1/*  Part of the SWI-Prolog HTTP package
    2
    3    Author:        Jan Wielemaker
    4    E-mail:        J.Wielemaker@vu.nl
    5    WWW:           http://www.swi-prolog.org
    6    Copyright (c)  2012-2014, University of Amsterdam,
    7                              VU University Amsterdam
    8    All rights reserved.
    9
   10    Redistribution and use in source and binary forms, with or without
   11    modification, are permitted provided that the following conditions
   12    are met:
   13
   14    1. Redistributions of source code must retain the above copyright
   15       notice, this list of conditions and the following disclaimer.
   16
   17    2. Redistributions in binary form must reproduce the above copyright
   18       notice, this list of conditions and the following disclaimer in
   19       the documentation and/or other materials provided with the
   20       distribution.
   21
   22    THIS SOFTWARE IS PROVIDED BY THE COPYRIGHT HOLDERS AND CONTRIBUTORS
   23    "AS IS" AND ANY EXPRESS OR IMPLIED WARRANTIES, INCLUDING, BUT NOT
   24    LIMITED TO, THE IMPLIED WARRANTIES OF MERCHANTABILITY AND FITNESS
   25    FOR A PARTICULAR PURPOSE ARE DISCLAIMED. IN NO EVENT SHALL THE
   26    COPYRIGHT OWNER OR CONTRIBUTORS BE LIABLE FOR ANY DIRECT, INDIRECT,
   27    INCIDENTAL, SPECIAL, EXEMPLARY, OR CONSEQUENTIAL DAMAGES (INCLUDING,
   28    BUT NOT LIMITED TO, PROCUREMENT OF SUBSTITUTE GOODS OR SERVICES;
   29    LOSS OF USE, DATA, OR PROFITS; OR BUSINESS INTERRUPTION) HOWEVER
   30    CAUSED AND ON ANY THEORY OF LIABILITY, WHETHER IN CONTRACT, STRICT
   31    LIABILITY, OR TORT (INCLUDING NEGLIGENCE OR OTHERWISE) ARISING IN
   32    ANY WAY OUT OF THE USE OF THIS SOFTWARE, EVEN IF ADVISED OF THE
   33    POSSIBILITY OF SUCH DAMAGE.
   34*/
   35
   36:- module(http_cors,
   37          [ cors_enable/0,
   38            cors_enable/2                       % +Requesy, +Options
   39          ]).   40:- use_module(library(settings)).   41
   42:- setting(http:cors, list(atom), [],
   43           'Enable CORS for the listed domains.  Use [*] for all domains').

my_handler(Request) :-
      ....,
      cors_enable,
      reply_json(Response, []).

my_handler(Request) :-
      option(method(options), Request), !,
      cors_enable(Request,
                  [ methods([get,post,delete])
                  ]),
      format('~n').                           % 200 with empty body

 cors_enable is det

Emit the HTTP header Access-Control-Allow-Origin using domains from the setting http:cors. This this setting is [] (default), nothing is written. This predicate is typically used for replying to API HTTP-request (e.g., replies to an AJAX request that typically serve JSON or XML).

   97cors_enable :-
   98    cors_enable_domain,
   99    !.
  100cors_enable.                            % CORS not enabled
  101
  102cors_enable_domain :-
  103    setting(http:cors, List),
  104    List \== [],
  105    !,
  106    format('Access-Control-Allow-Origin: ', []),
  107    write_domains(List),
  108    nl.
  109
  110write_domains([]).
  111write_domains([H|T]) :-
  112    write(H),
  113    (   T == []
  114    ->  true
  115    ;   write(' '),
  116        write_domains(T)
  117    ).

 cors_enable(+Request, +Options) is det

CORS reply to a Preflight OPTIONS request. Request is the HTTP request. Options provides:

methods(+List)

List of supported HTTP methods. The default is GET, only allowing for read requests.

headers(+List)

List of headers the client asks for and we allow. The default is to simply echo what has been requested for.

Both methods and headers may use Prolog friendly syntax, e.g., get for a method and content_type for a header.

See also

- http://www.html5rocks.com/en/tutorials/cors/

  136cors_enable(Request, Options) :-
  137    cors_enable_domain,
  138    !,
  139    option(methods(Methods), Options, [get]),
  140    cors_methods(Methods),
  141    (   option(headers(ReqHeaders), Options)
  142    ->  cors_request_headers(ReqHeaders)
  143    ;   option(access_control_request_headers(ReqHeader), Request)
  144    ->  format('Access-Control-Allow-Headers: ~w~n', [ReqHeader])
  145    ;   true
  146    ).
  147cors_enable(_, _).
  148
  149cors_methods([]) :- !.
  150cors_methods(Methods) :-
  151    format('Access-Control-Allow-Methods: '),
  152    write_methods(Methods),
  153    nl.
  154
  155write_methods([H|T]) :-
  156    upcase_atom(H, U),
  157    write(U),
  158    (   T == []
  159    ->  true
  160    ;   write(', '),
  161        write_methods(T)
  162    ).
  163
  164cors_request_headers([]) :- !.
  165cors_request_headers(ReqHeaders) :-
  166    phrase(field_names(ReqHeaders), String),
  167    format('Access-Control-Allow-Headers: ~s', String).
  168
  169
  170field_names([H|T]) -->
  171    http_header:field_name(H),
  172    (   {T==[]}
  173    ->  ""
  174    ;   ", ",
  175        field_names(T)
  176    )