100% CPU usage when too many streaming clients

[pjuhasz]

The following is a minimal streaming server that waits for a random interval between sending lines of text.

#!/usr/bin/perl

use Modern::Perl '2013';
use AnyEvent;

my $writers = {};
my $timer;

sub generate_log {
    $_->write("foo\n") for (values %$writers);
    $timer = AE::timer rand(1), 0, \&generate_log;
}

sub {
    my $env = shift;

    $timer = AE::timer rand(1), 0, \&generate_log;

    return sub {
        my $responder = shift;

        my $writer = $responder->([200, ['Content-Type', 'text/csv']]);
        $writers->{$writer} = $writer;
        $writer->{handle}->on_error(sub {delete($writers->{$writer}); warn "client closed connection\n"});
        return;
    }
}

The problem with it is that when too many clients (>1017) are connected, the server becomes unresponsive and begins to consume 100% CPU. This can be demonstrated with the following command:

for i in {1..1024}; do curl -s localhost:6000 > /dev/null & done

The specific number of connections can be explained by it being close to the default maximum number of file descriptors on Linux, so it is not a bug that the server can't serve more connections than this. However, the high CPU usage arguably is a bug, because an attacker can easily trigger it by simply starting many clients, potentially denying resources from other services running on the same machine.