Question

Creating a class which holds some threads, performing tasks and finally calling a callback-method is my current goal, nothing special on this road.

My experimental class does some connection-checks on specific ports of a given IP, to give me a status information.

So my attempt:

check = ConnectionChecker.new do | threads |
   # i am done callback
end

check.check_connectivity(ip0, port0, timeout0, identifier0)
check.check_connectivity(ip1, port1, timeout1, identifier1)
check.check_connectivity(ip2, port2, timeout2, identifier2)

sleep while not check.is_done

Maybe not the best approach, but in general it fits in my case.

So what’s happening:

In my Class I store a callback, perform actions and do internal stuff:

Thread.new -> success/failure -> mark as done, when all done -> call callback:

 class ConnectionChecker

   attr_reader :is_done

   def initialize(&callback)
     @callback     = callback
     @thread_count = 0
     @threads      = []
     @is_done      = false
   end

   def check_connectivity(host, port, timeout, ident)
     @thread_count += 1
     @threads << Thread.new do

       status = false
       pid    = Process.spawn("nc -z #{host} #{port} >/dev/null")

       begin
         Timeout.timeout(timeout) do
           Process.wait(pid)
           status = true
         end
       rescue Process::TimeoutError => e
         Process.kill('TERM', pid)
       end

       mark_as_done
       #returnvalue for the callback.
       [status, ident]
     end
   end

   # one less to go..
   def mark_as_done
     @thread_count -= 1
     if @thread_count.zero?
       @is_done = true
       @callback.call(@threads)
     end
   end
 end

This code – yes, I know there is no start method so I have to trust that I call it all quite instantly – works fine.

But when I swap these 2 lines:

  @is_done = true
  @callback.call(@threads)

to

  @callback.call(@threads)
  @is_done = true

then the very last line,

sleep while not check.is_done

becomes an endless loop. Debugging shows me that the callback is called properly, when I check for the value of is_done, it really always is false. Since I don’t put it into a closure, I wonder why this is happening.

The callback itself can also be empty, is_done remains false (so there is no mis-caught exception).

In this case I noticed that the last thread was at status running. Since I did not ask for the thread’s value, I just don’t get the hang here.

Any documentation/information regarding this problem? Also, a name for it would be fine.

Answer 1

Try using Mutex to ensure thread safety 🙂