"Impossible" socket error return

dd-b has asked for the wisdom of the Perl Monks concerning the following question:

In a fairly simple server (simulator; the actual guts aren't there, just the server infrastructure) using INET sockets running on Centos, I'm getting <$sock> returning undef with $! blank. SIGPIPE is ignored. I'm running out of ideas.

This chunk of code is in a subroutine, called to do all the input from the client:

$iologger->debug("socket blocking state ", $sock->blocking());
my $l = <$sock>;
my $err = $!;
$iologger->debug("Client input call err ", safe($err),
         " line ", safe($l));
if (!defined($l)) {
    my $d = Dumper($sock);
    $iologger->debug("Socket at error: $d");
    die "eiread failed: $err";
}
$iologger->info("Client input: ", safe($l));
return $l;
[download]

("safe()" is a function that turns undef into "[undef]" and an empty string into "[empty]" and passes through a non-empty string; so the log shows more clearly what's happening.)

Log output below shows it reading a number of lines, down to "h1,h2", and then on the next read call getting undef back. How can it get undef back, without $! being set? (The "Bad file descriptor" messages are lingering $! values, being reported when <$sock> returned a value, and hence don't mean anythying. The code is logging the $! value even when it's not meaningful. Lazy debug code, okay?)

2011-09-27 17:23:14,723 INFO  simkserver: Accepted cmdcnt 1 starttime 
+17:23:14
2011-09-27 17:23:14,744 DEBUG simkserver.io: socket blocking state 1
2011-09-27 17:23:14,745 DEBUG simkserver.io: Client input call err Bad
+ file desc
riptor line Proxy,127.0.0.1
2011-09-27 17:23:14,746 INFO  simkserver.io: Client input: Proxy,127.0
+.0.1
2011-09-27 17:23:14,746 INFO  simkserver: Connection is from proxy for
+ 127.0.0.1
2011-09-27 17:23:14,747 DEBUG simkserver.io: socket blocking state 1
2011-09-27 17:23:14,747 DEBUG simkserver.io: Client input call err Bad
+ file desc
riptor line foo,3
2011-09-27 17:23:14,748 INFO  simkserver.io: Client input: foo,3
2011-09-27 17:23:14,748 DEBUG simkserver: command foo arg1 3
2011-09-27 17:23:14,749 DEBUG simkserver.io: socket blocking state 1
2011-09-27 17:23:16,557 DEBUG simkserver.io: Client input call err Bad
+ file desc
riptor line h1,h2
2011-09-27 17:23:16,574 INFO  simkserver.io: Client input: h1,h2
2011-09-27 17:23:16,575 DEBUG simkserver.io: socket blocking state 1
2011-09-27 17:23:16,575 DEBUG simkserver.io: Client input call err [em
+pty] line 
[undef]
2011-09-27 17:23:16,576 DEBUG simkserver.io: Socket at error: $VAR1 = 
+bless( \*S
ymbol::GEN1, 'IO::Socket::INET' );
2011-09-27 17:23:16,577 ERROR simkserver: Aborted transaction on excep
+tion eirea
d failed:  at ./simkserver line 103, <GEN1> line 3.
[download]

(The logging calls are log4perl.)

Comment on "Impossible" socket error return Select or Download Code

Replies are listed 'Best First'.
Re: "Impossible" socket error return by ikegami (Patriarch) on Sep 27, 2011 at 23:36 UTC
How can it get undef back, without $! being set? `readline` (for which `<$fh>` is a shortcut) returns undef on end of file as well as on error. By the way, there's no way to know if `$!` is meaningful after `readline` because `readline` has no way of signaling an error occurred.	[reply] [d/l] [select]
Re^2: "Impossible" socket error return by dd-b (Monk) on Sep 28, 2011 at 14:55 UTC
Thanks! Yes, it was indeed the obvious. Bug in another bit entirely was causing it to disconnect at an inopportune moment.	[reply]
Re: "Impossible" socket error return by onelesd (Pilgrim) on Sep 27, 2011 at 22:51 UTC
Looks to me like the client finished sending and disconnected, and you get `undef` when that happens.	[reply] [d/l]

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