From: Robert Klemme <shortcutter@...>
Date: 2008-12-24T00:16:23+09:00
Subject: Re: Are all Ruby built-in objects thread safe?

On 23.12.2008 10:24, Just Another Victim of the Ambient Morality wrote:
>     Are all built-in objects thread safe?  For example, if I have an array 
> and one thread is constant appending to it while another thread is shifting 
> elements off of it and there's no synchronization going on, can the array 
> object ever get corrupted?  What about a similar scenario for hashes?  These 
> are surely complicated objects with internal state that must be maintained. 
> Are they implemented to be thread safe?

The answer will sound a bit odd: they are not built to be thread safe 
but it may well be that they are.  In fact, it may well depend whether 
they are thread safe in practical terms depending on the Ruby version 
you are using.  Since the classic interpreter uses green threads, i.e. 
no preemption is happening, it may be that some or all operations are 
atomic and thus thread safe.

The bottom line is: you should not rely on this and assume they are not 
thread safe!

In your example, you should be using class Queue which is thread safe 
and can be imported by requiring "thread"

$ irb -r thread
irb(main):001:0> q = Queue.new
=> #<Queue:0x7ff6afc0>
irb(main):002:0> t = Thread.new(q) {|qq| while (o = qq.deq) != qq; p o; end}
=> #<Thread:0x7ff61c68 sleep>
irb(main):003:0> 5.times {|i| q.enq i}; q.enq q; t.join
0
1
2
3
4
=> #<Thread:0x7ff61c68 dead>

Then there are also classes Mutex and Monitor plus module MonitorMixin. 
  The difference is reentrance:

irb(main):009:0> require 'monitor'
=> true
irb(main):010:0> m=Mutex.new
=> #<Mutex:0x7ff3e100>
irb(main):011:0> m.synchronize { m.synchronize { 1 } }
ThreadError: stopping only thread
         note: use sleep to stop forever
         from (irb):11:in `synchronize'
         from (irb):11
         from (irb):11:in `synchronize'
         from (irb):11
         from (null):0
irb(main):012:0> m=Monitor.new
=> #<Monitor:0x7ff329f4 @mon_count=0, @mon_owner=nil, 
@mon_waiting_queue=[], @mon_entering_queue=[]>
irb(main):013:0> m.synchronize { m.synchronize { 1 } }
=> 1
irb(main):014:0>

Kind regards

	robert