From: "Ara.T.Howard" <ahoward@...>
Date: 2004-04-28T06:54:03+09:00
Subject: Re: $stdin, thread blocking and DRb (for lack of a better subject :)

On Tue, 27 Apr 2004, Ken Hilton wrote:

> Greetings friends in Ruby-land,
> 
> 
> 
> I've crafted a Ruby application and its giving me a little trouble and I'm
> hoping someone out there can give me some insight into the problem.
> 
> 
> The app is composed of 3 threads (other than the primary thread, which waits
> for the others to finish once its created them):  the first thread is bound
> to a code block which reads email via NET::POP3; the 2nd thread is that of a
> simple DRb server; and the third thread is bound to a code block that
> implements a simple command shell which reads lines from $stdin.
> 
> 
> The application works wonderfully but w/the following problem: threads 1 and
> 2 block when the command shell thread reads from $stdin.  Adjusting thread
> priorities had no affect on the problem.  When the command shell thread is
> stopped all works as expected (email is read in the background and in
> parallel w/the DRb server accepting requests from an test harness running as
> a separate ruby session.)  I've checked all the Ruby books and docs I have
> and can't find an explanation as to why this should be happening.  My
> suspicion is that because $stdin belongs to the ruby application main thread
> and is being used by the command shell thread, that for some reason the
> blocking read of $stdin is holding off the other threads.  This speculation
> is supported by the fact that entering a command to the shell allows all 3
> threads to run in parallel until the command is completed and the next read
> of $stdin occurs.
> 
> Any insight ya'll can lend will be much appreciated.
> 
> Ken.

in ruby, if the process goes to sleep all threads stop because they are
non-native threads...

this program, however, seems to work in 1.6.8 or 1.8.0 so perhaps you have
another problem though?

~ > cat a.rb
#!/usr/bin/env ruby
require 'thread'

begin
  require 'readline'
  def prompt s
    Readline::readline s, true 
  end
rescue
  STDOUT.sync = true
  def prompt
    print s 
    gets
  end
end

threads = []
data = {} 
mutex = Mutex.new

3.times do
  threads <<
    Thread.new(threads.size) do |tid|
      loop{mutex.synchronize{(data[tid] ||= []) << [tid, Time.now]}; sleep 1}
    end
end


loop do
  cmd = prompt "select a thread #{ (0...threads.size).to_a.join ',' } > " 
  case cmd
    when /exit/io
      exit
    when /\d+/io
      values = mutex.synchronize{data[cmd.to_i]}
      (values || []).each{|val| p val}
  end
end

~ > ruby a.rb
select a thread 0,1,2 > 1
[1, Tue Apr 27 15:46:46 MDT 2004]
[1, Tue Apr 27 15:46:47 MDT 2004]
select a thread 0,1,2 > 1
[1, Tue Apr 27 15:46:46 MDT 2004]
[1, Tue Apr 27 15:46:47 MDT 2004]
[1, Tue Apr 27 15:46:48 MDT 2004]
[1, Tue Apr 27 15:46:49 MDT 2004]
select a thread 0,1,2 > exit


so, how do you KNOW that all three threads are not running, if you run this
program you'll see that they do seem to - even with the blocking read...  i
think ruby tries very hard not to block the process...

-a
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