From: Brian Candler <B.Candler@...>
Date: 2004-09-11T01:42:31+09:00
Subject: Re: Variable scoping

> This is because you have never read ruby-man-1.4, and it's very bad :-)

You mean very bad of me not to read it? Fair enough. I _have_ read quite a
lot though :-) Besides, we can all make mistakes.

| while TRUE
|   ns = gs.accept
|   print(ns, " is accepted\n")
|   Thread.start do
|     s = ns			# save to dynamic variable

But isn't there a race there? That is, if the interpreter context-switches
after the Thread.start, goes back around the loop, and ns is assigned to a
new object before "s = ns" is executed.

I think for safety you should do

  Thread.start(ns) do |s|

But that's the trouble with this thread-safety stuff. Maybe 999 out of 1000
times the previous code would work. It's almost certainly not possible to
catch that sort of problem with unit-testing, unless you splatter "sleep"s
all over the code when testing.

Some sort of static indication of a possible problem would be very useful.

It would have to be able to tell that:
  - you are using a variable within a Thread .... block
  - the variable may be assigned to again by the other thread
    (which, in this case, is an *earlier* assignment because it's in a loop)

Regards,

Brian.