Issue #4181 has been updated by Hiroshi NAKAMURA.


r4424 at 2003-08-03 is the bad commit.

Shyouhei, can I commit this?

  Index: lib/singleton.rb
  ===================================================================
  --- lib/singleton.rb	(revision 32126)
  +++ lib/singleton.rb	(working copy)
  @@ -94,9 +94,12 @@
           @__instance__ = new
         ensure
           if @__instance__
  -          class <<self
  -            remove_method :instance
  -            def instance; @__instance__ end
  +          # Check method existence to reduce redefinition warning.
  +          unless self.respond_to?(:instance)
  +            # It's doing test-then-set without a lock so there's a race to
  +            # override existing method that causes the redefinition warning.
  +            # We allow it since method redefinition is thread-safe for calling.
  +            def self.instance; @__instance__ end
             end
           else
             @__instance__ = nil #  failed instance creation
  @@ -111,9 +114,9 @@
           @__instance__ = new
         ensure
           if @__instance__
  -          class <<self
  -            remove_method :instance
  -            def instance; @__instance__ end
  +          # See the comment for self.instance above.
  +          unless self.respond_to?(:instance)
  +            def self.instance; @__instance__ end
             end
           else
             @__instance__ = nil

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

Author: Charles Nutter
Status: Assigned
Priority: Immediate
Assignee: Hiroshi NAKAMURA
Category: 
Target version: Ruby 1.8.7
ruby -v: Any Ruby 1.8 version


=begin
 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
=end



-- 
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