From: Brent Roman <brent@...>
Date: 2007-12-14T11:08:02+09:00
Subject: [PATCH] Continuations on dead threads crash ruby during GC


This patch is against Ruby 1.8.6-p110
The bug appears in 1.8.6 and 1.6.8.

Note:
I have not yet verified whether this bug occurs in the 1.9 series.
(It would be nice if someone tracking Ruby 1.9 development
 could help me there)

Here is the 1.8.6 Changlog entry:

Fri Dec 14 01:03:59 2007  Brent Roman  <brent@mbari.org>

        * eval.c:  fixes bug described in [ruby-core:13889]
          Added cc_mark for GC marking Continuations to prevent
          BLOCKs in use by those continuations from being prematurely
          freed when their parent thread dies

Basically, we've got to prevent threads that are preserved in Continuations
from being treated as "live" by the garbage collector if the thread
referenced by that continuation has already died.  There is nothing
that can be done with such a "dead" Continuation, so its stack frames, etc.
should be freed.  Only the core Continuation object must be maintained
as long as the script references it.

Here is a test script that causes the crash until the attached patch is
applied:   http://www.nabble.com/file/p14328573/cont18.patch cont18.patch 

#!/usr/bin/env ruby
#Crashes 1.8.6 & 1.6.8 as of Dec 13th, 2007
#submitted by brent@mbari.org

def nestedThreadBug
  inner = nil
  outer = Thread.new(Thread.current) do |passedThread|
  $c4 = callcc {|c| c}
    inner = Thread.new passedThread do
    end
  end
  yield(outer)
  outer.value
  inner
end

(tries=100000).times {
  inner = nestedThreadBug do |thread|
    thread.value
  end
  inner.join
}
puts "Survived spawning #{tries} Threads with inner Continuations!!!"

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