ArrayChannel.kt

/*
 * Copyright 2016-2021 JetBrains s.r.o. Use of this source code is governed by the Apache 2.0 license.
 */

package kotlinx.coroutines.channels

import kotlinx.atomicfu.*
import kotlinx.coroutines.*
import kotlinx.coroutines.internal.*
import kotlinx.coroutines.selects.TrySelectDetailedResult.*
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<E>(
    /**
     * Buffer capacity.
     */
    private val capacity: Int,
    private val onBufferOverflow: BufferOverflow,
    onUndeliveredElement: OnUndeliveredElement<E>?
) : AbstractChannel<E>(onUndeliveredElement) {
    init {
        // This check is actually used by the Channel(...) constructor function which checks only for known
        // capacities and calls ArrayChannel constructor for everything else.
        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<Any?> = arrayOfNulls<Any?>(min(capacity, 8)).apply { fill(EMPTY) }

    private var head: Int = 0
    private val size = atomic(0) // Invariant: size <= capacity

    protected final override val isBufferAlwaysEmpty: Boolean get() = false
    protected final override val isBufferEmpty: Boolean get() = size.value == 0
    protected final override val isBufferAlwaysFull: Boolean get() = false
    protected final override val isBufferFull: Boolean get() = size.value == capacity && onBufferOverflow == BufferOverflow.SUSPEND

    override val isEmpty: Boolean get() = lock.withLock { isEmptyImpl }
    override val isClosedForReceive: Boolean get() = lock.withLock { super.isClosedForReceive }

    // result is `OFFER_SUCCESS | OFFER_FAILED | Closed`
    protected override fun offerInternal(element: E): Any {
        var receive: ReceiveOrClosed<E>? = null
        lock.withLock {
            val size = this.size.value
            closedForSend?.let { return it }
            // update size before checking queue (!!!)
            updateBufferSize(size)?.let { return it }
            // 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.value = size // restore size
                        return receive!!
                    }
                    val token = receive!!.tryResumeReceive(element)
                    if (token != null) {
                        assert { token === RESUME_TOKEN }
                        this.size.value = size // restore size
                        return@withLock
                    }
                }
            }
            enqueueElement(size, element)
            return OFFER_SUCCESS
        }
        // breaks here if offer meets receiver
        receive!!.completeResumeReceive(element)
        return receive!!.offerResult
    }

    override fun enqueueSend(send: Send): Any? = lock.withLock {
        super.enqueueSend(send)
    }

    // Guarded by lock
    // Result is `OFFER_SUCCESS | OFFER_FAILED | null`
    private fun updateBufferSize(currentSize: Int): Symbol? {
        if (currentSize < capacity) {
            size.value = currentSize + 1 // tentatively put it into the buffer
            return null // proceed
        }
        // buffer is full
        return when (onBufferOverflow) {
            BufferOverflow.SUSPEND -> OFFER_FAILED
            BufferOverflow.DROP_LATEST -> OFFER_SUCCESS
            BufferOverflow.DROP_OLDEST -> null // proceed, will drop oldest in enqueueElement
        }
    }

    // Guarded by lock
    private fun enqueueElement(currentSize: Int, element: E) {
        if (currentSize < capacity) {
            ensureCapacity(currentSize)
            buffer[(head + currentSize) % buffer.size] = element // actually queue element
        } else {
            // buffer is full
            assert { onBufferOverflow == BufferOverflow.DROP_OLDEST } // the only way we can get here
            buffer[head % buffer.size] = null // drop oldest element
            buffer[(head + currentSize) % buffer.size] = element // actually queue element
            head = (head + 1) % buffer.size
        }
    }

    // Guarded by lock
    private fun ensureCapacity(currentSize: Int) {
        if (currentSize >= buffer.size) {
            val newSize = min(buffer.size * 2, capacity)
            val newBuffer = arrayOfNulls<Any?>(newSize)
            for (i in 0 until currentSize) {
                newBuffer[i] = buffer[(head + i) % buffer.size]
            }
            newBuffer.fill(EMPTY, currentSize, newSize)
            buffer = newBuffer
            head = 0
        }
    }

    // result is `E | POLL_FAILED | Closed`
    protected override fun pollInternal(): Any? {
        var send: Send? = null
        var resumed = false
        var result: Any? = null
        lock.withLock {
            val size = this.size.value
            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.value = 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
                    val token = send!!.tryResumeSend(null)
                    if (token != null) {
                        assert { token === RESUME_TOKEN }
                        resumed = true
                        replacement = send!!.pollResult
                        break@loop
                    }
                    // Too late, already cancelled, but we removed it from the queue and need to notify on undelivered element.
                    // The only exception is when this "send" operation is an `onSend` clause that has to be re-registered
                    // in the corresponding `select` invocation.
                    send!!.apply {
                        if (!(this is SendElementSelectWithUndeliveredHandler<*> && trySelectResult == REREGISTER))
                            undeliveredElement()
                    }
                }
            }
            if (replacement !== POLL_FAILED && replacement !is Closed<*>) {
                this.size.value = size // restore size
                buffer[(head + size) % buffer.size] = replacement
            }
            head = (head + 1) % buffer.size
        }
        // complete send the we're taken replacement from
        if (resumed)
            send!!.completeResumeSend()
        return result
    }

    override fun enqueueReceiveInternal(receive: Receive<E>): Boolean = lock.withLock {
        super.enqueueReceiveInternal(receive)
    }

    // Note: this function is invoked when channel is already closed
    override fun onCancelIdempotent(wasClosed: Boolean) {
        // clear buffer first, but do not wait for it in helpers
        val onUndeliveredElement = onUndeliveredElement
        var undeliveredElementException: UndeliveredElementException? = null // first cancel exception, others suppressed
        lock.withLock {
            repeat(size.value) {
                val value = buffer[head]
                if (onUndeliveredElement != null && value !== EMPTY) {
                    @Suppress("UNCHECKED_CAST")
                    undeliveredElementException = onUndeliveredElement.callUndeliveredElementCatchingException(value as E, undeliveredElementException)
                }
                buffer[head] = EMPTY
                head = (head + 1) % buffer.size
            }
            size.value = 0
        }
        // then clean all queued senders
        super.onCancelIdempotent(wasClosed)
        undeliveredElementException?.let { throw it } // throw UndeliveredElementException at the end if there was one
    }

    // ------ debug ------

    override val bufferDebugString: String
        get() = "(buffer:capacity=$capacity,size=${size.value})"
}