Rubinius: can't spawn a Thread from within an FFI::Function if the FFI::Function is called from another thread

callback.c

#include "callback.h"
#include <pthread.h>
#include <assert.h>
#include <stdlib.h>

static void *
s_thread_shim (void *args)
{
	assert (args);
	callback_fn* callback = (callback_fn *) args;
	printf("libcallback: calling callback at %p ...\n", callback);
	(* callback)();
	printf("libcallback: callback %p has returned.\n", callback);
	return NULL;
}

void
call_callback(callback_fn *callback)
{
	pthread_t thread;
	pthread_create (&thread, NULL, s_thread_shim, callback);
	pthread_detach (thread);
}

callback.h

#ifndef __CALLBACK_BUG_H_INCLUDED__
#define __CALLBACK_BUG_H_INCLUDED__

#include <stdio.h>
typedef void (callback_fn) (void);

void
call_callback(callback_fn *callback);
#endif

Makefile

libcallback.dylib: callback.h callback.c
	clang -dynamiclib -std=gnu99 callback.c -o $@

rbx_ffi-callback_spawn_thread.rb

require 'ffi'

module Callback
  extend ::FFI::Library

  lib_name = 'libcallback'
  ffi_lib "#{lib_name}.#{::FFI::Platform::LIBSUFFIX}"

  attach_function :call_callback, [:pointer], :void, blocking: true
end

ffi_function = FFI::Function.new(:void, [], blocking: true) do
  puts "BEGIN ffi_function"
  Thread.new do
    puts "hello from inner thread"
  end
  puts "END ffi_function"
end

Callback.call_callback(ffi_function)

sleep 1
__END__

###
# EXPECTED:
# Output on CRuby 2.2.4:
libcallback: calling callback at 0x10242b000 ...
BEGIN ffi_function
END ffi_function
libcallback: callback 0x10242b000 has returned.
hello from inner thread


###
# ACTUAL
# Output on Rubinius 2.8.5
libcallback: calling callback at 0x1088eb000 ...
BEGIN ffi_function
libcallback: callback 0x1088eb000 has returned.
#