sysvar: add INSTANCE scope for system variable

errno/errcode.go

@@ -1016,6 +1016,7 @@ const (
 	ErrInconsistentHandle                  = 8139
 	ErrInconsistentIndexedValue            = 8140
 	ErrAssertionFailed                     = 8141
+	ErrInstanceScope                       = 8142
 
 	// Error codes used by TiDB ddl package
 	ErrUnsupportedDDLOperation            = 8200

errno/errname.go

@@ -1014,6 +1014,7 @@ var MySQLErrName = map[uint16]*mysql.ErrMessage{
 	ErrInconsistentHandle:            mysql.Message("writing inconsistent data in table: %s, index: %s, index-handle:%#v != record-handle:%#v, index: %#v, record: %#v", []int{2, 3, 4, 5}),
 	ErrInconsistentIndexedValue:      mysql.Message("writing inconsistent data in table: %s, index: %s, col: %s, indexed-value:{%s} != record-value:{%s}", []int{3, 4}),
 	ErrAssertionFailed:               mysql.Message("assertion failed: key: %s, assertion: %s, start_ts: %v, existing start ts: %v, existing commit ts: %v", []int{0}),
+	ErrInstanceScope:                 mysql.Message("allowing variable %s with INSTANCE scope change to SESSION scope while setting for now, but its deprecated", nil),
 
 	ErrWarnOptimizerHintInvalidInteger:  mysql.Message("integer value is out of range in '%s'", nil),
 	ErrWarnOptimizerHintUnsupportedHint: mysql.Message("Optimizer hint %s is not supported by TiDB and is ignored", nil),

executor/errors.go

@@ -53,6 +53,7 @@ var (
 	ErrIllegalPrivilegeLevel         = dbterror.ClassExecutor.NewStd(mysql.ErrIllegalPrivilegeLevel)
 	ErrInvalidSplitRegionRanges      = dbterror.ClassExecutor.NewStd(mysql.ErrInvalidSplitRegionRanges)
 	ErrViewInvalid                   = dbterror.ClassExecutor.NewStd(mysql.ErrViewInvalid)
+	ErrInstanceScope                 = dbterror.ClassExecutor.NewStd(mysql.ErrInstanceScope)
 
 	ErrBRIEBackupFailed      = dbterror.ClassExecutor.NewStd(mysql.ErrBRIEBackupFailed)
 	ErrBRIERestoreFailed     = dbterror.ClassExecutor.NewStd(mysql.ErrBRIERestoreFailed)

executor/set.go

@@ -115,6 +115,14 @@ func (e *SetExecutor) setSysVariable(ctx context.Context, name string, v *expres
 		}
 		return variable.ErrUnknownSystemVar.GenWithStackByArgs(name)
 	}
+
+	if sysVar.HasInstanceScope() && !v.IsGlobal && sessionVars.EnableLegacyInstanceScope {
+		// For backward compatibility we will change the v.IsGlobal to true,
+		// and append a warning saying this will not be supported in future.
+		v.IsGlobal = true
+		sessionVars.StmtCtx.AppendWarning(ErrInstanceScope.GenWithStackByArgs(sysVar.Name))
+	}
+
 	if v.IsGlobal {
 		valStr, err := e.getVarValue(v, sysVar)
 		if err != nil {

executor/set_test.go

@@ -416,19 +416,19 @@ func TestSetVar(t *testing.T) {
 	tk.MustQuery(`select @@global.tidb_enable_parallel_apply`).Check(testkit.Rows("0"))
 	tk.MustQuery(`select @@tidb_enable_parallel_apply`).Check(testkit.Rows("1"))
 
-	tk.MustQuery(`select @@session.tidb_general_log;`).Check(testkit.Rows("0"))
+	tk.MustQuery(`select @@global.tidb_general_log;`).Check(testkit.Rows("0"))
 	tk.MustQuery(`show variables like 'tidb_general_log';`).Check(testkit.Rows("tidb_general_log OFF"))
 	tk.MustExec("set tidb_general_log = 1")
-	tk.MustQuery(`select @@session.tidb_general_log;`).Check(testkit.Rows("1"))
+	tk.MustQuery(`select @@global.tidb_general_log;`).Check(testkit.Rows("1"))
 	tk.MustQuery(`show variables like 'tidb_general_log';`).Check(testkit.Rows("tidb_general_log ON"))
 	tk.MustExec("set tidb_general_log = 0")
-	tk.MustQuery(`select @@session.tidb_general_log;`).Check(testkit.Rows("0"))
+	tk.MustQuery(`select @@global.tidb_general_log;`).Check(testkit.Rows("0"))
 	tk.MustQuery(`show variables like 'tidb_general_log';`).Check(testkit.Rows("tidb_general_log OFF"))
 	tk.MustExec("set tidb_general_log = on")
-	tk.MustQuery(`select @@session.tidb_general_log;`).Check(testkit.Rows("1"))
+	tk.MustQuery(`select @@global.tidb_general_log;`).Check(testkit.Rows("1"))
 	tk.MustQuery(`show variables like 'tidb_general_log';`).Check(testkit.Rows("tidb_general_log ON"))
 	tk.MustExec("set tidb_general_log = off")
-	tk.MustQuery(`select @@session.tidb_general_log;`).Check(testkit.Rows("0"))
+	tk.MustQuery(`select @@global.tidb_general_log;`).Check(testkit.Rows("0"))
 	tk.MustQuery(`show variables like 'tidb_general_log';`).Check(testkit.Rows("tidb_general_log OFF"))
 	require.Error(t, tk.ExecToErr("set tidb_general_log = abc"))
 	require.Error(t, tk.ExecToErr("set tidb_general_log = 123"))
@@ -794,6 +794,7 @@ func TestValidateSetVar(t *testing.T) {
 	tk.MustQuery("select @@tidb_constraint_check_in_place;").Check(testkit.Rows("1"))
 
 	tk.MustExec("set @@tidb_general_log=0;")
+	tk.MustQuery(`show warnings`).Check(testkit.Rows(fmt.Sprintf("Warning %d allowing variable tidb_general_log with INSTANCE scope change to SESSION scope while setting for now, but its deprecated", errno.ErrInstanceScope)))
 	tk.MustQuery("select @@tidb_general_log;").Check(testkit.Rows("0"))
 
 	tk.MustExec("set @@tidb_pprof_sql_cpu=1;")
@@ -1568,3 +1569,20 @@ func TestSetTopSQLVariables(t *testing.T) {
 	tk.MustQuery("show variables like '%top_sql%'").Check(testkit.Rows("tidb_enable_top_sql OFF", "tidb_top_sql_max_meta_count 5000", "tidb_top_sql_max_time_series_count 20"))
 	tk.MustQuery("show global variables like '%top_sql%'").Check(testkit.Rows("tidb_enable_top_sql OFF", "tidb_top_sql_max_meta_count 5000", "tidb_top_sql_max_time_series_count 20"))
 }
+
+func TestInstanceScopeSwitching(t *testing.T) {
+
+	store, clean := testkit.CreateMockStore(t)
+	defer clean()
+	tk := testkit.NewTestKit(t, store)
+	tk.MustExec("use test")
+
+	// enable 'switching' to SESSION variables
+	tk.MustExec("set tidb_enable_legacy_instance_scope = 1")
+	tk.MustExec("set tidb_general_log = 1")
+	tk.MustQuery(`show warnings`).Check(testkit.Rows(fmt.Sprintf("Warning %d allowing variable tidb_general_log with INSTANCE scope change to SESSION scope while setting for now, but its deprecated", errno.ErrInstanceScope)))
+
+	// disable 'switching' to SESSION variables
+	tk.MustExec("set tidb_enable_legacy_instance_scope = 0")
+	tk.MustGetErrCode("set tidb_general_log = 1", errno.ErrGlobalVariable)
+}

planner/core/expression_rewriter.go

@@ -1292,7 +1292,7 @@ func (er *expressionRewriter) rewriteVariable(v *ast.VariableExpr) {
 		er.b.visitInfo = appendDynamicVisitInfo(er.b.visitInfo, "RESTRICTED_VARIABLES_ADMIN", false, err)
 	}
 	if v.ExplicitScope && !sysVar.HasNoneScope() {
-		if v.IsGlobal && !sysVar.HasGlobalScope() {
+		if v.IsGlobal && !(sysVar.HasGlobalScope() || sysVar.HasInstanceScope()) {
 			er.err = variable.ErrIncorrectScope.GenWithStackByArgs(name, "SESSION")
 			return
 		}

session/session.go

@@ -1321,6 +1321,7 @@ func (s *session) GetGlobalSysVar(name string) (string, error) {
 }
 
 // SetGlobalSysVar implements GlobalVarAccessor.SetGlobalSysVar interface.
+// it is called (but skipped) when setting instance scope
 func (s *session) SetGlobalSysVar(name, value string) (err error) {
 	sv := variable.GetSysVar(name)
 	if sv == nil {
@@ -1332,6 +1333,9 @@ func (s *session) SetGlobalSysVar(name, value string) (err error) {
 	if err = sv.SetGlobalFromHook(s.sessionVars, value, false); err != nil {
 		return err
 	}
+	if sv.HasInstanceScope() { // skip for INSTANCE scope
+		return nil
+	}
 	if sv.GlobalConfigName != "" {
 		domain.GetDomain(s).NotifyGlobalConfigChange(sv.GlobalConfigName, variable.OnOffToTrueFalse(value))
 	}
@@ -1348,6 +1352,9 @@ func (s *session) SetGlobalSysVarOnly(name, value string) (err error) {
 	if err = sv.SetGlobalFromHook(s.sessionVars, value, true); err != nil {
 		return err
 	}
+	if sv.HasInstanceScope() { // skip for INSTANCE scope
+		return nil
+	}
 	return s.replaceGlobalVariablesTableValue(context.TODO(), sv.Name, value)
 }
 

session/session_test.go

@@ -731,6 +731,41 @@ func (s *testSessionSuite) TestGlobalVarAccessor(c *C) {
 	c.Assert(v, Equals, "OFF")
 }
 
+func (s *testSessionSuite) TestSetInstanceSysvarBySetGlobalSysVar(c *C) {
+
+	varName := "tidb_general_log"
+	defaultValue := "OFF" // This is the default value for tidb_general_log
+
+	tk := testkit.NewTestKitWithInit(c, s.store)
+	se := tk.Se.(variable.GlobalVarAccessor)
+
+	// Get globalSysVar twice and get the same default value
+	v, err := se.GetGlobalSysVar(varName)
+	c.Assert(err, IsNil)
+	c.Assert(v, Equals, defaultValue)
+	v, err = se.GetGlobalSysVar(varName)
+	c.Assert(err, IsNil)
+	c.Assert(v, Equals, defaultValue)
+
+	// session.GetGlobalSysVar would not get the value which session.SetGlobalSysVar writes,
+	// because SetGlobalSysVar calls SetGlobalFromHook, which uses TiDBGeneralLog's SetGlobal,
+	// but GetGlobalSysVar could not access TiDBGeneralLog's GetGlobal.
+
+	// set to "1"
+	err = se.SetGlobalSysVar(varName, "ON")
+	v, err = se.GetGlobalSysVar(varName)
+	tk.MustQuery("select @@global.tidb_general_log").Check(testkit.Rows("1"))
+	c.Assert(err, IsNil)
+	c.Assert(v, Equals, defaultValue)
+
+	// set back to "0"
+	err = se.SetGlobalSysVar(varName, defaultValue)
+	v, err = se.GetGlobalSysVar(varName)
+	tk.MustQuery("select @@global.tidb_general_log").Check(testkit.Rows("0"))
+	c.Assert(err, IsNil)
+	c.Assert(v, Equals, defaultValue)
+}
+
 func (s *testSessionSuite) TestMatchIdentity(c *C) {
 	tk := testkit.NewTestKitWithInit(c, s.store)
 	tk.MustExec("CREATE USER `useridentity`@`%`")
@@ -6058,3 +6093,18 @@ func (s *testSessionSuite) TestSysdateIsNow(c *C) {
 	tk.MustQuery("show variables like '%sysdate_is_now%'").Check(testkit.Rows("sysdate_is_now ON"))
 	c.Assert(tk.Se.GetSessionVars().SysdateIsNow, IsTrue)
 }
+
+func (s *testSessionSuite) TestEnableLegacyInstanceScope(c *C) {
+	tk := testkit.NewTestKit(c, s.store)
+
+	// enable 'switching' to SESSION variables
+	tk.MustExec("set tidb_enable_legacy_instance_scope = 1")
+	tk.MustExec("set tidb_general_log = 1")
+	tk.MustQuery(`show warnings`).Check(testkit.Rows(fmt.Sprintf("Warning %d allowing variable tidb_general_log with INSTANCE scope change to SESSION scope while setting for now, but its deprecated", errno.ErrInstanceScope)))
+	c.Assert(tk.Se.GetSessionVars().EnableLegacyInstanceScope, IsTrue)
+
+	// disable 'switching' to SESSION variables
+	tk.MustExec("set tidb_enable_legacy_instance_scope = 0")
+	tk.MustGetErrCode("set tidb_general_log = 1", errno.ErrGlobalVariable)
+	c.Assert(tk.Se.GetSessionVars().EnableLegacyInstanceScope, IsFalse)
+}

sessionctx/variable/session.go

@@ -999,6 +999,10 @@ type SessionVars struct {
 	// EnablePaging indicates whether enable paging in coprocessor requests.
 	EnablePaging bool
 
+	// EnableLegacyInstanceScope says if SET SESSION can be used to set an instance
+	// scope variable. The default is TRUE.
+	EnableLegacyInstanceScope bool
+
 	// ReadConsistency indicates the read consistency requirement.
 	ReadConsistency ReadConsistencyLevel
 
@@ -1246,6 +1250,7 @@ func NewSessionVars() *SessionVars {
 		MPPStoreFailTTL:             DefTiDBMPPStoreFailTTL,
 		Rng:                         utilMath.NewWithTime(),
 		StatsLoadSyncWait:           StatsLoadSyncWait.Load(),
+		EnableLegacyInstanceScope:   DefEnableLegacyInstanceScope,
 	}
 	vars.KVVars = tikvstore.NewVariables(&vars.Killed)
 	vars.Concurrency = Concurrency{

sessionctx/variable/sysvar.go

@@ -769,10 +769,10 @@ var defaultSysVars = []*SysVar{
 		return nil
 	}},
 	/* The following variable is defined as session scope but is actually server scope. */
-	{Scope: ScopeSession, Name: TiDBGeneralLog, Value: BoolToOnOff(DefTiDBGeneralLog), Type: TypeBool, skipInit: true, SetSession: func(s *SessionVars, val string) error {
+	{Scope: ScopeInstance, Name: TiDBGeneralLog, Value: BoolToOnOff(DefTiDBGeneralLog), Type: TypeBool, skipInit: true, SetGlobal: func(s *SessionVars, val string) error {
 		ProcessGeneralLog.Store(TiDBOptOn(val))
 		return nil
-	}, GetSession: func(s *SessionVars) (string, error) {
+	}, GetGlobal: func(s *SessionVars) (string, error) {
 		return BoolToOnOff(ProcessGeneralLog.Load()), nil
 	}},
 	{Scope: ScopeSession, Name: TiDBLogFileMaxDays, Value: strconv.Itoa(config.GetGlobalConfig().Log.File.MaxDays), Type: TypeInt, MinValue: 0, MaxValue: math.MaxInt32, skipInit: true, SetSession: func(s *SessionVars, val string) error {
@@ -1327,6 +1327,10 @@ var defaultSysVars = []*SysVar{
 		s.EnablePaging = TiDBOptOn(val)
 		return nil
 	}},
+	{Scope: ScopeGlobal | ScopeSession, Name: TiDBEnableLegacyInstanceScope, Value: BoolToOnOff(DefEnableLegacyInstanceScope), Type: TypeBool, SetSession: func(s *SessionVars, val string) error {
+		s.EnableLegacyInstanceScope = TiDBOptOn(val)
+		return nil
+	}},
 	{Scope: ScopeGlobal, Name: TiDBPersistAnalyzeOptions, Value: BoolToOnOff(DefTiDBPersistAnalyzeOptions), skipInit: true, Type: TypeBool,
 		GetGlobal: func(s *SessionVars) (string, error) {
 			return BoolToOnOff(PersistAnalyzeOptions.Load()), nil

sessionctx/variable/sysvar_test.go

@@ -246,22 +246,32 @@ func TestScope(t *testing.T) {
 	sv := SysVar{Scope: ScopeGlobal | ScopeSession, Name: "mynewsysvar", Value: On, Type: TypeEnum, PossibleValues: []string{"OFF", "ON", "AUTO"}}
 	require.True(t, sv.HasSessionScope())
 	require.True(t, sv.HasGlobalScope())
+	require.False(t, sv.HasInstanceScope())
 	require.False(t, sv.HasNoneScope())
 
 	sv = SysVar{Scope: ScopeGlobal, Name: "mynewsysvar", Value: On, Type: TypeEnum, PossibleValues: []string{"OFF", "ON", "AUTO"}}
 	require.False(t, sv.HasSessionScope())
 	require.True(t, sv.HasGlobalScope())
+	require.False(t, sv.HasInstanceScope())
 	require.False(t, sv.HasNoneScope())
 
 	sv = SysVar{Scope: ScopeSession, Name: "mynewsysvar", Value: On, Type: TypeEnum, PossibleValues: []string{"OFF", "ON", "AUTO"}}
 	require.True(t, sv.HasSessionScope())
 	require.False(t, sv.HasGlobalScope())
+	require.False(t, sv.HasInstanceScope())
 	require.False(t, sv.HasNoneScope())
 
 	sv = SysVar{Scope: ScopeNone, Name: "mynewsysvar", Value: On, Type: TypeEnum, PossibleValues: []string{"OFF", "ON", "AUTO"}}
 	require.False(t, sv.HasSessionScope())
 	require.False(t, sv.HasGlobalScope())
+	require.False(t, sv.HasInstanceScope())
 	require.True(t, sv.HasNoneScope())
+
+	sv = SysVar{Scope: ScopeInstance, Name: "mynewsysvar", Value: On, Type: TypeEnum, PossibleValues: []string{"OFF", "ON", "AUTO"}}
+	require.False(t, sv.HasSessionScope())
+	require.False(t, sv.HasGlobalScope())
+	require.True(t, sv.HasInstanceScope())
+	require.False(t, sv.HasNoneScope())
 }
 
 func TestBuiltInCase(t *testing.T) {
@@ -696,7 +706,7 @@ func TestSettersandGetters(t *testing.T) {
 			require.Nil(t, sv.SetSession)
 			require.Nil(t, sv.GetSession)
 		}
-		if !sv.HasGlobalScope() {
+		if !sv.HasGlobalScope() && !sv.HasInstanceScope() {
 			require.Nil(t, sv.SetGlobal)
 			if sv.Name == Timestamp {
 				// The Timestamp sysvar will have GetGlobal func even though it does not have global scope.
@@ -845,6 +855,59 @@ func TestDefaultCharsetAndCollation(t *testing.T) {
 	require.Equal(t, val, mysql.DefaultCollationName)
 }
 
+func TestInstanceScope(t *testing.T) {
+
+	// Instance scope used to be settable via "SET SESSION", which is weird to any MySQL user.
+	// It is now settable via SET GLOBAL, but to work correctly a sysvar can only ever
+	// be INSTANCE scoped or GLOBAL scoped, never *both* at the same time (at least for now).
+	// Otherwise the semantics are confusing to users for how precedence applies.
+
+	for _, sv := range GetSysVars() {
+		require.False(t, sv.HasGlobalScope() && sv.HasInstanceScope(), "sysvar %s has both instance and global scope", sv.Name)
+		if sv.HasInstanceScope() {
+			require.NotNil(t, sv.GetGlobal)
+			require.NotNil(t, sv.SetGlobal)
+		}
+	}
+
+	count := len(GetSysVars())
+	sv := SysVar{Scope: ScopeInstance, Name: "newinstancesysvar", Value: On, Type: TypeBool,
+		SetGlobal: func(s *SessionVars, val string) error {
+			return fmt.Errorf("set should fail")
+		},
+		GetGlobal: func(s *SessionVars) (string, error) {
+			return "", fmt.Errorf("get should fail")
+		},
+	}
+
+	RegisterSysVar(&sv)
+	require.Len(t, GetSysVars(), count+1)
+
+	sysVar := GetSysVar("newinstancesysvar")
+	require.NotNil(t, sysVar)
+
+	vars := NewSessionVars()
+
+	// It is a boolean, try to set it to a bogus value
+	_, err := sysVar.Validate(vars, "ABCD", ScopeInstance)
+	require.Error(t, err)
+
+	// Boolean oN or 1 converts to canonical ON or OFF
+	normalizedVal, err := sysVar.Validate(vars, "oN", ScopeInstance)
+	require.Equal(t, "ON", normalizedVal)
+	require.NoError(t, err)
+	normalizedVal, err = sysVar.Validate(vars, "0", ScopeInstance)
+	require.Equal(t, "OFF", normalizedVal)
+	require.NoError(t, err)
+
+	err = sysVar.SetGlobalFromHook(vars, "OFF", true) // default is on
+	require.Equal(t, "set should fail", err.Error())
+
+	// Test unregistration restores previous count
+	UnregisterSysVar("newinstancesysvar")
+	require.Equal(t, len(GetSysVars()), count)
+}
+
 func TestIndexMergeSwitcher(t *testing.T) {
 	vars := NewSessionVars()
 	vars.GlobalVarsAccessor = NewMockGlobalAccessor4Tests()

sessionctx/variable/tidb_vars.go

@@ -615,6 +615,9 @@ const (
 	// TiDBTmpTableMaxSize indicates the max memory size of temporary tables.
 	TiDBTmpTableMaxSize = "tidb_tmp_table_max_size"
 
+	// TiDBEnableLegacyInstanceScope indicates if instance scope can be set with SET SESSION
+	TiDBEnableLegacyInstanceScope = "tidb_enable_legacy_instance_scope"
+
 	// TiDBTableCacheLease indicates the read lock lease of a cached table.
 	TiDBTableCacheLease = "tidb_table_cache_lease"
 
@@ -819,6 +822,7 @@ const (
 	DefTiDBStmtSummaryMaxSQLLength        = 4096
 	DefTiDBCapturePlanBaseline            = Off
 	DefTiDBEnableIndexMerge               = true
+	DefEnableLegacyInstanceScope          = true
 	DefTiDBTableCacheLease                = 3 // 3s
 	DefTiDBPersistAnalyzeOptions          = true
 	DefTiDBEnableColumnTracking           = false

sessionctx/variable/variable.go

@@ -38,6 +38,8 @@ const (
 	ScopeGlobal ScopeFlag = 1 << 0
 	// ScopeSession means the system variable can only be changed in current session.
 	ScopeSession ScopeFlag = 1 << 1
+	// ScopeInstance means it is similar to global but doesn't propagate to other TiDB servers.
+	ScopeInstance ScopeFlag = 1 << 2
 
 	// TypeStr is the default
 	TypeStr TypeFlag = 0
@@ -248,6 +250,11 @@ func (sv *SysVar) HasGlobalScope() bool {
 	return sv.Scope&ScopeGlobal != 0
 }
 
+// HasInstanceScope returns true if the scope for the sysVar includes instance
+func (sv *SysVar) HasInstanceScope() bool {
+	return sv.Scope&ScopeInstance != 0
+}
+
 // Validate checks if system variable satisfies specific restriction.
 func (sv *SysVar) Validate(vars *SessionVars, value string, scope ScopeFlag) (string, error) {
 	// Check that the scope is correct first.
@@ -303,7 +310,7 @@ func (sv *SysVar) validateScope(scope ScopeFlag) error {
 	if sv.ReadOnly || sv.Scope == ScopeNone {
 		return ErrIncorrectScope.FastGenByArgs(sv.Name, "read only")
 	}
-	if scope == ScopeGlobal && !sv.HasGlobalScope() {
+	if scope == ScopeGlobal && !(sv.HasGlobalScope() || sv.HasInstanceScope()) {
 		return errLocalVariable.FastGenByArgs(sv.Name)
 	}
 	if scope == ScopeSession && !sv.HasSessionScope() {