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Incorrect example of Net::HTTP::NB usage [rt.cpan.org #107770]

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Assessment

Difficulty
3/5
Estimated time
1-2 days
Newbie friendliness
35/100
Issue type
Documentation
Clarity
Mostly clear
Activity status
Stale
Tech stack
perl
Domain
documentation

Research direction

Start with the Net::HTTP::NB SYNOPSIS example described in the issue and reproduce it against the provided keep-alive test server. Review how read_response_headers and read_entity_body interact with the internal buffer, then update the documentation example so buffered response data is handled correctly. Confirm the revised example receives the body without timing out.

Written by the indexing model from the issue text.

Description

Migrated from rt.cpan.org#107770 (status was 'new')

Requestors:

From [email protected] on 2015-10-14 10:15:16:

In SYNOPSIS section we can see an example of how you can send request and read response with Net::HTTP::NB. But this example has potential problems. Let's see this test server:

use strict;
use IO::Socket;

my $serv = IO::Socket::INET->new(Listen => 10, LocalPort => 8080)
	or die $@;

my $body = 'This is the body';

my $header = join(
	"\r\n",
	"HTTP/1.1 200 OK",
	"Server: nginx/1.0.4",
	"Date: Thu, 06 Oct 2011 16:14:01 GMT",
	"Content-Type: text/html",
	"Content-Length: ".length($body),
	"Connection: keep-alive",
	"Vary: Accept-Encoding",
	"X-Powered-By: PHP/5.3.6",
	"\r\n"
);

while (warn("waiting for next request...\n") and my $client = $serv->accept()) {
	my $req;
	while ($req !~ /\r\n\r\n$/) {
		$client->sysread($req, 1024, length $req) or die $!;
	}
	
	$client->syswrite($header.$body);
	<$client>; # keep-alive ;)
}
__END__

And this client (a little reworked example from SYNOPSIS):

use strict;
use Net::HTTP::NB;

my $s = Net::HTTP::NB->new(Host => "localhost:8080") || die $@;
$s->write_request(GET => "/", 'User-Agent' => "Mozilla/5.0");

use IO::Select;
my $sel = IO::Select->new($s);
 
READ_HEADER: {
   die "Header timeout" unless $sel->can_read(10);
   my($code, $mess, %h) = $s->read_response_headers;
   redo READ_HEADER unless $code;
}

while (1) {
   die "Body timeout" unless $sel->can_read(10);
   my $buf;
   my $n = $s->read_entity_body($buf, 1024);
   last unless $n;
   print $buf;
}
__END__

And output of this client will be "Body timeout" error, instead of expected "This is the body" body content.
The problem is that Net::HTTP::Methods internally uses a buffer when reading data from the server. Here read_response_headers() call readed both headers and data, returned headers for us and stored body in the buffer. So, body now in the buffer instead of a socket and socket will not be available for read anymore (until server will close connection, but our uses keep-alive, so will not do it). This is why our can_read(10) call timed out after 10 seconds.

And this is how this example may looks like to work properly:

use strict;
use Net::HTTP::NB;
use Errno qw/EAGAIN EWOULDBLOCK/;

my $s = Net::HTTP::NB->new(Host => "localhost:8080", KeepAlive => 1) || die $@;
$s->write_request(GET => "/", 'User-Agent' => "Mozilla/5.0");

use IO::Select;
my $sel = IO::Select->new($s);
 
READ_HEADER: {
   die "Header timeout" unless $sel->can_read(10);
   my($code, $mess, %h) = $s->read_response_headers;
   redo READ_HEADER unless $code;
}

# Net::HTTP::NB uses internal buffer, so we should check it before
# socket check by calling read_entity_body()
# make socket non-blocking, so read_entity_body() will not block
$s->blocking(0);

while (1) {
   my $buf;
   my $n;
   # try to read until error or all data received
   while (1) {
		my $tmp_buf;
		$n = $s->read_entity_body($tmp_buf, 1024);
		if ($n == -1 || (!defined($n) && ($! == EWOULDBLOCK || $! == EAGAIN))) {
			last; # no data available this time
		}
		elsif ($n) {
			$buf .= $tmp_buf; # data received
		}
		elsif (defined $n) {
			last; # $n == 0, all readed
		}
		else {
			die "Read error occured: ", $!; # $n == undef
		}
   }
   
   print $buf if length $buf;
   last if defined $n && $n == 0; # all readed
   die "Body timeout" unless $sel->can_read(10); # wait for new data
}

__END__

The bad news is that most modules which uses Net::HTTP::NB doing it wrong, as showed in the example from the documentation.
Dominant language
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