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Bug #15262

Updated by larskanis (Lars Kanis) about 3 years ago

Given the following program: 

 ```ruby 
 require "weakref" 

 Thread.abort_on_exception = true 

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

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

   def add 
     @qu.push @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 

 1000.times 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 50% 80% with the following error: 
 ``` 
 $ ruby -W2 adder-test.rb  
 #<Thread:0x0000556e9e06f3f8@adder-test2.rb:6 #<Thread:0x0000562d5089fea8@adder-test.rb:6 run> terminated with exception (report_on_exception is true): 
 Traceback (most recent call last): 
 adder-test2.rb:7:in 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` works with a `WeakRef`, the `Adder` object should still be GC marked and therefore kept usable per `WeakRef` until marked, since `Adder.new` exited. This is because `Adder.new` waits for completion of doesn't return before all Threads calls to have called `start_adder`. `start_adder` finished. 

 If `Adder.start_adder` is changed to catch the `WeakRef` error like so: 

 ```ruby 
   def self.start_adder(obj, oid) 
     obj.add 
   rescue WeakRef::RefError 
   end 
 ``` 

 ... then the ruby VM detects a deadlock, which shows that `@qu.pop` is still executed within `initialize`. Therefore `WeakRef#add` should not raise a `WeakRef::RefError` to that point in time, but allow access to the object: 

 ``` 
 Traceback (most recent call last): 
         5: from adder-test2.rb:35:in `<main>' 
         4: from adder-test2.rb:35:in `times' 
         3: from adder-test2.rb:36:in `block in <main>' 
         2: from adder-test2.rb:32:in `test_adder' 
         1: from adder-test2.rb:32:in `each' 
 adder-test2.rb:32:in `join': No live threads left. Deadlock? (fatal) 
 2 threads, 2 sleeps current:0x000056520aa3cee0 main thread:0x000056520a5f2ff0 
 * #<Thread:0x000056520a644b48 sleep_forever> 
    rb_thread_t:0x000056520a5f2ff0 native:0x00007f7ccf535740 int:0 
    adder-test2.rb:32:in `join' 
    adder-test2.rb:32:in `each' 
    adder-test2.rb:32:in `test_adder' 
    adder-test2.rb:36:in `block in <main>' 
    adder-test2.rb:35:in `times' 
    adder-test2.rb:35:in `<main>' 
 * #<Thread:0x000056520a913040@adder-test2.rb:29 sleep_forever> 
    rb_thread_t:0x000056520a609fc0 native:0x00007f7ca54d4700 int:0 
     depended by: tb_thread_id:0x000056520a5f2ff0 
    adder-test2.rb:18:in `pop' 
    adder-test2.rb:18:in `block in initialize' 
    adder-test2.rb:17:in `times' 
    adder-test2.rb:17:in `initialize' 
    adder-test2.rb:30:in `new' 
    adder-test2.rb:30:in `block (2 levels) in test_adder' 
 ``` 

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

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