Reply

Help!
1

I’m using: SWI-Prolog version: 9.3.24 for x86_64-linux

I am writing a web server in Prolog and I want it to return an image file, but only if the user is authorized; otherwise I want to return an error response “Bad request”.

I have seen that some tutorials suggest to do “throw(http_reply(400))”, but I to don’t want to return an error page, because the application does not expect a page, but a binary file. I want it to return something like this:

HTTP/1.1 400 Bad request\r\n

What is the right way to do this?

This is a simplified version of my code:


:-http_handler(root('get_file'),  get_file, []).

get_file(Request):-
   ( 
       authorized(Request),
       get_file_name(Request,FileName)
   ->
       http_reply_file(FileName, [], Request)
  ;
       format("HTTP/1.1 400 Bad request\r\n\r\n",[])
  ).

This code does not work. I get

…error("goal unexpectedly failed: user:get_file(…

Please help. Thank you.

2

you have to send the http header first of all.
the header contains the information for the response, and the body length
for the message you would like to send.

3

I think that is mostly the problem with the application. It is quite common that a request for a location may return a (text) error document rather than the expected type.

And no, you cannot affect the first line of the reply this way. That is always generated. The files http_exception.pl and http_header.pl provide hooks that first map exceptions to a Prolog term expressing the reply and then into a concrete document. That can be used to formulate arbitrary replies for error and non-200 status pages. I’d stay away from this though and fix the client.

1 Like
4

Could you please explain in more details how to formulate arbitrary replies for error and non-200 status pages?

I see in http_header.pl:

http:map_exception_to_http_status_hook/4. % Exception, Reply, HdrExtra, Ctx

Is this the predicate that I have to define? I could not find a good documentation on how to use this.

5

Hello,

for testing, you can simply create a CGI like Application, that send
a http header.

:- use_module(library(http/http_client)).
:- use_module(library(http/http_open)).

send_http_header(URL, HeaderName, HeaderValue) :-
     setup_call_cleanup(
         http_open(URL, Stream, [request_header(HeaderName = HeaderValue)]),
         read_string(Stream, _, Response),
         close(Stream)
     ),
     format('Response: ~s~n', [Response]).

?- send_http_header('https://localhost.org/headers',
    'X-Custom-Header',
    'your value').

% http_open/3                    opens a HTTP connection
% request_header(Name = Value)   set a HTTP header
% setup_call_cleanup/3           close the stream
% read_string/3                  read server response as string

Happy proller
Jens

7

@paule32, thank you for looking into this, but it seems that either you don’t understand my question, or I don’t understand your answer.

My previous post was related to Jan’s comment: " The files http_exception.pl and http_header.pl provide hooks that first map exceptions to a Prolog term expressing the reply and then into a concrete document. That can be used to formulate arbitrary replies for error and non-200 status pages."

If possible, I would like Jan (or somebody else) to elaborate on this, and provide an example how would I use these hooks to generate a non-200 status reply.

8

You can check out the library(http/http_json), which uses these hooks to provide JSON error messages in case the headers indicate to prefer that.

Not you can emit any content with a non-200 code by sending the Status header first. For example:

format('Status: 404~n'),
format('Content-type: text/plain~n~n'),
format('No such document!~n').