CODEWITHSUNDEEP
httpprologdcg
Rodrigo Setti
Rodrigo Setti

Reputation: 180

phrase_from_stream/2 nontermination (Stream from http_open/3)

I'm building an application using Scryer Prolog that has a HTTP Client component.

It would be nice to streaming DCG to parse the data but phrase_from_stream/2 does not terminate when using the instantiated stream from http_open/3.

I appreciate debugging tips (how/where it doesn't terminate?).

Steps to reproduce in top-level scryer-prolog:

(Started a simple local HTTP server using python -m http.server 8000).

?- use_module(library(charsio)).
   true.
?- use_module(library(dcgs)).
   true.
?- use_module(library(http/http_open)).
   true.

% reading the entire stream before parsing works as expected:
?- http_open("http://127.0.0.1:8000", Stream, []), get_n_chars(Stream, N, Chars), phrase(seq(Data), Chars).
   Stream = '$stream'(0x600000636908), N = 726, Chars = "<!DOCTYPE HTML>\n<ht ...", Data = "<!DOCTYPE HTML>\n<ht ...".

?- use_module(library(pio)).
   true.

% phrase_from_stream/2 does not terminate in this case:
?- http_open("http://127.0.0.1:8000", Stream, []), phrase_from_stream(seq(Data), Stream).
% nontermination

This seems to happen with other grammar rules as well (I'm originally trying to parse JSON data), but I'm able to reproduce with any rule. I also attempted to unify with content size and set Connection: close header (using options) without success.

Similar code using SWI-Prolog works as expected:

?- use_module(library(http/http_open)).
true.
?- use_module(library(dcg/basics)).
true.
?- use_module(library(pio)).
true.
?- http_open("http://127.0.0.1:8000", Stream, []), phrase_from_stream(string(Data), Stream).
Stream = <stream>(0x600002954400,0x600002974b00),
Data = [60, 33, 68, 79, 67, 84, 89, 80, 69|...] ;
false.

Upvotes: 2

Views: 104

Answers (1)

Adri&#225;n Arroyo Calle
Adri&#225;n Arroyo Calle

Reputation: 719

This was a bug in Scryer Prolog, as the streaming code was incomplete, giving always the response that there were more incoming bytes even it wasn't the case.

There's PR with the fix

Upvotes: 2

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