[ruby-core:117216] [Ruby master Bug#20346] FiberScheduler.unblock not called by Thread#join when Thread body contains Ractor.take

18 Mar 2024


      Issue #20346 has been reported by forthoney (Seong-Heon Jung).

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Bug #20346: FiberScheduler.unblock not called by Thread#join when Thread body contains Ractor.take
https://bugs.ruby-lang.org/issues/20346

* Author: forthoney (Seong-Heon Jung)
* Status: Open
* Backport: 3.0: UNKNOWN, 3.1: UNKNOWN, 3.2: UNKNOWN, 3.3: UNKNOWN
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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
```ruby
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`


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