pengines_io does not support flush_output/0

Let server.pl consist of the following code:

:- module(server, []).

:- use_module(library(pengines)).
:- use_module(library(http/thread_httpd)).
:- use_module(library(http/http_dispatch)).
:- use_module(library(http/http_server_files)).
:- use_module(library(http/http_files)).
:- use_module(library(http/http_unix_daemon)).

:- pengine_application(myapp).
:- use_module(myapp:library(pengines_io)).
pengines:prepare_module(Module, myapp, _Options) :-
        pengines_io:pengine_bind_io_to_html(Module).

myapp:pred :-
        format("pred\n", []).

:- http_handler(/, http_reply_from_files(., []), [prefix]).

Let demo.html consist of:

<!DOCTYPE html>
<html lang="en">
  <head>
    <title>Demo</title>
    <script src="js/pengines.js"></script>
  </head>
  <body style="padding-left: 5%; padding-right: 5%">
    <br><br>

    <script>
    var p = new Pengine({
        oncreate: function () { p.ask('pred'); },
        onoutput: function () { console.log(this.data); },
        application: "myapp",
        server: "pengine",
    });

    </script>
  </body>
</html>

Start the server with:

$ swipl server.pl --port=5055 --interactive

and direct your browser to http://localhost:5055/demo.html.

If everything works, you see in the JavaScript console:

<pre class="console">
pred
</pre>

This means that pengines_io has done its job, because the output is decorated.

In contrast, if we change the definition of myapp:pred/0 to:

myapp:pred :-
        format("pred\n", []),
        flush_output.

Then we get:

No permission to call sandboxed `flush_output'
Reachable from:
      '93592351277906267204527080385574986579':pred

From the source of pengines_io, it seems that support for flush_output is intended. However, it seems as if it does not work.

SWI-Prolog / packages-pengines

pengines_io does not support flush_output/0 #37