From: Sam Dalton <mail@...>
Date: 2010-07-01T17:31:44+09:00
Subject: Synchronized Circular Buffer

I'm relatively new to Ruby, having switched from a PHP and Java
background, and I'm having some trouble with the Monitor class trying to
implement a synchronised circular buffer. The other posts here on
synchronised methods aren't quite relevant enough unfortunately.

My current implementation is the following

require 'monitor'

class SynchronisedBuffer < Monitor

   def initialize(capacity)
      @capacity = capacity
      @front = 0
      @back = 0
      @elements = Array.new(capacity)
      @empty_cond = new_cond
      @full_cond = new_cond
      super()
   end

   def get
       @empty_cond.wait_while {empty?}
       element = nil
       synchronize do
           element = @elements[@front]
           @elements[@front] = nil
           @front = (@front + 1) % @capacity
           @full_cond.signal
        end
        return element
   end

   def put(element)
       @full_cond.wait_while {full?}
       synchronize do
           @elements[@back] = element
           @back = (@back + 1) % @capacity
           @empty_cond.signal
        end
   end

   def full?
       result = false
       synchronize do
           result = (@front == @back and @elements[@front] != nil)
        end
        return result
   end

   def empty?
       result = false
       synchronize do
           result = (@front == @back and @elements[@front] == nil)
        end
        return result
   end

end

This has been adapted from a version I had written in Java. The problem
is that when I have a thread call 'get' on the buffer, I receive a
ThreadException saying 'current thread not owner'

This is being thrown by mon_check_owner, and a bit of poking around
shows that it fails because mon_owner is set to nil in the condition
'@mon_owner != Thread.current'.

I am using the buffer for a simple producer/consumer web server, like so

buffer = SynchronisedBuffer.new(10)
workers = []

for i in (1..10)
   workers[i] = Worker.new(buffer)
end

while socket = server.accept
    buffer.put(socket)
end

I'm writing this purely to gain a better understanding of some important
ruby classes like thread, monitor and socket, and am aware that there
are other ruby web servers I could use straight away :). I'm also trying
to understand the 'ruby way' of coding, so any comments as to how the
above code might better be written would be really appreciated.

Sam Dalton
-- 
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