From: Tassilo Horn <heimdall@...>
Date: 2004-10-09T00:14:44+09:00
Subject: Problems with my first multithreaded Rupy programm

Hi,

I'm trying to get into Ruby and today it's time for
multithreading. Therefore I wanted to write a program which does this:

        In a tearoom there are three thirsty drinkers. To cook a tea one
        needs water, tea and a cup. Every one of these three drinkers
        has one of those items with him.
        Every time the waiter is called he puts two items on the
        table. Now one drinker has all the items he needs and cooks his
        tea. After that he calls the waiter and the fun begins again...

Ok, here's what I wrote so far and what produces this output:

,----
| (%:~/tmp/Teadrinker)- ./TeaRoom.rb
| WAITER: Looking at the table!
| WAITER: Oh, nothing on the table!
| WAITER: Putting 1 and 2 on the table.
| deadlock 0x4023b798: sleep:J(0x4022603c) (main) - ./TeaRoom.rb:94
| deadlock 0x40225e70: sleep:-  - /usr/lib/ruby/1.8/thread.rb:96
| deadlock 0x40225efc: sleep:-  - /usr/lib/ruby/1.8/thread.rb:96
| deadlock 0x40225f88: sleep:-  - /usr/lib/ruby/1.8/thread.rb:96
| deadlock 0x4022603c: sleep:-  - ./TeaRoom.rb:31
| ./TeaRoom.rb:31: Thread(0x4022603c): deadlock (fatal)
`----

<----- CODE ----->

#! /usr/bin/env ruby

require 'thread'

# For synchronisation
mutex = Mutex.new
cv = ConditionVariable.new

# items on the table:
# 0: cup
# 1: water
# 2: tea
table = []

waiter = Thread.new {
  while true
    mutex.synchronize {
      puts "WAITER: Looking at the table!"
      if table.length == 0
        puts "WAITER: Oh, nothing on the table!"
        sleep 2
        while table[0] == table[1] do
          table = [rand(3), rand(3)]
        end
        $stdout << "WAITER: Putting " << table[0] << " and " << table[1]
  << " on the table.\n"
      else
        puts "WAITER: Why did you call me???"
      end
      # Call the waiting tea drinkers
      cv.broadcast
      Thread.stop
    }
  end
}    

# Mike has a cup (0)
mike = Thread.new { 
  while true
    mutex.synchronize {
      cv.wait(mutex)
      if table.include?(1) and table.include?(2)
        puts "MIKE: Water and tea on table. I'll cook my tea."
        table = []
        sleep 3
        puts "MIKE: Drinking my tea. Calling the waiter."
        # Wake up the waiter-Thread
        waiter.wakeup
        sleep 3
        puts "MIKE: Now I will read my newspaper."
      else
        puts "MIKE: Not what I need."
      end
    }
  end
}

# Al has water (1)
al = Thread.new { 
  while true
    mutex.synchronize {
      cv.wait(mutex)
      if table.include?(0) and table.include?(2)
        puts "AL: Cup and tea on table. I'll cook my tea."
        table = []
        sleep 3
        puts "AL: Drinking my tea. Calling the waiter."
        # Wake up the waiter-Thread
        waiter.wakeup
        sleep 3
        puts "AL: Now I will read my newspaper."
      else
        puts "AL: Not what I need."
      end
    }
  end
}
 
# Ed has tea (2)
ed = Thread.new { 
  while true
    mutex.synchronize {
      cv.wait(mutex)
      if table.include?(0) and table.include?(1)
        puts "ED: Cup and water on table. I'll cook my tea."
        table = []
        sleep 3
        puts "ED: Drinking my tea. Calling the waiter."
        # Wake up the waiter-Thread
        waiter.wakeup
        sleep 3
        puts "ED: Now I will read my newspaper."
      else
        puts "ED: Not what I need."
      end
    }
  end
}

waiter.join

<----- END OF CODE ----->

What's wrong with these lines?

Much thanks in advance,
Tassilo