Nonblocking sockets and Perl’s Net::Daemon

I was writing a Perl-based proxy for line-based SMTP protocol. The main reason doing this is because I receive unwanted e-mail bounces that are not filtered out by my SpamAssassin. The idea was to hook into the mail delivery chain and to collect e-mail addresses that I use. The proxy can later then filter out any bounce message that was not originated by myself.

I decided to use the Net::Daemon module which has a quite fancy interface. One just writes a single function which handles the client connection. As I didn’t want to learn every detail of SMTP protocol, I simply use non-blocking sockets. So whoever of the two parties wants to talk, it can do so and my proxy will just listen to the chat. The IO::Select documentation tells you to do this when you have multiple sockets to react on:

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# Prepare selecting
$select = new IO::Select();
$select->add($socket1);
$select->add($socket2);
 
# Run until timed out / select socket
while (@CANREAD = $select->can_read(30)) {
    foreach $s (@CANREAD) {
        #... read the socket and do your stuff
    }
}
# Whenever there is a problem (like talk ended) the loop exits here

However, this code doesn’t work as expected. The can_read() function will not return an empty list when the sockets closed. It still returns any socket and the loop goes on forever. In fact, as we are in non-blocking mode, the script now eats up CPU time. 🙁

There are two solutions to it. The first is to check whether the given socket is still connected and then exit the loop:

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# Prepare selecting
$select = new IO::Select();
$select->add($socket1);
$select->add($socket2);
 
# Run until timed out / select socket
while (@CANREAD = $select->can_read(30)) {
    foreach $s (@CANREAD) {
        if (!$s->connected()) {
            return;
        }
        #... It's safe now to read the socket and do your stuff
    }
}

The second and more clean method is just to remove the closed socket from the IO::Select object:

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# Prepare selecting
$select = new IO::Select();
$select->add($socket1);
$select->add($socket2);
 
# Run until timed out / select socket
while (@CANREAD = $select->can_read(30)) {
    foreach $s (@CANREAD) {
        if (!$s->connected()) {
            $select->remove($s);
            next;
        }
        #... It's safe now to read the socket and do your stuff
    }
}

Then the selector runs empty and will exit the loop as well.