stream: clone stream before execution

internal/expr/functions/scalar_definition.go

@@ -56,6 +56,18 @@ type ScalarFunction struct {
 	params []expr.Expr
 }
 
+func (sf *ScalarFunction) Clone() expr.Expr {
+	exprs := make([]expr.Expr, 0, len(sf.params))
+	for _, e := range sf.params {
+		exprs = append(exprs, expr.Clone(e))
+	}
+
+	return &ScalarFunction{
+		def:    sf.def,
+		params: exprs,
+	}
+}
+
 // Eval returns a row.Value based on the given environment and the underlying function
 // definition.
 func (sf *ScalarFunction) Eval(env *environment.Environment) (types.Value, error) {