From: joost baaij <joost@...>
Date: 2006-10-21T09:21:54+09:00
Subject: threaded Socket call blocks main thread

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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