Bug #4181: Backport Ruby 1.9 singleton.rb, since 1.8's is not thread-safe
http://redmine.ruby-lang.org/issues/show/4181

Author: Charles Nutter
Status: Open, Priority: Immediate
ruby -v: Any Ruby 1.8 version

Ruby 1.9 modified singleton.rb by eliminating much of the lazy init logic, using a real mutex instead of Thread.critical, and eliminating the redefinition of "instance" on first call. None of these changes have been backported into a 1.8 release, which means all 1.8 releases have a broken singleton.rb.

The following script breaks under any version of 1.8:

<code>
 require 'singleton'
 $jruby = RUBY_PLATFORM =~ /java/
 require 'jruby/synchronized' if $jruby
 
 loop do
   $inits = []
   $inits.extend JRuby::Synchronized if $jruby
   classes = []
   1000.times do
     classes << Class.new do
       include Singleton
     end
   end
   
   (0..10).map do
     Thread.new do
       classes.each do |cls|
         cls.instance
       end
     end
   end.map(&:join)
   puts "loop completed"
 end
</code>

Results:

~/projects/jruby  ruby -v singleton_killer.rb 
ruby 1.8.7 (2010-08-16 patchlevel 302) [i686-darwin10.4.0]
loop completed
loop completed
loop completed
loop completed
loop completed
loop completed
loop completed
loop completed
loop completed
singleton_killer.rb:18: undefined method `instance' for #<Class:0x1001896a0> (NoMethodError)
	from singleton_killer.rb:1:in `join'
	from singleton_killer.rb:1:in `to_proc'
	from singleton_killer.rb:21:in `map'
	from singleton_killer.rb:21
	from singleton_killer.rb:5:in `loop'
	from singleton_killer.rb:5


~/projects/jruby  ruby -v singleton_killer.rb 
ruby 1.8.7 (2009-06-12 patchlevel 174) [universal-darwin10.0]
loop completed
loop completed
loop completed
loop completed
singleton_killer.rb:18: undefined method `instance' for #<Class:0x100348c70> (NoMethodError)
	from singleton_killer.rb:1:in `join'
	from singleton_killer.rb:1:in `to_proc'
	from singleton_killer.rb:21:in `map'
	from singleton_killer.rb:21
	from singleton_killer.rb:5:in `loop'
	from singleton_killer.rb:5

This can lead to lazy failures in any library that uses singleton.rb. See also this commit to Nokogiri, where they had to stop using Singleton because of this issue:

https://github.com/tenderlove/nokogiri/commit/5eb036e39ea85a8e12eebee11bc5086b0e4ce6e3


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