Hi all. A blocking IO issue is puzzling me, maybe someone can offer any insights.
I have written a plugin for Rails that does dns requests (to real-time blackhole lists). It works quite well. I am doing the dns requests (using Socket.gethostbyname) in separate threads. Unfortunately, when the call to Socket blocks, the main thread is blocking as well. And even though I have specified a parameter to thread.join, the script won't kill the blocked thread.
This is only a problem when no dns servers can be found or when they're terribly slow, but I must harden the plugin for these cases.
I have read posts on this list and it seems that any blocked IO such as what I am doing will block the main thread in Ruby. Is there a different way for me to kill the blocked thread and just move on? As this plugin is meant for web applications, a timely response is quite important. One or two seconds delay is fine, but no more.
Any help greatly appreciated.
require 'socket'
module DNSBL_Check
$dnsbl_passed ||= []
DNSBLS = %w{list.dsbl.org bl.spamcop.net sbl-xbl.spamhaus.org}
private
# Filter to check if the client is listed. This will be run before all requests.
def dnsbl_check
return true if $dnsbl_passed.include? request.remote_addr
passed = true
threads = []
request.remote_addr =~ /(\d+).(\d+).(\d+).(\d+)/
# Check the remote address against each dnsbl in a separate thread
DNSBLS.each do |dnsbl|
threads << Thread.new("#$4.#$3.#$2.#$1.#{dnsbl}") do |host|
logger.warn("Checking DNSBL #{host}")
if Socket.gethostbyname("#{host}")[3][0,2]=="\177\000"
logger.info("#{request.remote_addr} found using DNSBL #{host}")
passed = false
end
end
end
threads.each {|thread| thread.join(2)} # join threads, but use timeout to kill blocked ones
# Add client ip to global passed cache if no dnsbls objected. else deny service.
if passed
$dnsbl_passed = $dnsbl_passed[0,49].unshift request.remote_addr
logger.warn("#{request.remote_addr} added to DNSBL passed cache")
else
render :text => 'Access denied', :status => 403
return false
end
end
end
···
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