This is an issue made because I had to revert the PR (Fix deadlock when tearing down runtime during debugging #4352). I don't know if I am thick but I can also trivially deadlock with this:

def foo
  1
end


2.times do
  puts "T"
  Thread.new {
    a = []
    [:call, :c_call, :return, :c_return].each do |n|
      t = TracePoint.new(n) { |tp| p tp.event }
      t.enable
    end
    loop do
      sleep 0.01
      foo
    end
  }
end

sleep 0.5
exit!

I think it is because both threads need to receive an event callback but the event callback is generating an event callback and we deadlock? In any case I threw this second deadlock into this issue...