sessionctx/variable: fix panic when set variable='' (pingcap#9533)

Author: crazycs520

expression/integration_test.go

@@ -30,6 +30,7 @@ import (
 	"github.com/pingcap/tidb/plan"
 	"github.com/pingcap/tidb/session"
 	"github.com/pingcap/tidb/sessionctx"
+	"github.com/pingcap/tidb/sessionctx/variable"
 	"github.com/pingcap/tidb/store/mockstore"
 	"github.com/pingcap/tidb/table"
 	"github.com/pingcap/tidb/terror"
@@ -3225,6 +3226,10 @@ func (s *testIntegrationSuite) TestSetVariables(c *C) {
 	_, err = tk.Exec("set global transaction read write;")
 	r = tk.MustQuery(`select @@session.tx_read_only, @@global.tx_read_only, @@session.transaction_read_only, @@global.transaction_read_only;`)
 	r.Check(testkit.Rows("0 0 0 0"))
+
+	_, err = tk.Exec("set @@global.max_user_connections='';")
+	c.Assert(err, NotNil)
+	c.Assert(err.Error(), Equals, variable.ErrWrongTypeForVar.GenByArgs("max_user_connections").Error())
 }
 
 func (s *testIntegrationSuite) TestIssues(c *C) {

sessionctx/variable/varsutil.go

@@ -169,6 +169,9 @@ func ValidateGetSystemVar(name string, isGlobal bool) error {
 }
 
 func checkUInt64SystemVar(name, value string, min, max uint64, vars *SessionVars) (string, error) {
+	if len(value) == 0 {
+		return value, ErrWrongTypeForVar.GenByArgs(name)
+	}
 	if value[0] == '-' {
 		_, err := strconv.ParseInt(value, 10, 64)
 		if err != nil {