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Bug #15262

Updated by nobu (Nobuyoshi Nakada) 5 months 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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