streampool.pl

/*  Part of SWI-Prolog

    Author:        Jan Wielemaker
    E-mail:        J.Wielemaker@vu.nl
    WWW:           http://www.swi-prolog.org
    Copyright (c)  2002-2018, University of Amsterdam
                              CWI, Amsterdam
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       notice, this list of conditions and the following disclaimer.

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*/

:- module(stream_pool,
          [ add_stream_to_pool/2,       % +Stream, :Goal
            delete_stream_from_pool/1,  % +Stream
            close_stream_pool/0,
            dispatch_stream_pool/1,     % +TimeOut
            stream_pool_main_loop/0
          ]).
:- use_module(library(debug),[debug/3]).

:- meta_predicate
    add_stream_to_pool(+, 0).

:- volatile
    pool/2.                         % sockets don't survive a saved-state
:- dynamic
    pool/2.                         % Stream, Action

/** <module> Input multiplexing

This libary allows a single thread to  monitor multiple streams and call
a goal if input is available on a stream.

@bug Note that if the processing   predicate blocks other input channals
are not processed. This may happen, for example, if a read/2 call blocks
due to incomplete input.
*/

%!   add_stream_to_pool(+Stream :Goal)
%
%    Call Goal whenever there is input on Stream.

add_stream_to_pool(Stream, Action) :-
    strip_module(Action, Module, Plain),
    register_stream(Stream, Module:Plain).

register_stream(Stream, Goal) :-
    assert(pool(Stream, Goal)).

%!  delete_stream_from_pool(+Stream)
%
%   Retract stream from the pool

delete_stream_from_pool(Stream) :-
    retractall(pool(Stream, _)).

%!  close_stream_pool
%
%   Close all streams in the   pool. This causes stream_pool_main_loop/0
%   to terminate.

close_stream_pool :-
    forall(retract(pool(Stream, _)),
           close(Stream, [force(true)])).

%!  dispatch_stream_pool(+TimeOut)
%
%   Wait for input on one or more streams   and handle that. Wait for at
%   most TimeOut seconds (0 means infinite).

dispatch_stream_pool(Timeout) :-
    findall(S, pool(S, _), Pool),
    debug(tcp, 'Select ~p ...', [Pool]),
    catch(wait_for_input(Pool, Ready, Timeout), E, true),
    debug(tcp, '    --> ~p (E=~p)', [Ready, E]),
    (   var(E)
    ->  actions(Ready)
    ;   E = error(existence_error(stream, Stream), _)
    ->  delete_stream_from_pool(Stream)
    ).

actions([]).
actions([H|T]) :-
    action(H),
    actions(T).

action(Stream) :-
    pool(Stream, Action),
    (   catch(Action, E, true)
    ->  (   var(E)
        ->  true
        ;   print_message(error, E)
        )
    ;   print_message(warning,
                      goal_failed(Action, stream_pool))
    ).

%!  stream_pool_main_loop
%
%   Keep handling input from the streams in the pool until they have all
%   died away.

stream_pool_main_loop :-
    pool(_, _),
    !,
    (   current_prolog_flag(windows, true)
    ->  dispatch_stream_pool(1)     % so we can break out easily
    ;   dispatch_stream_pool(0)
    ),
    stream_pool_main_loop.
stream_pool_main_loop.