From: lars@...
Date: 2018-10-27T17:12:23+00:00
Subject: [ruby-core:89579] [Ruby trunk Bug#15262] GCing of object in use

Issue #15262 has been reported by larskanis (Lars Kanis).

----------------------------------------
Bug #15262: GCing of object in use
https://bugs.ruby-lang.org/issues/15262

* Author: larskanis (Lars Kanis)
* Status: Open
* Priority: Normal
* Assignee: 
* Target version: 
* ruby -v: ruby 2.6.0dev (2018-10-27 trunk 65390) [x86_64-linux]
* Backport: 2.3: UNKNOWN, 2.4: UNKNOWN, 2.5: UNKNOWN
----------------------------------------
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 works with a WeakRef, the Adder object should still be GC marked, since Adder.new doesn't return before all calls to start_adder finished.

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




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