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Queue.scala
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Queue.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
package effect
package std
import cats.effect.kernel.{Async, Cont, Deferred, GenConcurrent, MonadCancelThrow, Poll, Ref}
import cats.effect.kernel.syntax.all._
import cats.syntax.all._
import scala.annotation.tailrec
import scala.collection.immutable.{Queue => ScalaQueue}
import scala.collection.mutable.ListBuffer
import java.util.concurrent.atomic.{AtomicLong, AtomicLongArray}
/**
* A purely functional, concurrent data structure which allows insertion and retrieval of
* elements of type `A` in a first-in-first-out (FIFO) manner.
*
* Depending on the type of queue constructed, the [[Queue#offer]] operation can block
* semantically until sufficient capacity in the queue becomes available.
*
* The [[Queue#take]] operation semantically blocks when the queue is empty.
*
* The [[Queue#tryOffer]] and [[Queue#tryTake]] allow for usecases which want to avoid fiber
* blocking a fiber.
*/
abstract class Queue[F[_], A] extends QueueSource[F, A] with QueueSink[F, A] { self =>
/**
* Modifies the context in which this queue is executed using the natural transformation `f`.
*
* @return
* a queue in the new context obtained by mapping the current one using `f`
*/
def mapK[G[_]](f: F ~> G): Queue[G, A] =
new Queue[G, A] {
def offer(a: A): G[Unit] = f(self.offer(a))
def tryOffer(a: A): G[Boolean] = f(self.tryOffer(a))
def size: G[Int] = f(self.size)
val take: G[A] = f(self.take)
val tryTake: G[Option[A]] = f(self.tryTake)
}
}
object Queue {
/**
* Constructs an empty, bounded queue holding up to `capacity` elements for `F` data types
* that are [[cats.effect.kernel.GenConcurrent]]. When the queue is full (contains exactly
* `capacity` elements), every next [[Queue#offer]] will be backpressured (i.e. the
* [[Queue#offer]] blocks semantically).
*
* @param capacity
* the maximum capacity of the queue
* @return
* an empty, bounded queue
*/
def bounded[F[_], A](capacity: Int)(implicit F: GenConcurrent[F, _]): F[Queue[F, A]] = {
assertNonNegative(capacity)
// async queue can't handle capacity == 1 and allocates eagerly, so cap at 64k
if (1 < capacity && capacity < Short.MaxValue.toInt * 2) {
F match {
case f0: Async[F] =>
boundedForAsync[F, A](capacity)(f0)
case _ =>
boundedForConcurrent[F, A](capacity)
}
} else if (capacity > 0) {
boundedForConcurrent[F, A](capacity)
} else {
synchronous[F, A]
}
}
private[effect] def boundedForConcurrent[F[_], A](capacity: Int)(
implicit F: GenConcurrent[F, _]): F[Queue[F, A]] =
F.ref(State.empty[F, A]).map(new BoundedQueue(capacity, _))
private[effect] def boundedForAsync[F[_], A](capacity: Int)(
implicit F: Async[F]): F[Queue[F, A]] =
F.delay(new BoundedAsyncQueue(capacity))
private[effect] def unboundedForConcurrent[F[_], A](
implicit F: GenConcurrent[F, _]): F[Queue[F, A]] =
boundedForConcurrent[F, A](Int.MaxValue)
private[effect] def unboundedForAsync[F[_], A](implicit F: Async[F]): F[Queue[F, A]] =
F.delay(new UnboundedAsyncQueue())
/**
* Constructs a queue through which a single element can pass only in the case when there are
* at least one taking fiber and at least one offering fiber for `F` data types that are
* [[cats.effect.kernel.GenConcurrent]]. Both [[Queue#offer]] and [[Queue#take]] semantically
* block until there is a fiber executing the opposite action, at which point both fibers are
* freed.
*
* @return
* a synchronous queue
*/
def synchronous[F[_], A](implicit F: GenConcurrent[F, _]): F[Queue[F, A]] =
F.ref(SyncState.empty[F, A]).map(new Synchronous(_))
/**
* Constructs an empty, unbounded queue for `F` data types that are
* [[cats.effect.kernel.GenConcurrent]]. [[Queue#offer]] never blocks semantically, as there
* is always spare capacity in the queue.
*
* @return
* an empty, unbounded queue
*/
def unbounded[F[_], A](implicit F: GenConcurrent[F, _]): F[Queue[F, A]] =
F match {
case f0: Async[F] =>
unboundedForAsync(f0)
case _ =>
unboundedForConcurrent
}
/**
* Constructs an empty, bounded, dropping queue holding up to `capacity` elements for `F` data
* types that are [[cats.effect.kernel.GenConcurrent]]. When the queue is full (contains
* exactly `capacity` elements), every next [[Queue#offer]] will be ignored, i.e. no other
* elements can be enqueued until there is sufficient capacity in the queue, and the offer
* effect itself will not semantically block.
*
* @param capacity
* the maximum capacity of the queue
* @return
* an empty, bounded, dropping queue
*/
def dropping[F[_], A](capacity: Int)(implicit F: GenConcurrent[F, _]): F[Queue[F, A]] = {
assertPositive(capacity, "Dropping")
F.ref(State.empty[F, A]).map(new DroppingQueue(capacity, _))
}
/**
* Constructs an empty, bounded, circular buffer queue holding up to `capacity` elements for
* `F` data types that are [[cats.effect.kernel.GenConcurrent]]. The queue always keeps at
* most `capacity` number of elements, with the oldest element in the queue always being
* dropped in favor of a new elements arriving in the queue, and the offer effect itself will
* not semantically block.
*
* @param capacity
* the maximum capacity of the queue
* @return
* an empty, bounded, sliding queue
*/
def circularBuffer[F[_], A](capacity: Int)(
implicit F: GenConcurrent[F, _]): F[Queue[F, A]] = {
assertPositive(capacity, "CircularBuffer")
F.ref(State.empty[F, A]).map(new CircularBufferQueue(capacity, _))
}
private def assertNonNegative(capacity: Int): Unit =
if (capacity < 0)
throw new IllegalArgumentException(
s"Bounded queue capacity must be non-negative, was: $capacity")
else ()
private def assertPositive(capacity: Int, name: String): Unit =
if (capacity <= 0)
throw new IllegalArgumentException(
s"$name queue capacity must be positive, was: $capacity")
else ()
private final class Synchronous[F[_], A](stateR: Ref[F, SyncState[F, A]])(
implicit F: GenConcurrent[F, _])
extends Queue[F, A] {
def offer(a: A): F[Unit] =
F.deferred[Deferred[F, A]] flatMap { latch =>
F uncancelable { poll =>
val cleanupF = stateR modify {
case SyncState(offerers, takers) =>
val fallback = latch.tryGet flatMap {
case Some(offerer) => offerer.complete(a).void
case None => F.unit
}
SyncState(offerers.filter(_ ne latch), takers) -> fallback
}
val modificationF = stateR modify {
case SyncState(offerers, takers) if takers.nonEmpty =>
val (taker, tail) = takers.dequeue
SyncState(offerers, tail) -> taker.complete(a).void
case SyncState(offerers, takers) =>
SyncState(offerers.enqueue(latch), takers) ->
poll(latch.get).onCancel(cleanupF.flatten).flatMap(_.complete(a).void)
}
modificationF.flatten
}
}
def tryOffer(a: A): F[Boolean] = {
val modificationF = stateR modify {
case SyncState(offerers, takers) if takers.nonEmpty =>
val (taker, tail) = takers.dequeue
SyncState(offerers, tail) -> taker.complete(a).as(true)
case st =>
st -> F.pure(false)
}
modificationF.flatten.uncancelable
}
val take: F[A] =
F.deferred[A] flatMap { latch =>
val modificationF = stateR modify {
case SyncState(offerers, takers) if offerers.nonEmpty =>
val (offerer, tail) = offerers.dequeue
SyncState(tail, takers) -> offerer.complete(latch).void
case SyncState(offerers, takers) =>
SyncState(offerers, takers.enqueue(latch)) -> F.unit
}
val cleanupF = stateR update {
case SyncState(offerers, takers) =>
SyncState(offerers, takers.filter(_ ne latch))
}
F uncancelable { poll => modificationF.flatten *> poll(latch.get).onCancel(cleanupF) }
}
val tryTake: F[Option[A]] = {
val modificationF = stateR modify {
case SyncState(offerers, takers) if offerers.nonEmpty =>
val (offerer, tail) = offerers.dequeue
SyncState(tail, takers) -> F
.deferred[A]
.flatMap(d => offerer.complete(d) *> d.get.map(_.some))
case st =>
st -> none[A].pure[F]
}
modificationF.flatten.uncancelable
}
val size: F[Int] = F.pure(0)
}
private final case class SyncState[F[_], A](
offerers: ScalaQueue[Deferred[F, Deferred[F, A]]],
takers: ScalaQueue[Deferred[F, A]])
private object SyncState {
def empty[F[_], A]: SyncState[F, A] = SyncState(ScalaQueue(), ScalaQueue())
}
private sealed abstract class AbstractQueue[F[_], A](
capacity: Int,
state: Ref[F, State[F, A]]
)(implicit F: GenConcurrent[F, _])
extends Queue[F, A] {
protected def onOfferNoCapacity(
s: State[F, A],
a: A,
offerer: Deferred[F, Unit],
poll: Poll[F],
recurse: => F[Unit]): (State[F, A], F[Unit])
protected def onTryOfferNoCapacity(s: State[F, A], a: A): (State[F, A], F[Boolean])
def offer(a: A): F[Unit] =
F uncancelable { poll =>
F.deferred[Unit] flatMap { offerer =>
val modificationF = state modify {
case State(queue, size, takers, offerers) if takers.nonEmpty =>
val (taker, rest) = takers.dequeue
State(queue.enqueue(a), size + 1, rest, offerers) -> taker.complete(()).void
case State(queue, size, takers, offerers) if size < capacity =>
State(queue.enqueue(a), size + 1, takers, offerers) -> F.unit
case s =>
onOfferNoCapacity(s, a, offerer, poll, offer(a))
}
modificationF.flatten
}
}
def tryOffer(a: A): F[Boolean] =
state
.modify {
case State(queue, size, takers, offerers) if takers.nonEmpty =>
val (taker, rest) = takers.dequeue
State(queue.enqueue(a), size + 1, rest, offerers) -> taker.complete(()).as(true)
case State(queue, size, takers, offerers) if size < capacity =>
State(queue.enqueue(a), size + 1, takers, offerers) -> F.pure(true)
case s =>
onTryOfferNoCapacity(s, a)
}
.flatten
.uncancelable
val take: F[A] =
F.uncancelable { poll =>
F.deferred[Unit] flatMap { taker =>
val modificationF = state modify {
case State(queue, size, takers, offerers) if queue.nonEmpty && offerers.isEmpty =>
val (a, rest) = queue.dequeue
State(rest, size - 1, takers, offerers) -> F.pure(a)
case State(queue, size, takers, offerers) if queue.nonEmpty =>
val (a, rest) = queue.dequeue
// if we haven't made enough space for a new offerer, don't disturb the water
if (size - 1 < capacity) {
val (release, tail) = offerers.dequeue
State(rest, size - 1, takers, tail) -> release.complete(()).as(a)
} else {
State(rest, size - 1, takers, offerers) -> F.pure(a)
}
case State(queue, size, takers, offerers) =>
/*
* In the case that we're notified as we're canceled and the cancelation wins the
* race, we need to not only remove ourselves from the queue but also grab the next
* in line and notify *them*. Since this scenario cannot be detected reliably, we
* just unconditionally notify. If the notification was spurious, the taker we notify
* will end up going to the back of the queue, violating fairness.
*/
val cleanup = state modify { s =>
val takers2 = s.takers.filter(_ ne taker)
if (takers2.isEmpty) {
s.copy(takers = takers2) -> F.unit
} else {
val (taker, rest) = takers2.dequeue
s.copy(takers = rest) -> taker.complete(()).void
}
}
val await = poll(taker.get).onCancel(cleanup.flatten) *>
poll(take).onCancel(notifyNextTaker.flatten)
val (fulfill, offerers2) = if (offerers.isEmpty) {
(await, offerers)
} else {
val (release, rest) = offerers.dequeue
(release.complete(()) *> await, rest)
}
// it would be safe to throw cleanup around *both* polls, but we micro-optimize slightly here
State(queue, size, takers.enqueue(taker), offerers2) -> fulfill
}
modificationF.flatten
}
}
val tryTake: F[Option[A]] =
state
.modify {
case State(queue, size, takers, offerers) if queue.nonEmpty && offerers.isEmpty =>
val (a, rest) = queue.dequeue
State(rest, size - 1, takers, offerers) -> F.pure(a.some)
case State(queue, size, takers, offerers) if queue.nonEmpty =>
val (a, rest) = queue.dequeue
val (release, tail) = offerers.dequeue
State(rest, size - 1, takers, tail) -> release.complete(()).as(a.some)
case s =>
s -> F.pure(none[A])
}
.flatten
.uncancelable
val size: F[Int] = state.get.map(_.size)
private[this] val notifyNextTaker = state modify { s =>
if (s.takers.isEmpty) {
s -> F.unit
} else {
val (taker, rest) = s.takers.dequeue
s.copy(takers = rest) -> taker.complete(()).void
}
}
}
private final class BoundedQueue[F[_], A](capacity: Int, state: Ref[F, State[F, A]])(
implicit F: GenConcurrent[F, _]
) extends AbstractQueue(capacity, state) {
require(capacity > 0)
protected def onOfferNoCapacity(
s: State[F, A],
a: A,
offerer: Deferred[F, Unit],
poll: Poll[F],
recurse: => F[Unit]): (State[F, A], F[Unit]) = {
val State(queue, size, takers, offerers) = s
val cleanup = state modify { s =>
val offerers2 = s.offerers.filter(_ ne offerer)
if (offerers2.isEmpty) {
s.copy(offerers = offerers2) -> F.unit
} else {
val (offerer, rest) = offerers2.dequeue
s.copy(offerers = rest) -> offerer.complete(()).void
}
}
State(queue, size, takers, offerers.enqueue(offerer)) ->
(poll(offerer.get) *> poll(recurse)).onCancel(cleanup.flatten)
}
protected def onTryOfferNoCapacity(s: State[F, A], a: A): (State[F, A], F[Boolean]) =
s -> F.pure(false)
}
private final class DroppingQueue[F[_], A](capacity: Int, state: Ref[F, State[F, A]])(
implicit F: GenConcurrent[F, _]
) extends AbstractQueue(capacity, state) {
protected def onOfferNoCapacity(
s: State[F, A],
a: A,
offerer: Deferred[F, Unit],
poll: Poll[F],
recurse: => F[Unit]
): (State[F, A], F[Unit]) =
s -> F.unit
protected def onTryOfferNoCapacity(s: State[F, A], a: A): (State[F, A], F[Boolean]) =
s -> F.pure(false)
}
private final class CircularBufferQueue[F[_], A](capacity: Int, state: Ref[F, State[F, A]])(
implicit F: GenConcurrent[F, _]
) extends AbstractQueue(capacity, state) {
protected def onOfferNoCapacity(
s: State[F, A],
a: A,
offerer: Deferred[F, Unit],
poll: Poll[F],
recurse: => F[Unit]): (State[F, A], F[Unit]) = {
// dotty doesn't like cats map on tuples
val (ns, fb) = onTryOfferNoCapacity(s, a)
(ns, fb.void)
}
protected def onTryOfferNoCapacity(s: State[F, A], a: A): (State[F, A], F[Boolean]) = {
val State(queue, size, takers, offerers) = s
val (_, rest) = queue.dequeue
State(rest.enqueue(a), size, takers, offerers) -> F.pure(true)
}
}
private final case class State[F[_], A](
queue: ScalaQueue[A],
size: Int,
takers: ScalaQueue[Deferred[F, Unit]],
offerers: ScalaQueue[Deferred[F, Unit]])
private object State {
def empty[F[_], A]: State[F, A] =
State(ScalaQueue.empty, 0, ScalaQueue.empty, ScalaQueue.empty)
}
private val EitherUnit: Either[Nothing, Unit] = Right(())
/*
* Does not correctly handle bound = 0 because take waiters are async[Unit]
*/
private final class BoundedAsyncQueue[F[_], A](capacity: Int)(implicit F: Async[F])
extends Queue[F, A] {
require(capacity > 1)
private[this] val buffer = new UnsafeBounded[A](capacity)
private[this] val takers = new UnsafeUnbounded[Either[Throwable, Unit] => Unit]()
private[this] val offerers = new UnsafeUnbounded[Either[Throwable, Unit] => Unit]()
private[this] val FailureSignal = cats.effect.std.FailureSignal // prefetch
// private[this] val takers = new ConcurrentLinkedQueue[AtomicReference[Either[Throwable, Unit] => Unit]]()
// private[this] val offerers = new ConcurrentLinkedQueue[AtomicReference[Either[Throwable, Unit] => Unit]]()
def offer(a: A): F[Unit] =
F uncancelable { poll =>
F defer {
try {
// attempt to put into the buffer; if the buffer is full, it will raise an exception
buffer.put(a)
// println(s"offered: size = ${buffer.size()}")
// we successfully put, if there are any takers, grab the first one and wake it up
notifyOne(takers)
F.unit
} catch {
case FailureSignal =>
// capture whether or not we were successful in our retry
var succeeded = false
// a latch blocking until some taker notifies us
val wait = F.async[Unit] { k =>
F delay {
// add ourselves to the listeners queue
val clear = offerers.put(k)
try {
// now that we're listening, re-attempt putting
buffer.put(a)
// it worked! clear ourselves out of the queue
clear()
// our retry succeeded
succeeded = true
// manually complete our own callback
// note that we could have a race condition here where we're already completed
// async will deduplicate these calls for us
// additionally, the continuation (below) is held until the registration completes
k(EitherUnit)
// we *might* have negated a notification by succeeding here
// unnecessary wake-ups are mostly harmless (only slight fairness loss)
notifyOne(offerers)
// technically it's possible to already have waiting takers. notify one of them
notifyOne(takers)
// we're immediately complete, so no point in creating a finalizer
None
} catch {
case FailureSignal =>
// our retry failed, meaning the queue is still full and we're listening, so suspend
// println(s"failed offer size = ${buffer.size()}")
Some(F.delay(clear()))
}
}
}
val notifyAnyway = F delay {
// we might have been awakened and canceled simultaneously
// try waking up another offerer just in case
notifyOne(offerers)
}
// suspend until the buffer put can succeed
// if succeeded is true then we've *already* put
// if it's false, then some taker woke us up, so race the retry with other offers
(poll(wait) *> F.defer(if (succeeded) F.unit else poll(offer(a))))
.onCancel(notifyAnyway)
}
}
}
def tryOffer(a: A): F[Boolean] = F delay {
try {
buffer.put(a)
notifyOne(takers)
true
} catch {
case FailureSignal =>
false
}
}
val size: F[Int] = F.delay(buffer.size())
val take: F[A] =
F uncancelable { poll =>
F defer {
try {
// attempt to take from the buffer. if it's empty, this will raise an exception
val result = buffer.take()
// println(s"took: size = ${buffer.size()}")
// still alive! notify an offerer that there's some space
notifyOne(offerers)
F.pure(result)
} catch {
case FailureSignal =>
// buffer was empty
// capture the fact that our retry succeeded and the value we were able to take
var received = false
var result: A = null.asInstanceOf[A]
// a latch to block until some offerer wakes us up
// we need to use cont to avoid calling `get` on the double-check
val wait = F.cont[Unit, Unit](new Cont[F, Unit, Unit] {
override def apply[G[_]](implicit G: MonadCancelThrow[G]) = { (k, get, lift) =>
G uncancelable { poll =>
val lifted = lift {
F delay {
// register ourselves as a listener for offers
val clear = takers.put(k)
try {
// now that we're registered, retry the take
result = buffer.take()
// it worked! clear out our listener
clear()
// we got a result, so received should be true now
received = true
// we *might* have negated a notification by succeeding here
// unnecessary wake-ups are mostly harmless (only slight fairness loss)
notifyOne(takers)
// it's technically possible to already have queued offerers. wake up one of them
notifyOne(offerers)
// we skip `get` here because we already have a value
// this is the thing that `async` doesn't allow us to do
G.unit
} catch {
case FailureSignal =>
// println(s"failed take size = ${buffer.size()}")
// our retry failed, we're registered as a listener, so suspend
poll(get).onCancel(lift(F.delay(clear())))
}
}
}
lifted.flatten
}
}
})
val notifyAnyway = F delay {
// we might have been awakened and canceled simultaneously
// try waking up another taker just in case
notifyOne(takers)
}
// suspend until an offerer wakes us or our retry succeeds, then return a result
(poll(wait) *> F.defer(if (received) F.pure(result) else poll(take)))
.onCancel(notifyAnyway)
}
}
}
val tryTake: F[Option[A]] = F delay {
try {
val back = buffer.take()
notifyOne(offerers)
Some(back)
} catch {
case FailureSignal =>
None
}
}
override def tryTakeN(limit: Option[Int])(implicit F0: Monad[F]): F[List[A]] = {
QueueSource.assertMaxNPositive(limit)
F delay {
val _ = F0
val back = buffer.drain(limit.getOrElse(Int.MaxValue))
@tailrec
def loop(i: Int): Unit = {
if (i >= 0) {
var took = false
val f =
try {
val back = offerers.take()
took = true
back
} catch {
case FailureSignal => null
}
if (took) {
if (f != null) {
f(EitherUnit)
loop(i - 1)
} else {
loop(i)
}
}
}
}
// notify up to back.length offerers
loop(back.length)
back
}
}
def debug(): Unit = {
println(s"buffer: ${buffer.debug()}")
// println(s"takers: ${takers.debug()}")
// println(s"offerers: ${offerers.debug()}")
}
// TODO could optimize notifications by checking if buffer is completely empty on put
@tailrec
private[this] def notifyOne(
waiters: UnsafeUnbounded[Either[Throwable, Unit] => Unit]): Unit = {
// capture whether or not we should loop (structured in this way to avoid nested try/catch, which has a performance cost)
val retry =
try {
val f =
waiters
.take() // try to take the first waiter; if there are none, raise an exception
// we didn't get an exception, but the waiter may have been removed due to cancelation
if (f == null) {
// it was removed! loop and retry
true
} else {
// it wasn't removed, so invoke it
// taker may have already been invoked due to the double-check pattern, in which case this will be idempotent
f(EitherUnit)
// don't retry
false
}
} catch {
// there are no takers, so don't notify anything
case FailureSignal => false
}
if (retry) {
// loop outside of try/catch
notifyOne(waiters)
}
}
}
private final class UnboundedAsyncQueue[F[_], A]()(implicit F: Async[F]) extends Queue[F, A] {
private[this] val buffer = new UnsafeUnbounded[A]()
private[this] val takers = new UnsafeUnbounded[Either[Throwable, Unit] => Unit]()
private[this] val FailureSignal = cats.effect.std.FailureSignal // prefetch
def offer(a: A): F[Unit] = F delay {
buffer.put(a)
notifyOne()
}
def tryOffer(a: A): F[Boolean] = F delay {
buffer.put(a)
notifyOne()
true
}
val size: F[Int] = F.delay(buffer.size())
val take: F[A] = F defer {
try {
// attempt to take from the buffer. if it's empty, this will raise an exception
F.pure(buffer.take())
} catch {
case FailureSignal =>
// buffer was empty
// capture the fact that our retry succeeded and the value we were able to take
var received = false
var result: A = null.asInstanceOf[A]
// a latch to block until some offerer wakes us up
val wait = F.async[Unit] { k =>
F delay {
// register ourselves as a listener for offers
val clear = takers.put(k)
try {
// now that we're registered, retry the take
result = buffer.take()
// it worked! clear out our listener
clear()
// we got a result, so received should be true now
received = true
// complete our own callback. see notes in offer about raced redundant completion
k(EitherUnit)
// we *might* have negated a notification by succeeding here
// unnecessary wake-ups are mostly harmless (only slight fairness loss)
notifyOne()
// don't bother with a finalizer since we're already complete
None
} catch {
case FailureSignal =>
// println(s"failed take size = ${buffer.size()}")
// our retry failed, we're registered as a listener, so suspend
Some(F.delay(clear()))
}
}
}
// suspend until an offerer wakes us or our retry succeeds, then return a result
wait *> F.defer(if (received) F.pure(result) else take)
}
}
val tryTake: F[Option[A]] = F delay {
try {
Some(buffer.take())
} catch {
case FailureSignal =>
None
}
}
@tailrec
private[this] def notifyOne(): Unit = {
// capture whether or not we should loop (structured in this way to avoid nested try/catch, which has a performance cost)
val retry =
try {
val f =
takers.take() // try to take the first waiter; if there are none, raise an exception
// we didn't get an exception, but the waiter may have been removed due to cancelation
if (f == null) {
// it was removed! loop and retry
true
} else {
// it wasn't removed, so invoke it
// taker may have already been invoked due to the double-check pattern, in which case this will be idempotent
f(EitherUnit)
// don't retry
false
}
} catch {
// there are no takers, so don't notify anything
case FailureSignal => false
}
if (retry) {
// loop outside of try/catch
notifyOne()
}
}
}
// ported with love from https://github.com/JCTools/JCTools/blob/master/jctools-core/src/main/java/org/jctools/queues/MpmcArrayQueue.java
private[effect] final class UnsafeBounded[A](bound: Int) {
require(bound > 1)
private[this] val buffer = new Array[AnyRef](bound)
private[this] val sequenceBuffer = new AtomicLongArray(bound)
private[this] val head = new AtomicLong(0)
private[this] val tail = new AtomicLong(0)
private[this] val LookAheadStep = Math.max(2, Math.min(bound / 4, 4096)) // TODO tunable
private[this] val FailureSignal = cats.effect.std.FailureSignal // prefetch
0.until(bound).foreach(i => sequenceBuffer.set(i, i.toLong))
def debug(): String = buffer.mkString("[", ", ", "]")
@tailrec
def size(): Int = {
val before = head.get()
val currentTail = tail.get()
val after = head.get()
if (before == after) {
val size = currentTail - after
if (size < 0)
0
else
size.toInt
} else {
size()
}
}
def put(data: A): Unit = {
@tailrec
def loop(currentHead: Long): Long = {
val currentTail = tail.get()
val seq = sequenceBuffer.get(project(currentTail))
if (seq < currentTail) {
if (currentTail - bound >= currentHead) {
val currentHead2 = head.get()
if (currentTail - bound >= currentHead2)
throw FailureSignal
else
loop(currentHead2)
} else {
loop(currentHead)
}
} else {
if (seq == currentTail && tail.compareAndSet(currentTail, currentTail + 1))
currentTail
else
loop(currentHead)
}
}
val currentTail = loop(Long.MinValue)
buffer(project(currentTail)) = data.asInstanceOf[AnyRef]
sequenceBuffer.incrementAndGet(project(currentTail))
()
}
def take(): A = {
@tailrec
def loop(currentTail: Long): Long = {
val currentHead = head.get()
val seq = sequenceBuffer.get(project(currentHead))
if (seq < currentHead + 1) {
if (currentHead >= currentTail) {
val currentTail2 = tail.get()
if (currentHead == currentTail2)
throw FailureSignal
else
loop(currentTail2)
} else {
loop(currentTail)
}
} else {
if (seq == currentHead + 1 && head.compareAndSet(currentHead, currentHead + 1))
currentHead
else
loop(currentTail)
}
}
val currentHead = loop(-1)
val back = buffer(project(currentHead)).asInstanceOf[A]
buffer(project(currentHead)) = null
sequenceBuffer.set(project(currentHead), currentHead + bound)
back
}
def drain(limit: Int): List[A] = {
val back = new ListBuffer[A]()
@tailrec
def loopOne(consumed: Int): Unit = {
if (consumed < limit) {
val next =
try {
back += take()
true
} catch {
case FailureSignal => false
}
if (next) {
loopOne(consumed + 1)
}
}
}
val maxLookAheadStep = Math.min(LookAheadStep, limit)
@tailrec
def loopMany(consumed: Int): Unit = {
if (consumed < limit) {
val remaining = limit - consumed
val step = Math.min(remaining, maxLookAheadStep)
val currentHead = head.get()
val lookAheadIndex = currentHead + step - 1