autoload & require deadlock

readme.txt

*** Tested using JRuby 1.7.21 ***
Save all the files into the same directory and run

# jruby main.rb

as any good deadlock, it won't happen all the time so you can do something like this

# while [ true ] ; do jruby main.rb  ; done

and wait till it hangs.

You can confirm the dealock with jstack
Once the process is hung, get its pid and run

jstack -l <pid>

you'll see something like this

Found one Java-level deadlock:
=============================
"Ruby-0-Thread-3: main.rb:1":
  waiting for ownable synchronizer 0x00000007b65b9a58, (a java.util.concurrent.locks.ReentrantLock$NonfairSync),
  which is held by "Ruby-0-Thread-2: main.rb:1"
"Ruby-0-Thread-2: main.rb:1":
  waiting to lock monitor 0x00007f8832523fb8 (object 0x00000007b63ff810, a java.lang.Object),
  which is held by "Ruby-0-Thread-3: main.rb:1"

Java stack information for the threads listed above:
===================================================
"Ruby-0-Thread-3: main.rb:1":
	at sun.misc.Unsafe.park(Native Method)
	- parking to wait for  <0x00000007b65b9a58> (a java.util.concurrent.locks.ReentrantLock$NonfairSync)
	at java.util.concurrent.locks.LockSupport.park(LockSupport.java:175)
	at java.util.concurrent.locks.AbstractQueuedSynchronizer.parkAndCheckInterrupt(AbstractQueuedSynchronizer.java:836)
	at java.util.concurrent.locks.AbstractQueuedSynchronizer.acquireQueued(AbstractQueuedSynchronizer.java:870)
	at java.util.concurrent.locks.AbstractQueuedSynchronizer.acquire(AbstractQueuedSynchronizer.java:1199)
	at java.util.concurrent.locks.ReentrantLock$NonfairSync.lock(ReentrantLock.java:209)
	at java.util.concurrent.locks.ReentrantLock.lock(ReentrantLock.java:285)

...	

"Ruby-0-Thread-2: main.rb:1":
	at org.jruby.RubyModule$Autoload.getConstant(RubyModule.java:3825)
	- waiting to lock <0x00000007b63ff810> (a java.lang.Object)
	at org.jruby.RubyModule.getAutoloadConstant(RubyModule.java:3621)
	at org.jruby.RubyModule.resolveUndefConstant(RubyModule.java:3108)
	at org.jruby.RubyModule.getConstantAtSpecial(RubyModule.java:2968)
	at org.jruby.RubyModule.defineOrGetClassUnder(RubyModule.java:1281)
	at org.jruby.ast.ClassNode.interpret(ClassNode.java:134)
	at org.jruby.ast.NewlineNode.interpret(NewlineNode.java:105)
	at org.jruby.ast.BlockNode.interpret(BlockNode.java:71)
	at org.jruby.evaluator.ASTInterpreter.INTERPRET_CLASS(ASTInterpreter.java:103)
	at org.jruby.evaluator.ASTInterpreter.evalClassDefinitionBody(ASTInterpreter.java:280)
	at org.jruby.ast.ModuleNode.interpret(ModuleNode.java:120)
	at org.jruby.ast.NewlineNode.interpret(NewlineNode.java:105)
	at org.jruby.ast.RootNode.interpret(RootNode.java:129)
	at org.jruby.evaluator.ASTInterpreter.INTERPRET_ROOT(ASTInterpreter.java:121)
	at org.jruby.Ruby.runInterpreter(Ruby.java:894)
	at org.jruby.Ruby.loadFile(Ruby.java:2860)	
	
...

another_class_base.rb

module DeadlockTest
  class AnotherClassBase
  end
end

class_a.rb

module DeadlockTest
  autoload :AnotherClassBase, 'another_class_base'

  class ClassA

    class AnotherClass < AnotherClassBase

    end
  end
end

deadlock_test.rb

module DeadlockTest
  autoload :ClassA, 'class_a'
end

main.rb

require 'thread'
require 'deadlock_test'

puts("Starting, JRuby version: #{JRUBY_VERSION}")
ta = Thread.new{
  puts("Thread A #{JRuby.reference(Thread.current).native_thread}\n")

  # This is not a usual thing for people to do but it can happen if an external component is loading
  # your ruby code
  # I first run into this problem in a storm topology (via RedStorm), which does
  # a similar thing. I'm sure there are other ways to reproduce the same problem
  # without a require but this
  require 'class_a'
}

tb = Thread.new{
  puts("Thread B #{JRuby.reference(Thread.current).native_thread}\n")

  o = DeadlockTest::ClassA.new
  puts('if you see this, try again :) ')
}

ta.join
tb.join