[ruby-core:89643] [Ruby trunk Bug#15262] WeakRef::RefError for object that is still in use

[lars@greiz-reinsdorf.de]

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


@nobu I updated the bug report, so that it should be more understandable now, what's going wrong.

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Bug #15262: WeakRef::RefError for object that is still in use
https://bugs.ruby-lang.org/issues/15262#change-74683

* Author: larskanis (Lars Kanis)
* Status: Feedback
* 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, oid)
    obj.add
  end

  def initialize
    @qu = Queue.new
    count = 10
    count.times do
      Thread.new(WeakRef.new(self), object_id, &self.class.method(:start_adder))
    end
    count.times do
      @qu.pop
    end
  end

  def add
    @qu.push true
  end
end

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

1000.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% with the following error:
```
$ ruby -W2 adder-test.rb 
#<Thread:0x0000556e9e06f3f8@adder-test2.rb:6 run> terminated with exception (report_on_exception is true):
Traceback (most recent call last):
adder-test2.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 `Adder.new` exited. This is because `Adder.new` waits for completion of all Threads to have called `start_adder`.

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 the same behavior on all older MRI versions.


---Files--------------------------------
adder-test2.rb (476 Bytes)


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