session, sessionctx/variable: fix validation recursion bug

session/session.go

@@ -1115,11 +1115,9 @@ func (s *session) GetGlobalSysVar(name string) (string, error) {
 			return sv.Value, nil
 		}
 	}
-	// It might have been written from an earlier TiDB version, so we should run the validation function
-	// To normalize the value to be safe for this version of TiDB. This also happens for session scoped
-	// variables in loadCommonGlobalVariablesIfNeeded -> SetSystemVarWithRelaxedValidation
-	sysVar = sv.ValidateWithRelaxedValidation(s.GetSessionVars(), sysVar, variable.ScopeGlobal)
-	return sysVar, nil
+	// It might have been written from an earlier TiDB version, so we should do type validation
+	// See https://github.com/pingcap/tidb/issues/30255 for why we don't do full validation.
+	return sv.ValidateFromType(s.GetSessionVars(), sysVar, variable.ScopeGlobal)
 }
 
 // SetGlobalSysVar implements GlobalVarAccessor.SetGlobalSysVar interface.

session/session_test.go

@@ -2212,6 +2212,22 @@ func (s *testSchemaSerialSuite) TestLoadSchemaFailed(c *C) {
 	tk2.MustExec("commit")
 }
 
+func (s *testSchemaSerialSuite) TestValidationRecursion(c *C) {
+	// We have to expect that validation functions will call GlobalVarsAccessor.GetGlobalSysVar().
+	// This tests for a regression where GetGlobalSysVar() can not safely call the validation
+	// function because it might cause infinite recursion.
+	// See: https://github.com/pingcap/tidb/issues/30255
+	sv := variable.SysVar{Scope: variable.ScopeGlobal, Name: "mynewsysvar", Value: "test", Validation: func(vars *variable.SessionVars, normalizedValue string, originalValue string, scope variable.ScopeFlag) (string, error) {
+		return vars.GlobalVarsAccessor.GetGlobalSysVar("mynewsysvar")
+	}}
+	variable.RegisterSysVar(&sv)
+
+	tk := testkit.NewTestKitWithInit(c, s.store)
+	val, err := sv.Validate(tk.Se.GetSessionVars(), "test2", variable.ScopeGlobal)
+	c.Assert(err, IsNil)
+	c.Assert(val, Equals, "test")
+}
+
 func (s *testSchemaSerialSuite) TestSchemaCheckerSQL(c *C) {
 	tk := testkit.NewTestKitWithInit(c, s.store)
 	tk1 := testkit.NewTestKitWithInit(c, s.store)

sessionctx/variable/sysvar.go

@@ -265,7 +265,7 @@ func (sv *SysVar) Validate(vars *SessionVars, value string, scope ScopeFlag) (st
 	}
 	// Normalize the value and apply validation based on type.
 	// i.e. TypeBool converts 1/on/ON to ON.
-	normalizedValue, err := sv.validateFromType(vars, value, scope)
+	normalizedValue, err := sv.ValidateFromType(vars, value, scope)
 	if err != nil {
 		return normalizedValue, err
 	}
@@ -276,8 +276,8 @@ func (sv *SysVar) Validate(vars *SessionVars, value string, scope ScopeFlag) (st
 	return normalizedValue, nil
 }
 
-// validateFromType provides automatic validation based on the SysVar's type
-func (sv *SysVar) validateFromType(vars *SessionVars, value string, scope ScopeFlag) (string, error) {
+// ValidateFromType provides automatic validation based on the SysVar's type
+func (sv *SysVar) ValidateFromType(vars *SessionVars, value string, scope ScopeFlag) (string, error) {
 	// The string "DEFAULT" is a special keyword in MySQL, which restores
 	// the compiled sysvar value. In which case we can skip further validation.
 	if strings.EqualFold(value, "DEFAULT") {
@@ -329,9 +329,9 @@ func (sv *SysVar) validateScope(scope ScopeFlag) error {
 func (sv *SysVar) ValidateWithRelaxedValidation(vars *SessionVars, value string, scope ScopeFlag) string {
 	warns := vars.StmtCtx.GetWarnings()
 	defer func() {
-		vars.StmtCtx.SetWarnings(warns) // RelaxedVaidation = trim warnings too.
+		vars.StmtCtx.SetWarnings(warns) // RelaxedValidation = trim warnings too.
 	}()
-	normalizedValue, err := sv.validateFromType(vars, value, scope)
+	normalizedValue, err := sv.ValidateFromType(vars, value, scope)
 	if err != nil {
 		return normalizedValue
 	}