Bug #20346: FiberScheduler.unblock not called by Thread#join when Thread body contains Ractor.take

• Status: Open
• Assignee: -
• Target version: -
• Backport: 3.0: UNKNOWN, 3.1: UNKNOWN, 3.2: UNKNOWN, 3.3: UNKNOWN

[forthoney (Seong-Heon Jung)]

When using a Ractor.take inside a different thread, Thread#join on the thread running Ractor.take fails to call FiberScheduler.unblock. The below code can replicate this behavior

require "async"
class RactorWrapper
  def initialize
    @ractor = Ractor.new do
      Ractor.recv # Ractor doesn't start until explicitly told to
      # Do some calculations
      fib = ->(x) { x < 2 ? 1 : fib.call(x - 1) + fib.call(x - 2) }
      fib.call(20)
    end
  end
  
  def take_async
    @ractor.send(nil)
    Thread.new { @ractor.take }.join.value
  end
end

Async do |task|
  10000.times do |i|
    task.async do
      RactorWrapper.new.take_async
      puts i
    end
  end
end

The above code deadlocks, and when we leave a debugging print statement inside of Async's scheduler's block and unblock method, we can confirm that we only call Scheduler.block, and never Scheduler.unblock