racePair.scala

def racePair[F[_]: Concurrent, A, B](fa: F[A], fb: F[B]): F[Either[(Outcome[F, E, A], Fiber[F, E, B]), (Fiber[F, E, A], Outcome[F, E, B])]] =
  MonadCancel[F] uncancelable { poll =>
    for {
      fibADef <- Concurrent[F].deferred[Fiber[F, E, A]]
      fibBDef <- Concurrent[F].deferred[Fiber[F, E, B]]

      result <- Concurrent[F].deferred[Either[(Outcome[F, E, A], Fiber[F, E, B]), (Fiber[F, E, A], Outcome[F, E, B])]]

      fibA <- fa guarantee { oc =>
        fibBDef.get flatMap { fibB =>
          result.complete(Left((oc, fibB))).void
        }
      }

      fibB <- fb guarantee { oc =>
        fibADef.get flatMap { fibA =>
          result.complete(Right((fibA, oc))).void
        }
      }

      _ <- fibADef.complete(fibA)
      _ <- fibBDef.complete(fibB)

      back <- poll(result.get) onCancel {
        for {
          done <- Concurrent[F].deferred[Unit]
          _ <- fibA.cancel.guarantee(_ => done.complete(()))
          _ <- fibB.cancel.guarantee(_ => done.complete(()))
          _ <- done.get
        } yield ()
      }
    } yield back
  }