Hi. I worked with a perl blocking sockets and found a weird thing.

Looks like IO::Select works bad when you read from sockets with <$fh> in case someone on the other end wrote to the socket _two_ or more lines. I.e. <$fh> will return one line. Second line will be read too by perl process but kept in perlio layer buffers. So IO::Select will return no sockets too read.

In practice this can produce hardly-to catch errors on application. I.e application can work _fine_ for a long time until it receive two lines at once cause of a big delay when handling previous lines.

I think this is lack in perl documentation. There is no word about this. Nothing about possible _read_ buffering problem. And lots of perl code all over the internet with io::select and <$fh>.

What do you think ?

Thanks.

Code (proof-of-concept):

#!/usr/bin/perl -w 

use strict; 
use IO::Select; 
use IO::Pipe;

  my $fromchild = new IO::Pipe;
  my $tochild = new IO::Pipe;
  my $pid;
  my $parent_pid = $$;
  if($pid = fork()) { # Parent
   $fromchild->reader();
   $fromchild->autoflush(1);
   $fromchild->blocking(1);
   binmode $fromchild;
   $tochild->writer();
   $tochild->autoflush(1);
   $tochild->blocking(1);
   binmode $tochild;

   my $read_set = new IO::Select(); # create handle set for reading
   $read_set->add($fromchild);

   while(1) {
    print "before select\n";
    my ($rh_set, undef, $ex_set) = IO::Select->select($read_set, undef
+, $read_set, 30);
    print "after select\n";
    for my $rh (@$rh_set) {
        my $s =  <$rh>;
        print "command: $s";
    }
   }
  } elsif (defined ($pid)) { # Child
   $fromchild->writer();
   $fromchild->autoflush(1);
   $fromchild->blocking(1);
   binmode $fromchild;
   $tochild->reader();
   $tochild->autoflush(1);
   $tochild->blocking(1);
   binmode $tochild;
   print $fromchild "abc\n";
   #sleep(1);  ### IF you uncomment this line it will work
   print $fromchild "def\n";

   sleep(86400);
   die;
  }

__END__

=Output=:
$ ./poc1.pl
before select
after select
command: abc
before select

(process hangs here)
[download]

---

---

Posts are HTML formatted. Put <p> </p> tags around your paragraphs. Put <code> </code> tags around your code and data!

Titles consisting of a single word are discouraged, and in most cases are disallowed outright.

Read Where should I post X? if you're not absolutely sure you're posting in the right place.

Please read these before you post! —

• How do I compose an effective node title?
• How do I post a question effectively?
• Markup in the Monastery

Posts may use any of the Perl Monks Approved HTML tags:

a, abbr, b, big, blockquote, br, caption, center, col, colgroup, dd, del, details, div, dl, dt, em, font, h1, h2, h3, h4, h5, h6, hr, i, ins, li, ol, p, pre, readmore, small, span, spoiler, strike, strong, sub, summary, sup, table, tbody, td, tfoot, th, thead, tr, tt, u, ul, wbr

You may need to use entities for some characters, as follows. (Exception: Within code tags, you can put the characters literally.)

	For:		Use:
	&		&
	<		&lt;
	>		&gt;
	[		&#91;
	]		&#93;

Link using PerlMonks shortcuts! What shortcuts can I use for linking?

See Writeup Formatting Tips and other pages linked from there for more info.