Sharing an enumerator across threads

Question

I want to call a common enumerator from different threads. When I do the following,

enum = (0..1000).to_enum
t1 = Thread.new do
  p enum.next
  sleep(1)
end
t2 = Thread.new do
  p enum.next
  sleep(1)
end
t1.join
t2.join

it raises an error:

Fiber called across threads.

when enum is called from t2 after once being called from t1.

Why is Ruby designed to not allow an enumerator (or fiber) to be called across threads, and
Is there an alternative way to provide a similar function?

I am guessing that atomicity of an operation on an enumerator/fiber is relevant here, but am not fully sure. If that is the issue, then exclusively-locking the enumerator/fiber while in use shall solve the problem, and I don't know why calling an enumerator/fiber across threads is prohibited in general. If an alternative can be provided by using locking, that would satisfy my need.

Uri Agassi Uri Agassi · Accepted Answer · 2014-02-23T12:58:59

You can use Queue

queue = Queue.new
(0..1000).map(&queue.method(:push))

t1 = Thread.new do
  while !queue.empty?
    p queue.pop(true)
    sleep(0.1)
  end
end
t2 = Thread.new do
  while !queue.empty?
    p queue.pop(true)
    sleep(0.1)
  end
end
t1.join
t2.join

Sharing an enumerator across threads

1 Answers