As no one has said anything so far, I'll just give it a shot... :)
One potential problem here could be that the "socket" you open to the
real socket's file descriptor is not really an IO::Socket::INET object/handle,
but rather a regular filehandle. As such it doesn't know about the
autoflush setting of the IO::Socket handle (for example), not stored
within the descriptor.
Autoflush is automatically turned on by IO::Socket (as of version
1.18). The other handle, however, will not "inherit" this setting, so in
the one case (using the original socket handle) it's on, but in the
other case it's off. This might account for different behaviour... Proof-of-concept demo:
#!/usr/bin/perl
use strict;
use warnings;
use IO::Socket;
my $pid = fork(); die "couldn't fork" unless defined($pid);
if ($pid) { # SERVER
my $server = IO::Socket::INET->new(
Proto => 'tcp',
LocalPort => 9999,
Listen => 1,
ReuseAddr => 1,
);
die "couldn't setup server" unless $server;
my $client = $server->accept();
$client->autoflush(1);
while (<$client>) {
if (/hello/) { print $client "nice to meet you\n" }
elsif (/quit/) { print $client "bye\n"; last }
else { print $client "??\n" }
}
close $client;
} else { # CLIENT
sleep 1; # wait for server to get ready
my $sock1 = IO::Socket::INET->new(
Proto => 'tcp',
PeerAddr => "localhost",
PeerPort => 9999,
);
die "couldn't connect" unless $sock1;
open my $sock2, '+<&='.$sock1->fileno() or die $!;
printf "sock1 = %s\n", $sock1;
printf "fileno(sock1) = %d\n", $sock1->fileno();
printf "autoflush = %s\n\n", $sock1->autoflush() ? "ON":"OFF";
printf "sock2 = %s\n", $sock2;
printf "fileno(sock2) = %d\n", $sock2->fileno();
printf "autoflush = %s\n\n", $sock2->autoflush() ? "ON":"OFF";
my $resp;
print $sock2 "hello\n";
$resp = <$sock2>; print $resp;
print $sock2 "quit\n";
$resp = <$sock2>; print $resp;
exit;
}
As it is, it should print something like
$ ./751902.pl
sock1 = IO::Socket::INET=GLOB(0x7dc230)
fileno(sock1) = 4
autoflush = ON
sock2 = GLOB(0x604410)
fileno(sock2) = 4
autoflush = OFF
nice to meet you
bye
But if you comment out the line with the $sock2->autoflush(), it will
hang before "nice to meet you", because the client's print $sock2 "hello\n"; doesn't
make it to the server, due to not being flushed. (Note that ->autoflush() is
not a real "getter" method — i.e. it does return the current
state, but it's special in that it also turns it on! So, despite the
printout "OFF", it is in fact ON after the call... unless you comment out that line — you get the idea.)
I have no real explanation how these circumstances would produce
the exact behaviour you observe, but that's roughly where I would start looking
(e.g. try $socket->autoflush() after the open $socket, '+<&='...).
If that doesn't help, you could use strace to figure out
what's actually being exchanged over the socket.
HTH.
BTW, is there any specific reason you can't simply pass around the
original $socket itself, instead of its file descriptor number?
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