cats.effect.fiber: join and cancel

Join and Cancel on a fiber

When dealing with fibers you must be very careful not to cancel before joining, otherwise the join will leak.

Look at the following example:

for {
 fiber <- IO.sleep(2.seconds).start
 _ <- fiber.join.guarantee(putStrLn("canceled")).start
 _ <- IO.sleep(1.second) >> fiber.cancel
} yield ()

fiber.join will never terminate. You must be careful not to wait on a canceled fiber. The easiest solution is building a synchronize mechanism on top. An alternative is to build a higher level abstraction to avoid dealing with fiber like this await.

Cancelling a join action on a cancelled fiber works as expected:

for {
  fiber <- IO.never.start
  _ <- IO.race(fiber.join.guarantee(putStrLn("canceled")), IO.sleep(1.second) >> fiber.cancel)
 } yield ()

Oleg Pyzhcov has this article about fibers safety that I would recommend reading.

monadplus/fibers.md

Join and Cancel on a fiber