Ruby

The following program will fail with FiberError, and is difficult to properly clean up:

root_fiber = Fiber.current f1 = Fiber.new do root_fiber.transfer end f2 = Fiber.new do f1.resume end f2.transfer f2.raise("error") # => `raise': attempt to transfer to a resuming fiber (FiberError) 

This program deliberately set's up a scenario where f2 is resuming f1. Trying to raise an exception on f2 is impossible, because the only way control flow can return to it, is when f1 yields or exits.

We can avoid this problem, by raising the exception on f1, and we can do this automatically using the following logic:

static VALUE fiber_raise(rb_fiber_t *fiber, VALUE exception) { // Add this recursive step: if (fiber->resuming_fiber) { return fiber_raise(fiber->resuming_fiber, exception); } // Existing code ... else if (FIBER_SUSPENDED_P(fiber) && !fiber->yielding) { return fiber_transfer_kw(fiber, -1, &exception, RB_NO_KEYWORDS); } else { return fiber_resume_kw(fiber, -1, &exception, RB_NO_KEYWORDS); } } 

This makes Fiber#raise much more robust and useful for the purpose of stopping fibers, without knowing exactly what they are doing.