Bug #15262
closedWeakRef::RefError for object that is still in use
Description
Given the following program:
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:
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