Good! You mean this change right? Getting rid of those sleep is 
Maybe the segfault was coming from rescheduling a fiber in an invalid state, so it’s definitely better to use Channel for synchronization.
On that note, maybe you want the cancel channel to have a capacity of 1 (instead of 0) so the #cancel method will never block. I think right now it could in some race condition. The cancel.send will block until a receiver gets the message, but that is not what you want here.