*: fix the sysvar value may be corrupted after set by subquery

executor/set.go

@@ -284,7 +284,16 @@ func (e *SetExecutor) getVarValue(ctx context.Context, v *expression.VarAssignme
  if err != nil || nativeVal.IsNull() {
  return "", err
  }
- return nativeVal.ToString()
+
+ value, err = nativeVal.ToString()
+ if err != nil {
+ return "", err
+ }
+
+ // We need to clone the string because the value is constructed by `hack.String` in Datum which reuses the under layer `[]byte`
+ // instead of allocating some new spaces. The `[]byte` in Datum will be reused in `chunk.Chunk` by different statements in session.
+ // If we do not clone the value, the system variable will have a risk to be modified by other statements.
+ return strings.Clone(value), nil
 }
 
 func (e *SetExecutor) loadSnapshotInfoSchemaIfNeeded(name string, snapshotTS uint64) error {

server/tidb_test.go

@@ -2950,3 +2950,38 @@ func TestSandBoxMode(t *testing.T) {
  require.NoError(t, err)
  }
 }
+
+func TestIssue40979(t *testing.T) {
+ ts := createTidbTestSuite(t)
+
+ db, err := sql.Open("mysql", ts.getDSN())
+ require.NoError(t, err)
+ defer func() {
+ require.NoError(t, db.Close())
+ }()
+
+ conn, err := db.Conn(context.Background())
+ require.NoError(t, err)
+ defer func() {
+ require.NoError(t, conn.Close())
+ }()
+
+ rs, err := conn.QueryContext(context.Background(), "show tables in test")
+ ts.Rows(t, rs)
+
+ _, err = conn.ExecContext(context.Background(), "set @@time_zone=(select 'Asia/Shanghai')")
+ require.NoError(t, err)
+
+ rs, err = conn.QueryContext(context.Background(), "select TIDB_TABLE_ID from information_schema.tables where TABLE_SCHEMA='aaaa'")
+ ts.Rows(t, rs)
+
+ rs, err = conn.QueryContext(context.Background(), "select @@time_zone")
+ require.NoError(t, err)
+ defer func() {
+ require.NoError(t, rs.Close())
+ }()
+
+ rows := ts.Rows(t, rs)
+ require.Equal(t, 1, len(rows))
+ require.Equal(t, "Asia/Shanghai", rows[0])
+}