Re: A problem involving threads and http streams

On 09/14/2013 12:39 PM, Torbj�rn Lager wrote:
> Dear all,
>
> In connection with my work on Pengine I have run into an issue I don't
> understand.
>
> The help you see the problem I have attached a zip archive with three
> small files, two servers and one client:
>
> test_server_buggy.pl
> test_server_working.pl
> test_client.pl
>
> The difference between test_server_working.pl and test_server_buggy.pl
> is very small. In test_server_working.pl I use a message queue with a
> hard coded name 'out'. This works but isn't good enough since my
> application requires that the name of the message queue is unique. In
> test_server_buggy.pl I use http_session_assert/1 and
> http_session_data/1 to remember the unique message queue name as given
> by message_queue_create/1. This doesn't work, since an error
>
> error(existence_error(stream,<stream>(0x7fb2d9201fb0)),context(http_stream:is_cgi_stream/1,_G132))
>
> is thrown. I don't understand why this happens and how to work around it.

Normally, you'd find this using the graphical tracer and Edit/Exceptions
menu.  There you specify that you want to trap the debugger on all
exceptions and then you run the goal.

The problem is with this:

output(Term) :-
     http_session_data(out(Out)),
     thread_send_message(Out, Term).

This thing is executed in a new thread that is not connected to
the session.  It calls http_session_data/1, which uses the CGI
stream to find the session id, but there is no CGI stream for
this thread.  So, you need to start this as

	thread_create(test_goal(Out), _, [])

and then somehow make sure the Out remains accessible in the thread,
either by passing it, putting it in a global variable or putting it
in a thread-local predicate.

	Cheers --- Jan


> I use the latest SWI Prolog developer version on a Mac.
>
> Best regards,
> Torbj�rn
>

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