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List: ruby-core
Subject: [ruby-core:89658] [Ruby trunk Bug#15262] WeakRef::RefError for object that is still in use
From: lars () greiz-reinsdorf ! de
Date: 2018-10-31 10:07:27
Message-ID: redmine.journal-74700.20181031100725.55101057456768dd () ruby-lang ! org
[Download RAW message or body]
Issue #15262 has been updated by larskanis (Lars Kanis).
The reference error doesn't appear when `WeakRef` is replaced by `_id2ref` like so:
```ruby
def self.start_adder(oid)
ObjectSpace._id2ref(oid).add
end
```
So it appears to be a bug in `ObjectSpace::WeakMap`.
----------------------------------------
Bug #15262: WeakRef::RefError for object that is still in use
https://bugs.ruby-lang.org/issues/15262#change-74700
* 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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