Re: Get Timout with LWP

[tachyon]

in reply to Get Timout with LWP

LWP::Simple is just that. Simple. It will hang if it can open a socket to a server but the server then plays dead and does not respond. LWP::UserAgent is much more sophisticated with its timeouts and will return if it times out.......

BUT THERE IS A CAVEAT: Because of the way it is implemented in LWP::UserAgent the timeout is best termed as *fuzzy*. It will timeout but if you have a connection to a (say) overloaded server that trickle feeds data back to your client application it may take orders of magnitude longer to timeout than the number you specify. If you absolutely need a hard limit use alarm.

Here is an example. Although the timeout is set to 2 seconds LWP will wait 30 seconds to get the trickle data. Effectively what the timeout means to LWP is 1) make the connection within TIMEOUT seconds and 2) get at least one byte of data from a non blocking socket every TIMEOUT seconds. It will return if either of these are not fulfilled but as you can see you can still kinda hang it.

As an aside it you are scraping sites, and they run anti-scraping response throttling this scenario if quite realistic.

# this is the test server that does a tricke feed response

#!/usr/bin/perl
use IO::Socket;
use IO::Select;
$lsn = IO::Socket::INET->new( Listen    => 1,
                        LocalAddr => 'localhost',
                        LocalPort => 9000,);

my $client = new IO::Select( $lsn );

while( my @ready = $client->can_read ) {
    for my $fh (@ready) {
        if($fh == $lsn) {
            warn "Accepted new socket\n";
            my $new = $lsn->accept;
            $client->add($new);
        }
        else {
            # Process socket
            warn "Getting data\n";
            $data = <$fh>;  # yeah yeah this is only a toy app
            warn "Got $data\nDoing stuff slowly!\n";
            my @response = split '', "HTTP/1.1 200 OK\n\nHello World!\
+n";
            for( @response) {
                warn $_;
                sleep 1;
                print {$fh} $_;
            }
            $client->remove($fh);
            $fh->close(); 
        }
    }
}
   

# this is the test client

#!/usr/bin/perl
use LWP::UserAgent;
use Data::Dumper;
my $ua = LWP::UserAgent->new( timeout => 2 );
$response = $ua->get('http://localhost:9000/');
print Dumper $response;

[IlyaM]

LWP::Simple is just that. Simple. It will hang if it can open a socket to a server but the server then plays dead and does not respond. LWP::UserAgent is much more sophisticated with its timeouts and will return if it times out.......

This is quite wrong. LWP::Simple is just a wrapper on top of LWP::UserAgent so it behaves exactly same way as LWP::UserAgent concerning timeout handling. The only exception is that LWP::Simple always uses default timeout value (180 seconds) and with LWP::UserAgent timeout can be set.

[tachyon]

Unfortunately I am not wrong. The hanging behaviour is real. The underlying reason is LWP::Simple uses (for http) the _http_trivial_get() function, not LWP::UserAgent. This implements a 60 second IO::Socket::INET make connection timeout, but no timeout on data recieve. Modifying the test code to use LWP::Simple proves the point (LWP::Simple will wait 500 seconds to get the data - it gets an instant socket, then nothing for 500 seconds). This function remains active in the latest LWP::Simple.

I agree it should really be a subclass of LWP::UserAgent but for http it is not. The timeout behaviour for LWP::Simple and LWP::UserAgent could IMHO use some work.

C:\>type server.pl
#!/usr/bin/perl
use IO::Socket;
use IO::Select;
$lsn = IO::Socket::INET->new( Listen    => 1,
                        LocalAddr => 'localhost',
                        LocalPort => 9000,);

my $client = new IO::Select( $lsn );

while( my @ready = $client->can_read ) {
    for my $fh (@ready) {
        if($fh == $lsn) {
            warn "Accepted new socket\n";
            my $new = $lsn->accept;
            $client->add($new);
        }
        else {
            # Process socket
            warn "Getting data\n";
            $data = <$fh>;  # yeah yeah
            warn "Got $data\nDoing nothing forever!\n";
            my @response = split '', "HTTP/1.1 200 OK\n\nHello World!\
+n";
            sleep 500;
            print {$fh} @response;
            $client->remove($fh);
            $fh->close();
        }
    }
}


C:\>type lwp.pl
#!/usr/bin/perl
$|++;
use LWP::Simple;
$start = time();
print "Begin at $start\n";
$response = get('http://localhost:9000/');
$end = time();
my $time = $end - $start;
print "Done at $end\nTook $time seconds\nGot:\n$response\n";



C:\>perl lwp.pl
Begin at 1087868116
Done at 1087868616
Took 500 seconds
Got:
Hello World!


C:\>

[tachyon]

Further to the previous, here is a possible patch to implement timeout behaviour in LWP::Simple. There is no way to set $timeout passed to _trivial_http_get() included. A global would be easiest I guess given the very simple interface. Defaults to 180 seconds.

$ diff -u Perl/site/lib/LWP/Simple.pm Perl/site/lib/LWP/Simple-patch.p
+m
--- Perl/site/lib/LWP/Simple.pm Tue Jun 22 12:40:14 2004
+++ Perl/site/lib/LWP/Simple-patch.pm   Tue Jun 22 12:42:15 2004
@@ -154,9 +154,10 @@

 sub _trivial_http_get
 {
-   my($host, $port, $path) = @_;
+   my($host, $port, $path, $timeout) = @_;
    #print "HOST=$host, PORT=$port, PATH=$path\n";
-
+   $timeout ||=180;
+   my $hard_timeout = time() + $timeout;
    require IO::Socket;
    local($^W) = 0;
    my $sock = IO::Socket::INET->new(PeerAddr => $host,
@@ -174,8 +175,15 @@

    my $buf = "";
    my $n;
-   1 while $n = sysread($sock, $buf, 8*1024, length($buf));
-   return undef unless defined($n);
+   while( 1 ) {
+       my $remaining = $hard_timeout - time();
+       return undef if $remaining <= 0;
+       if ( can_read( $sock, $remaining ) ) {
+           $n = sysread($sock, $buf, 8*1024, length($buf));
+         return undef unless defined($n);
+         last if $n == 0; # we are eof
+       }
+   }

    if ($buf =~ m,^HTTP/\d+\.\d+\s+(\d+)[^\012]*\012,) {
        my $code = $1;
@@ -191,6 +199,15 @@
    }

    return $buf;
+}
+
+sub can_read {
+    my($sock, $timeout) = @_;
+    my $fbits = '';
+    vec($fbits, fileno($sock), 1) = 1;
+    my $nfound = select($fbits, undef, undef, $timeout);
+    die "select failed: $!" unless defined $nfound;
+    return $nfound > 0;
 }



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