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CyclicBarrier.scala
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CyclicBarrier.scala
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/*
* Copyright 2020-2023 Typelevel
*
* Licensed under the Apache License, Version 2.0 (the "License");
* you may not use this file except in compliance with the License.
* You may obtain a copy of the License at
*
* http://www.apache.org/licenses/LICENSE-2.0
*
* Unless required by applicable law or agreed to in writing, software
* distributed under the License is distributed on an "AS IS" BASIS,
* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
* See the License for the specific language governing permissions and
* limitations under the License.
*/
package cats.effect.std
import cats.effect.kernel.{Deferred, GenConcurrent}
import cats.effect.kernel.syntax.all._
import cats.syntax.all._
import cats.~>
/**
* A synchronization abstraction that allows a set of fibers to wait until they all reach a
* certain point.
*
* A cyclic barrier is initialized with a positive integer capacity n and a fiber waits by
* calling [[await]], at which point it is semantically blocked until a total of n fibers are
* blocked on the same cyclic barrier.
*
* At this point all the fibers are unblocked and the cyclic barrier is reset, allowing it to be
* used again.
*/
abstract class CyclicBarrier[F[_]] { self =>
/**
* Possibly semantically block until the cyclic barrier is full
*/
def await: F[Unit]
/**
* Modifies the context in which this cyclic barrier is executed using the natural
* transformation `f`.
*
* @return
* a cyclic barrier in the new context obtained by mapping the current one using the natural
* transformation `f`
*/
def mapK[G[_]](f: F ~> G): CyclicBarrier[G] =
new CyclicBarrier[G] {
def await: G[Unit] = f(self.await)
}
}
object CyclicBarrier {
def apply[F[_]](capacity: Int)(implicit F: GenConcurrent[F, _]): F[CyclicBarrier[F]] = {
if (capacity < 1)
throw new IllegalArgumentException(
s"Cyclic barrier constructed with capacity $capacity. Must be > 0")
case class State(awaiting: Int, epoch: Long, unblock: Deferred[F, Unit])
F.deferred[Unit].map(State(capacity, 0, _)).flatMap(F.ref).map { state =>
new CyclicBarrier[F] {
val await: F[Unit] =
F.deferred[Unit].flatMap { gate =>
state.flatModifyFull {
case (poll, State(awaiting, epoch, unblock)) =>
val awaitingNow = awaiting - 1
if (awaitingNow == 0)
State(capacity, epoch + 1, gate) -> unblock.complete(()).void
else {
val newState = State(awaitingNow, epoch, unblock)
// reincrement count if this await gets canceled,
// but only if the barrier hasn't reset in the meantime
val cleanup = state.update { s =>
if (s.epoch == epoch) s.copy(awaiting = s.awaiting + 1)
else s
}
newState -> poll(unblock.get).onCancel(cleanup)
}
}
}
}
}
}
}