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Coroutines support libraries for Kotlin
/*
* Copyright 2016-2019 JetBrains s.r.o. Use of this source code is governed by the Apache 2.0 license.
*/
package kotlinx.coroutines.channels
import kotlinx.coroutines.*
import kotlinx.coroutines.internal.*
import kotlinx.coroutines.selects.*
import kotlin.jvm.*
import kotlin.math.*
/**
* Channel with array buffer of a fixed [capacity].
* Sender suspends only when buffer is full and receiver suspends only when buffer is empty.
*
* This channel is created by `Channel(capacity)` factory function invocation.
*
* This implementation uses lock to protect the buffer, which is held only during very short buffer-update operations.
* The lists of suspended senders or receivers are lock-free.
**/
internal open class ArrayChannel(
/**
* Buffer capacity.
*/
val capacity: Int
) : AbstractChannel() {
init {
require(capacity >= 1) { "ArrayChannel capacity must be at least 1, but $capacity was specified" }
}
private val lock = ReentrantLock()
/*
* Guarded by lock.
* Allocate minimum of capacity and 16 to avoid excess memory pressure for large channels when it's not necessary.
*/
private var buffer: Array = arrayOfNulls(min(capacity, 8))
private var head: Int = 0
@Volatile
private var size: Int = 0 // Invariant: size <= capacity
protected final override val isBufferAlwaysEmpty: Boolean get() = false
protected final override val isBufferEmpty: Boolean get() = size == 0
protected final override val isBufferAlwaysFull: Boolean get() = false
protected final override val isBufferFull: Boolean get() = size == capacity
// result is `OFFER_SUCCESS | OFFER_FAILED | Closed`
protected override fun offerInternal(element: E): Any {
var receive: ReceiveOrClosed? = null
var token: Any? = null
lock.withLock {
val size = this.size
closedForSend?.let { return it }
if (size < capacity) {
// tentatively put element to buffer
this.size = size + 1 // update size before checking queue (!!!)
// check for receivers that were waiting on empty queue
if (size == 0) {
loop@ while (true) {
receive = takeFirstReceiveOrPeekClosed() ?: break@loop // break when no receivers queued
if (receive is Closed) {
this.size = size // restore size
return receive!!
}
token = receive!!.tryResumeReceive(element, idempotent = null)
if (token != null) {
this.size = size // restore size
return@withLock
}
}
}
ensureCapacity(size)
buffer[(head + size) % buffer.size] = element // actually queue element
return OFFER_SUCCESS
}
// size == capacity: full
return OFFER_FAILED
}
// breaks here if offer meets receiver
receive!!.completeResumeReceive(token!!)
return receive!!.offerResult
}
// result is `ALREADY_SELECTED | OFFER_SUCCESS | OFFER_FAILED | Closed`
protected override fun offerSelectInternal(element: E, select: SelectInstance<*>): Any {
var receive: ReceiveOrClosed? = null
var token: Any? = null
lock.withLock {
val size = this.size
closedForSend?.let { return it }
if (size < capacity) {
// tentatively put element to buffer
this.size = size + 1 // update size before checking queue (!!!)
// check for receivers that were waiting on empty queue
if (size == 0) {
loop@ while (true) {
val offerOp = describeTryOffer(element)
val failure = select.performAtomicTrySelect(offerOp)
when {
failure == null -> { // offered successfully
this.size = size // restore size
receive = offerOp.result
token = offerOp.resumeToken
assert { token != null }
return@withLock
}
failure === OFFER_FAILED -> break@loop // cannot offer -> Ok to queue to buffer
failure === ALREADY_SELECTED || failure is Closed<*> -> {
this.size = size // restore size
return failure
}
else -> error("performAtomicTrySelect(describeTryOffer) returned $failure")
}
}
}
// let's try to select sending this element to buffer
if (!select.trySelect(null)) { // :todo: move trySelect completion outside of lock
this.size = size // restore size
return ALREADY_SELECTED
}
ensureCapacity(size)
buffer[(head + size) % buffer.size] = element // actually queue element
return OFFER_SUCCESS
}
// size == capacity: full
return OFFER_FAILED
}
// breaks here if offer meets receiver
receive!!.completeResumeReceive(token!!)
return receive!!.offerResult
}
// Guarded by lock
private fun ensureCapacity(currentSize: Int) {
if (currentSize >= buffer.size) {
val newSize = min(buffer.size * 2, capacity)
val newBuffer = arrayOfNulls(newSize)
for (i in 0 until currentSize) {
newBuffer[i] = buffer[(head + i) % buffer.size]
}
buffer = newBuffer
head = 0
}
}
// result is `E | POLL_FAILED | Closed`
protected override fun pollInternal(): Any? {
var send: Send? = null
var token: Any? = null
var result: Any? = null
lock.withLock {
val size = this.size
if (size == 0) return closedForSend ?: POLL_FAILED // when nothing can be read from buffer
// size > 0: not empty -- retrieve element
result = buffer[head]
buffer[head] = null
this.size = size - 1 // update size before checking queue (!!!)
// check for senders that were waiting on full queue
var replacement: Any? = POLL_FAILED
if (size == capacity) {
loop@ while (true) {
send = takeFirstSendOrPeekClosed() ?: break
token = send!!.tryResumeSend(idempotent = null)
if (token != null) {
replacement = send!!.pollResult
break@loop
}
}
}
if (replacement !== POLL_FAILED && replacement !is Closed<*>) {
this.size = size // restore size
buffer[(head + size) % buffer.size] = replacement
}
head = (head + 1) % buffer.size
}
// complete send the we're taken replacement from
if (token != null)
send!!.completeResumeSend(token!!)
return result
}
// result is `ALREADY_SELECTED | E | POLL_FAILED | Closed`
protected override fun pollSelectInternal(select: SelectInstance<*>): Any? {
var send: Send? = null
var token: Any? = null
var result: Any? = null
lock.withLock {
val size = this.size
if (size == 0) return closedForSend ?: POLL_FAILED
// size > 0: not empty -- retrieve element
result = buffer[head]
buffer[head] = null
this.size = size - 1 // update size before checking queue (!!!)
// check for senders that were waiting on full queue
var replacement: Any? = POLL_FAILED
if (size == capacity) {
loop@ while (true) {
val pollOp = describeTryPoll()
val failure = select.performAtomicTrySelect(pollOp)
when {
failure == null -> { // polled successfully
send = pollOp.result
token = pollOp.resumeToken
assert { token != null }
replacement = send!!.pollResult
break@loop
}
failure === POLL_FAILED -> break@loop // cannot poll -> Ok to take from buffer
failure === ALREADY_SELECTED -> {
this.size = size // restore size
buffer[head] = result // restore head
return failure
}
failure is Closed<*> -> {
send = failure
token = failure.tryResumeSend(idempotent = null)
replacement = failure
break@loop
}
else -> error("performAtomicTrySelect(describeTryOffer) returned $failure")
}
}
}
if (replacement !== POLL_FAILED && replacement !is Closed<*>) {
this.size = size // restore size
buffer[(head + size) % buffer.size] = replacement
} else {
// failed to poll or is already closed --> let's try to select receiving this element from buffer
if (!select.trySelect(null)) { // :todo: move trySelect completion outside of lock
this.size = size // restore size
buffer[head] = result // restore head
return ALREADY_SELECTED
}
}
head = (head + 1) % buffer.size
}
// complete send the we're taken replacement from
if (token != null)
send!!.completeResumeSend(token!!)
return result
}
// Note: this function is invoked when channel is already closed
override fun cleanupSendQueueOnCancel() {
// clear buffer first
lock.withLock {
repeat(size) {
buffer[head] = 0
head = (head + 1) % buffer.size
}
size = 0
}
// then clean all queued senders
super.cleanupSendQueueOnCancel()
}
// ------ debug ------
override val bufferDebugString: String
get() = "(buffer:capacity=$capacity,size=$size)"
}