fix charset error

pkg/proxy/backend/client/conn.go

@@ -7,6 +7,7 @@ import (
 	"strings"
 	"time"
 
+	"github.com/pingcap-incubator/weir/pkg/proxy/constant"
 	"github.com/pingcap/errors"
 	. "github.com/siddontang/go-mysql/mysql"
 	"github.com/siddontang/go-mysql/packet"
@@ -65,8 +66,8 @@ func Connect(addr string, user string, password string, dbName string, options .
 	c.db = dbName
 	c.proto = proto
 
-	//use default charset here, utf-8
-	c.charset = DEFAULT_CHARSET
+	//use default charset here
+	c.charset = constant.DefaultCharset
 
 	// Apply configuration functions.
 	for i := range options {

pkg/proxy/constant/charset.go

@@ -0,0 +1,8 @@
+package constant
+
+import "github.com/pingcap/parser/charset"
+
+const (
+	DefaultCharset = charset.CharsetUTF8MB4
+	DefaultCollationID = charset.CollationUTF8MB4
+)

pkg/proxy/driver/sessionvars.go

@@ -38,6 +38,9 @@ func (s *SessionVarsWrapper) SetSystemVarAST(name string, v *ast.VariableAssignm
 }
 
 func (s *SessionVarsWrapper) CheckSessionSysVarValid(name string) error {
+	if name == ast.SetNames || name == ast.SetCharset {
+		return nil
+	}
 	sysVar := variable.GetSysVar(name)
 	if sysVar == nil {
 		return fmt.Errorf("%s is not a valid sysvar", name)