Channel.kt

/*
 * Copyright 2016-2018 JetBrains s.r.o. Use of this source code is governed by the Apache 2.0 license.
 */

@file:Suppress("FunctionName")

package kotlinx.coroutines.channels

import kotlinx.coroutines.*
import kotlinx.coroutines.channels.Channel.Factory.CONFLATED
import kotlinx.coroutines.channels.Channel.Factory.RENDEZVOUS
import kotlinx.coroutines.channels.Channel.Factory.UNLIMITED
import kotlinx.coroutines.selects.*
import kotlin.jvm.*

/**
 * Sender's interface to [Channel].
 */
public interface SendChannel<in E> {
    /**
     * Returns `true` if this channel was closed by invocation of [close] and thus
     * the [send] and [offer] attempts throws exception.
     *
     * **Note: This is an experimental api.** This property may change its semantics and/or name in the future.
     */
    @ExperimentalCoroutinesApi
    public val isClosedForSend: Boolean

    /**
     * Returns `true` if the channel is full (out of capacity) and the [send] attempt will suspend.
     * This function returns `false` for [isClosedForSend] channel.
     *
     * **Note: This is an experimental api.** This property may change its semantics and/or name in the future.
     */
    @ExperimentalCoroutinesApi
    public val isFull: Boolean

    /**
     * Adds [element] into to this channel, suspending the caller while this channel [isFull],
     * or throws exception if the channel [isClosedForSend] (see [close] for details).
     *
     * Note, that closing a channel _after_ this function had suspended does not cause this suspended send invocation
     * to abort, because closing a channel is conceptually like sending a special "close token" over this channel.
     * All elements that are sent over the channel are delivered in first-in first-out order. The element that
     * is being sent will get delivered to receivers before a close token.
     *
     * This suspending function is cancellable. If the [Job] of the current coroutine is cancelled or completed while this
     * function is suspended, this function immediately resumes with [CancellationException].
     *
     * *Cancellation of suspended send is atomic* -- when this function
     * throws [CancellationException] it means that the [element] was not sent to this channel.
     * As a side-effect of atomic cancellation, a thread-bound coroutine (to some UI thread, for example) may
     * continue to execute even after it was cancelled from the same thread in the case when this send operation
     * was already resumed and the continuation was posted for execution to the thread's queue.
     *
     * Note, that this function does not check for cancellation when it is not suspended.
     * Use [yield] or [CoroutineScope.isActive] to periodically check for cancellation in tight loops if needed.
     *
     * This function can be used in [select] invocation with [onSend] clause.
     * Use [offer] to try sending to this channel without waiting.
     */
    public suspend fun send(element: E)

    /**
     * Clause for [select] expression of [send] suspending function that selects when the element that is specified
     * as parameter is sent to the channel. When the clause is selected the reference to this channel
     * is passed into the corresponding block.
     *
     * The [select] invocation fails with exception if the channel [isClosedForSend] (see [close] for details).
     */
    public val onSend: SelectClause2<E, SendChannel<E>>

    /**
     * Adds [element] into this queue if it is possible to do so immediately without violating capacity restrictions
     * and returns `true`. Otherwise, it returns `false` immediately
     * or throws exception if the channel [isClosedForSend] (see [close] for details).
     */
    public fun offer(element: E): Boolean

    /**
     * Closes this channel.
     * This is an idempotent operation -- repeated invocations of this function have no effect and return `false`.
     * Conceptually, its sends a special "close token" over this channel.
     *
     * Immediately after invocation of this function
     * [isClosedForSend] starts returning `true`. However, [isClosedForReceive][ReceiveChannel.isClosedForReceive]
     * on the side of [ReceiveChannel] starts returning `true` only after all previously sent elements
     * are received.
     *
     * A channel that was closed without a [cause] throws [ClosedSendChannelException] on attempts to send or receive.
     * A channel that was closed with non-null [cause] is called a _failed_ channel. Attempts to send or
     * receive on a failed channel throw the specified [cause] exception.
     */
    public fun close(cause: Throwable? = null): Boolean

    /**
     * Registers handler which is synchronously invoked once the channel is [closed][close]
     * or receiving side of this channel is [cancelled][ReceiveChannel.cancel].
     * Only one handler can be attached to the channel during channel's lifetime.
     * Handler is invoked when [isClosedForSend] starts to return `true`.
     * If channel is already closed, handler is invoked immediately.
     *
     * The meaning of `cause` that is passed to the handler:
     * * `null` if channel was closed or cancelled without corresponding argument
     * * close or cancel cause otherwise.
     *
     * Example of usage (exception handling is omitted):
     * ```
     * val events = Channel(UNLIMITED)
     * callbackBasedApi.registerCallback { event ->
     *   events.offer(event)
     * }
     *
     * val uiUpdater = launch(UI, parent = UILifecycle) {
     *    events.consume {}
     *    events.cancel()
     * }
     *
     * events.invokeOnClose { callbackBasedApi.stop() }
     *
     * ```
     *
     * **Note: This is an experimental api.** This function may change its semantics, parameters or return type in the future.
     *
     * @throws UnsupportedOperationException if underlying channel doesn't support [invokeOnClose].
     * Implementation note: currently, [invokeOnClose] is unsupported only by Rx-like integrations
     *
     * @throws IllegalStateException if another handler was already registered
     */
    @ExperimentalCoroutinesApi
    public fun invokeOnClose(handler: (cause: Throwable?) -> Unit)
}

/**
 * Receiver's interface to [Channel].
 */
public interface ReceiveChannel<out E> {
    /**
     * Returns `true` if this channel was closed by invocation of [close][SendChannel.close] on the [SendChannel]
     * side and all previously sent items were already received, so that the [receive] attempt
     * throws [ClosedReceiveChannelException]. If the channel was closed because of the exception, it
     * is considered closed, too, but it is called a _failed_ channel. All suspending attempts to receive
     * an element from a failed channel throw the original [close][SendChannel.close] cause exception.
     *
     * **Note: This is an experimental api.** This property may change its semantics and/or name in the future.
     */
    @ExperimentalCoroutinesApi
    public val isClosedForReceive: Boolean

    /**
     * Returns `true` if the channel is empty (contains no elements) and the [receive] attempt will suspend.
     * This function returns `false` for [isClosedForReceive] channel.
     *
     * **Note: This is an experimental api.** This property may change its semantics and/or name in the future.
     */
    @ExperimentalCoroutinesApi
    public val isEmpty: Boolean

    /**
     * Retrieves and removes the element from this channel suspending the caller while this channel [isEmpty]
     * or throws [ClosedReceiveChannelException] if the channel [isClosedForReceive].
     * If the channel was closed because of the exception, it is called a _failed_ channel and this function
     * throws the original [close][SendChannel.close] cause exception.
     *
     * This suspending function is cancellable. If the [Job] of the current coroutine is cancelled or completed while this
     * function is suspended, this function immediately resumes with [CancellationException].
     *
     * *Cancellation of suspended receive is atomic* -- when this function
     * throws [CancellationException] it means that the element was not retrieved from this channel.
     * As a side-effect of atomic cancellation, a thread-bound coroutine (to some UI thread, for example) may
     * continue to execute even after it was cancelled from the same thread in the case when this receive operation
     * was already resumed and the continuation was posted for execution to the thread's queue.
     *
     * Note, that this function does not check for cancellation when it is not suspended.
     * Use [yield] or [CoroutineScope.isActive] to periodically check for cancellation in tight loops if needed.
     *
     * This function can be used in [select] invocation with [onReceive] clause.
     * Use [poll] to try receiving from this channel without waiting.
     */
    public suspend fun receive(): E

    /**
     * Clause for [select] expression of [receive] suspending function that selects with the element that
     * is received from the channel.
     * The [select] invocation fails with exception if the channel
     * [isClosedForReceive] (see [close][SendChannel.close] for details).
     */
    public val onReceive: SelectClause1<E>

    /**
     * Retrieves and removes the element from this channel suspending the caller while this channel [isEmpty]
     * or returns `null` if the channel is [closed][isClosedForReceive] without cause
     * or throws the original [close][SendChannel.close] cause exception if the channel has _failed_.
     *
     * This suspending function is cancellable. If the [Job] of the current coroutine is cancelled or completed while this
     * function is suspended, this function immediately resumes with [CancellationException].
     *
     * *Cancellation of suspended receive is atomic* -- when this function
     * throws [CancellationException] it means that the element was not retrieved from this channel.
     * As a side-effect of atomic cancellation, a thread-bound coroutine (to some UI thread, for example) may
     * continue to execute even after it was cancelled from the same thread in the case when this receive operation
     * was already resumed and the continuation was posted for execution to the thread's queue.
     *
     * Note, that this function does not check for cancellation when it is not suspended.
     * Use [yield] or [CoroutineScope.isActive] to periodically check for cancellation in tight loops if needed.
     *
     * This function can be used in [select] invocation with [onReceiveOrNull] clause.
     * Use [poll] to try receiving from this channel without waiting.
     */
    public suspend fun receiveOrNull(): E?

    /**
     * Clause for [select] expression of [receiveOrNull] suspending function that selects with the element that
     * is received from the channel or selects with `null` if if the channel
     * [isClosedForReceive] without cause. The [select] invocation fails with
     * the original [close][SendChannel.close] cause exception if the channel has _failed_.
     *
     * **Note: This is an experimental api.** This function may be replaced with a better on in the future.
     */
    @ExperimentalCoroutinesApi
    public val onReceiveOrNull: SelectClause1<E?>

    /**
     * Retrieves and removes the element from this channel, or returns `null` if this channel [isEmpty]
     * or is [isClosedForReceive] without cause.
     * It throws the original [close][SendChannel.close] cause exception if the channel has _failed_.
     */
    public fun poll(): E?

    /**
     * Returns new iterator to receive elements from this channels using `for` loop.
     * Iteration completes normally when the channel is [isClosedForReceive] without cause and
     * throws the original [close][SendChannel.close] cause exception if the channel has _failed_.
     */
    public operator fun iterator(): ChannelIterator<E>

    /**
     * Cancels reception of remaining elements from this channel. This function closes the channel
     * and removes all buffered sent elements from it.
     *
     * Immediately after invocation of this function [isClosedForReceive] and
     * [isClosedForSend][SendChannel.isClosedForSend]
     * on the side of [SendChannel] start returning `true`, so all attempts to send to this channel
     * afterwards will throw [ClosedSendChannelException], while attempts to receive will throw
     * [ClosedReceiveChannelException].
     */
    public fun cancel(): Unit

    /**
     * @suppress
     */
    @Suppress("INAPPLICABLE_JVM_NAME")
    @Deprecated(level = DeprecationLevel.HIDDEN, message = "Left here for binary compatibility")
    @JvmName("cancel")
    public fun cancel0(): Boolean = cancel(null)

    /**
     * @suppress
     */
    @ObsoleteCoroutinesApi
    @Deprecated(level = DeprecationLevel.WARNING, message = "Use cancel without cause", replaceWith = ReplaceWith("cancel()"))
    public fun cancel(cause: Throwable? = null): Boolean
}

/**
 * Iterator for [ReceiveChannel]. Instances of this interface are *not thread-safe* and shall not be used
 * from concurrent coroutines.
 */
public interface ChannelIterator<out E> {
    /**
     * Returns `true` if the channel has more elements suspending the caller while this channel
     * [isEmpty][ReceiveChannel.isEmpty] or returns `false` if the channel
     * [isClosedForReceive][ReceiveChannel.isClosedForReceive] without cause.
     * It throws the original [close][SendChannel.close] cause exception if the channel has _failed_.
     *
     * This function retrieves and removes the element from this channel for the subsequent invocation
     * of [next].
     *
     * This suspending function is cancellable. If the [Job] of the current coroutine is cancelled or completed while this
     * function is suspended, this function immediately resumes with [CancellationException].
     *
     * *Cancellation of suspended receive is atomic* -- when this function
     * throws [CancellationException] it means that the element was not retrieved from this channel.
     * As a side-effect of atomic cancellation, a thread-bound coroutine (to some UI thread, for example) may
     * continue to execute even after it was cancelled from the same thread in the case when this receive operation
     * was already resumed and the continuation was posted for execution to the thread's queue.
     *
     * Note, that this function does not check for cancellation when it is not suspended.
     * Use [yield] or [CoroutineScope.isActive] to periodically check for cancellation in tight loops if needed.
     */
    public suspend operator fun hasNext(): Boolean

    /**
     * Retrieves and removes the element from this channel suspending the caller while this channel
     * [isEmpty][ReceiveChannel.isEmpty] or throws [ClosedReceiveChannelException] if the channel
     * [isClosedForReceive][ReceiveChannel.isClosedForReceive] without cause.
     * It throws the original [close][SendChannel.close] cause exception if the channel has _failed_.
     *
     * This suspending function is cancellable. If the [Job] of the current coroutine is cancelled or completed while this
     * function is suspended, this function immediately resumes with [CancellationException].
     *
     * *Cancellation of suspended receive is atomic* -- when this function
     * throws [CancellationException] it means that the element was not retrieved from this channel.
     * As a side-effect of atomic cancellation, a thread-bound coroutine (to some UI thread, for example) may
     * continue to execute even after it was cancelled from the same thread in the case when this receive operation
     * was already resumed and the continuation was posted for execution to the thread's queue.
     *
     * Note, that this function does not check for cancellation when it is not suspended.
     * Use [yield] or [CoroutineScope.isActive] to periodically check for cancellation in tight loops if needed.
     */
    public suspend operator fun next(): E
}

/**
 * Channel is a non-blocking primitive for communication between sender using [SendChannel] and receiver using [ReceiveChannel].
 * Conceptually, a channel is similar to [BlockingQueue][java.util.concurrent.BlockingQueue],
 * but it has suspending operations instead of blocking ones and it can be closed.
 *
 * `Channel(capacity)` factory function is used to create channels of different kind depending on
 * the value of `capacity` integer:
 *
 * * When `capacity` is 0 -- it creates `RendezvousChannel`.
 *   This channel does not have any buffer at all. An element is transferred from sender
 *   to receiver only when [send] and [receive] invocations meet in time (rendezvous), so [send] suspends
 *   until another coroutine invokes [receive] and [receive] suspends until another coroutine invokes [send].
 *
 * * When `capacity` is [Channel.UNLIMITED] -- it creates `LinkedListChannel`.
 *   This is a channel with linked-list buffer of a unlimited capacity (limited only by available memory).
 *   Sender to this channel never suspends and [offer] always returns `true`.
 *
 * * When `capacity` is [Channel.CONFLATED] -- it creates `ConflatedChannel`.
 *   This channel buffers at most one element and conflates all subsequent `send` and `offer` invocations,
 *   so that the receiver always gets the most recently sent element.
 *   Back-to-send sent elements are _conflated_ -- only the the most recently sent element is received,
 *   while previously sent elements **are lost**.
 *   Sender to this channel never suspends and [offer] always returns `true`.
 *
 * * When `capacity` is positive, but less than [UNLIMITED] -- it creates array-based channel with given capacity.
 *   This channel has an array buffer of a fixed `capacity`.
 *   Sender suspends only when buffer is fully and receiver suspends only when buffer is empty.
 */
public interface Channel<E> : SendChannel<E>, ReceiveChannel<E> {
    /**
     * Constants for channel factory function `Channel()`.
     */
    public companion object Factory {
        /**
         * Requests channel with unlimited capacity buffer in `Channel(...)` factory function
         */
        public const val UNLIMITED = Int.MAX_VALUE

        /**
         * Requests rendezvous channel in `Channel(...)` factory function -- the `RendezvousChannel` gets created.
         */
        public const val RENDEZVOUS = 0

        /**
         * Requests conflated channel in `Channel(...)` factory function -- the `ConflatedChannel` gets created.
         */
        public const val CONFLATED = -1
    }
}

/**
 * Creates a channel with the specified buffer capacity (or without a buffer by default).
 * See [Channel] interface documentation for details.
 *
 * @throws IllegalArgumentException when [capacity] < -1
 */
public fun <E> Channel(capacity: Int = RENDEZVOUS): Channel<E> =
    when (capacity) {
        RENDEZVOUS -> RendezvousChannel()
        UNLIMITED -> LinkedListChannel()
        CONFLATED -> ConflatedChannel()
        else -> ArrayChannel(capacity)
    }

/**
 * Indicates attempt to [send][SendChannel.send] on [isClosedForSend][SendChannel.isClosedForSend] channel
 * that was closed without a cause. A _failed_ channel rethrows the original [close][SendChannel.close] cause
 * exception on send attempts.
 */
public class ClosedSendChannelException(message: String?) : CancellationException(message)

/**
 * Indicates attempt to [receive][ReceiveChannel.receive] on [isClosedForReceive][ReceiveChannel.isClosedForReceive]
 * channel that was closed without a cause. A _failed_ channel rethrows the original [close][SendChannel.close] cause
 * exception on receive attempts.
 *
 * This exception is subclass of [NoSuchElementException] to be consistent with plain collections.
 */
public class ClosedReceiveChannelException(message: String?) : NoSuchElementException(message)