, value string) (string,
 		if v <= 0 {
 			return value, errors.Errorf("tidb_wait_split_region_timeout(%d) cannot be smaller than 1", v)
 		}
+	case TiDBAllowRemoveAutoInc:
+		switch {
+		case strings.EqualFold(value, "ON") || value == "1":
+			return "on", nil
+		case strings.EqualFold(value, "OFF") || value == "0":
+			return "off", nil
+		}
+		return value, ErrWrongValueForVar.GenWithStackByArgs(name, value)
 	}
 	return value, nil
 }