Ruby

Bug #15262

Updated by nobu (Nobuyoshi Nakada) about 7 years ago

Given the following program: 

 ```ruby 
 require "weakref" 

 Thread.abort_on_exception = true 

 class Adder 
   def self.start_adder(obj) 
     obj.add 
   end 

   def initialize 
     @qu = Queue.new 
     @mutex = Mutex.new 
     @mutex.lock 
     begin 
       @count = 10 
       @count.times do 
         Thread.new(WeakRef.new(self), &self.class.method(:start_adder)) 
       end 
     ensure 
       @mutex.unlock 
     end 
     @qu.deq 
   end 

   def add 
     @mutex.lock 
     begin 
       @count-=1 
       if @count == 0 
         @qu.enq true 
       elsif @count < 0 
         raise "shouldn't happen" 
       end 
     ensure 
       @mutex.unlock 
     end 
   end 
 end 

 def test_adder 
   10.times.map do 
     Thread.new do 
       Adder.new 
     end 
   end.each(&:join) 
 end 

 100.times do 
   test_adder 
 end 
 ``` 

 ## Expected behaviour: 

 The program should simply execute without error. This is the case on JRuby but not on MRI. 

 ## Actual behavior: 

 The program stops with a probability of approximately 80% with the following error: 
 ``` 
 $ ruby -W2 adder-test.rb  
 #<Thread:0x0000562d5089fea8@adder-test.rb:6 run> terminated with exception (report_on_exception is true): 
 Traceback (most recent call last): 
 adder-test.rb:7:in `start_adder': Invalid Reference - probably recycled (WeakRef::RefError) 
 Traceback (most recent call last): 
 adder-test.rb:7:in `start_adder': Invalid Reference - probably recycled (WeakRef::RefError) 
 ``` 

 Although `start_adder` start_adder works with a `WeakRef`, WeakRef, the `Adder` Adder object should still be GC marked, since `Adder.new` Adder.new doesn't return before all calls to `start_adder` start_adder finished. 

 I verified this on ruby-trunk, but get mostly the same behavior on all older MRI versions. 

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