From: Sean O'Halpin <sean.ohalpin@...>
Date: 2013-02-24T23:12:29+09:00
Subject: Re: Ruby Multithreaded producer-consumer problem

On Sun, Feb 24, 2013 at 1:30 AM, Abhijit Sarkar <lists@ruby-forum.com> wrote:
> Bump!
> A problem unsolved keeps bugging me, I seriously hope the great minds
> here would have something for me.
>

Hi,

I've created a gist at https://gist.github.com/seanohalpin/5023887
which attempts to make your program do what I think it's trying to do.
I've tried to keep as close to your original as possible so you can
see the differences more easily. There are other things I would change
but they are not relevant to this discussion. This is running on ruby
1.9.3p194.

A couple of points:

- it's really not clear what you're trying to do - a problem
definition would help.
- as Robert pointed out, using Thread.abort_on_exception = true would
help. You would have found that Math does not have a rand method for
example.
- the only shared state you need to protect access to is the
@orange_count variable - it's unnecessary to use synchronize around
the whole loop. Also, that logic would probably be better encapsulated
in the OrangeTree class itself.
- you don't check whether the exit condition for the orange_picker
thread is true before waiting on the CV. This will halt your thread
indefinitely when the age_increaser thread exits before the
orange_picker thread starts the wait. Multiple checking of conditions
is an unfortunate but necessary aspect of multi-threaded programming
with mutexes, etc.
- lastly, before you go down the rabbit hole of mutexes and condition
variables, I would strongly advise you adopt the Actor model to
isolate access to state and use queues to communicate between threads.
You will save yourself a world of pain. BTW Tony's advice to look at
DCell is good advice!

Regards,
Sean