fix: transaction_isolation to be applied at session level

go/test/endtoend/vtgate/transaction/tx_test.go

@@ -209,6 +209,31 @@ func TestTransactionAccessModes(t *testing.T) {
 	utils.Exec(t, conn, "rollback")
 }
 
+// TestTransactionIsolationInTx tests transaction isolation level inside transaction
+// and setting isolation level to different values.
+func TestTransactionIsolationInTx(t *testing.T) {
+	ctx := context.Background()
+
+	conn, err := mysql.Connect(ctx, &vtParams)
+	require.NoError(t, err)
+	defer conn.Close()
+
+	utils.Exec(t, conn, "set transaction isolation level read committed")
+	utils.Exec(t, conn, "begin")
+	utils.AssertMatches(t, conn, "select @@transaction_isolation", `[[VARCHAR("READ-COMMITTED")]]`)
+	utils.Exec(t, conn, "commit")
+
+	utils.Exec(t, conn, "set transaction isolation level serializable")
+	utils.Exec(t, conn, "begin")
+	utils.AssertMatches(t, conn, "select @@transaction_isolation", `[[VARCHAR("SERIALIZABLE")]]`)
+	utils.Exec(t, conn, "commit")
+
+	utils.Exec(t, conn, "set transaction isolation level read committed")
+	utils.Exec(t, conn, "begin")
+	utils.AssertMatches(t, conn, "select @@transaction_isolation", `[[VARCHAR("READ-COMMITTED")]]`)
+	utils.Exec(t, conn, "commit")
+}
+
 func start(t *testing.T) func() {
 	deleteAll := func() {
 		conn, err := mysql.Connect(context.Background(), &vtParams)

go/vt/vtgate/engine/fake_vcursor_test.go

@@ -355,6 +355,8 @@ type loggingVCursor struct {
 
 	// map different shards to keyspaces in the test.
 	ksShardMap map[string][]string
+
+	shardSession []*srvtopo.ResolvedShard
 }
 
 type tableRoutes struct {
@@ -420,7 +422,7 @@ func (f *loggingVCursor) InReservedConn() bool {
 }
 
 func (f *loggingVCursor) ShardSession() []*srvtopo.ResolvedShard {
-	return nil
+	return f.shardSession
 }
 
 func (f *loggingVCursor) ExecuteVSchema(context.Context, string, *sqlparser.AlterVschema) error {

go/vt/vtgate/engine/set.go

@@ -313,7 +313,7 @@ func (svs *SysVarReservedConn) Execute(ctx context.Context, vcursor VCursor, env
 	queries := make([]*querypb.BoundQuery, len(rss))
 	for i := 0; i < len(rss); i++ {
 		queries[i] = &querypb.BoundQuery{
-			Sql:           fmt.Sprintf("set @@%s = %s", svs.Name, svs.Expr),
+			Sql:           fmt.Sprintf("set %s = %s", svs.Name, svs.Expr),
 			BindVariables: env.BindVars,
 		}
 	}

go/vt/vtgate/engine/set_test.go

@@ -23,6 +23,7 @@ import (
 	"testing"
 
 	"vitess.io/vitess/go/vt/sqlparser"
+	"vitess.io/vitess/go/vt/srvtopo"
 
 	"github.com/stretchr/testify/require"
 
@@ -59,6 +60,7 @@ func TestSetSystemVariableAsString(t *testing.T) {
 			),
 			"foobar",
 		)},
+		shardSession: []*srvtopo.ResolvedShard{{Target: &querypb.Target{Keyspace: "ks", Shard: "-20"}}},
 	}
 	_, err := set.TryExecute(context.Background(), vc, map[string]*querypb.BindVariable{}, false)
 	require.NoError(t, err)
@@ -68,6 +70,7 @@ func TestSetSystemVariableAsString(t *testing.T) {
 		"ExecuteMultiShard ks.-20: select dummy_expr from dual where @@x != dummy_expr {} false false",
 		"SysVar set with (x,'foobar')",
 		"Needs Reserved Conn",
+		"ExecuteMultiShard ks.-20: set x = dummy_expr {} false false",
 	})
 }