in_root_fiber.rb

# frozen_string_literal: true

require "fiber" # needed for transfer

RootFiberException  = Class.new(Exception)
RootFiberTestSignal = Class.new(RootFiberException)

# Warning: this is only safe to call from the root fiber for a thread.
# Unfortunately, your code may not know if it is in the root fiber.  This will
# raise an exception in the root fiber of the current thread.  If you are in the
# root fiber, it will also catch that exception. If not, the root fiber will
# most likely crash, bringing the thread down with it!
#
# The trick is that, unlike Fiber#resume, Fiber#transfer doesn't set a
# "return_fiber" pointer. So when the fiber returns (or yields or raises), it
# doesn't necessarily go back the the fiber that created it or the fiber that
# transfered into it. Instead, it transfers to the root fiber.
def check_in_root_fiber!
  msg = "Another fiber raised this exception (#{rand})"
  test_fiber = Fiber.new { raise RootFiberTestSignal, msg; }
  test_fiber.transfer # th
rescue RootFiberTestSignal => ex
  raise ex unless ex.message == msg
  true
else
  raise RootFiberException, "in_root_fiber? was called from a non-root fiber"
end

check_in_root_fiber!  # => true


nonroot = Fiber.new do
  Fiber.new do
    Fiber.new do
      Fiber.new do
        check_in_root_fiber!
      end.resume
    end.resume
  end.resume
end

nonroot.resume

# ~> RootFiberTestSignal
# ~> Another fiber raised this exception (0.06201568006087321)
# ~>
# ~> /tmp/seeing_is_believing_temp_dir20201027-2846277-176v8hz/program.rb:18:in `block in check_in_root_fiber!'