Job.kt

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

@file:JvmMultifileClass
@file:JvmName("JobKt")
@file:Suppress("DEPRECATION_ERROR", "RedundantUnitReturnType")

package kotlinx.coroutines

import kotlinx.coroutines.selects.*
import kotlin.coroutines.*
import kotlin.jvm.*

// --------------- core job interfaces ---------------

/**
 * A background job. Conceptually, a job is a cancellable thing with a life-cycle that
 * culminates in its completion.
 *
 * Jobs can be arranged into parent-child hierarchies where cancellation
 * of a parent leads to immediate cancellation of all its [children] recursively.
 * Failure of a child with an exception other than [CancellationException] immediately cancels its parent and,
 * consequently, all its other children. This behavior can be customized using [SupervisorJob].
 *
 * The most basic instances of `Job` interface are created like this:
 *
 * * **Coroutine job** is created with [launch][CoroutineScope.launch] coroutine builder.
 *   It runs a specified block of code and completes on completion of this block.
 * * **[CompletableJob]** is created with a `Job()` factory function.
 *   It is completed by calling [CompletableJob.complete].
 *
 * Conceptually, an execution of a job does not produce a result value. Jobs are launched solely for their
 * side-effects. See [Deferred] interface for a job that produces a result.
 *
 * ### Job states
 *
 * A job has the following states:
 *
 * | **State**                        | [isActive] | [isCompleted] | [isCancelled] |
 * | -------------------------------- | ---------- | ------------- | ------------- |
 * | _New_ (optional initial state)   | `false`    | `false`       | `false`       |
 * | _Active_ (default initial state) | `true`     | `false`       | `false`       |
 * | _Completing_ (transient state)   | `true`     | `false`       | `false`       |
 * | _Cancelling_ (transient state)   | `false`    | `false`       | `true`        |
 * | _Cancelled_ (final state)        | `false`    | `true`        | `true`        |
 * | _Completed_ (final state)        | `false`    | `true`        | `false`       |
 *
 * Usually, a job is created in the _active_ state (it is created and started). However, coroutine builders
 * that provide an optional `start` parameter create a coroutine in the _new_ state when this parameter is set to
 * [CoroutineStart.LAZY]. Such a job can be made _active_ by invoking [start] or [join].
 *
 * A job is _active_ while the coroutine is working or until [CompletableJob] is completed,
 * or until it fails or cancelled.
 *
 * Failure of an _active_ job with an exception makes it _cancelling_.
 * A job can be cancelled at any time with [cancel] function that forces it to transition to
 * the _cancelling_ state immediately. The job becomes _cancelled_  when it finishes executing its work and
 * all its children complete.
 *
 * Completion of an _active_ coroutine's body or a call to [CompletableJob.complete] transitions the job to
 * the _completing_ state. It waits in the _completing_ state for all its children to complete before
 * transitioning to the _completed_ state.
 * Note that _completing_ state is purely internal to the job. For an outside observer a _completing_ job is still
 * active, while internally it is waiting for its children.
 *
 * ```
 *                                       wait children
 * +-----+ start  +--------+ complete   +-------------+  finish  +-----------+
 * | New | -----> | Active | ---------> | Completing  | -------> | Completed |
 * +-----+        +--------+            +-------------+          +-----------+
 *                  |  cancel / fail       |
 *                  |     +----------------+
 *                  |     |
 *                  V     V
 *              +------------+                           finish  +-----------+
 *              | Cancelling | --------------------------------> | Cancelled |
 *              +------------+                                   +-----------+
 * ```
 *
 * A `Job` instance in the
 * [coroutineContext](https://kotlinlang.org/api/latest/jvm/stdlib/kotlin.coroutines/coroutine-context.html)
 * represents the coroutine itself.
 *
 * ### Cancellation cause
 *
 * A coroutine job is said to _complete exceptionally_ when its body throws an exception;
 * a [CompletableJob] is completed exceptionally by calling [CompletableJob.completeExceptionally].
 * An exceptionally completed job is cancelled and the corresponding exception becomes the _cancellation cause_ of the job.
 *
 * Normal cancellation of a job is distinguished from its failure by the type of this exception that caused its cancellation.
 * A coroutine that threw [CancellationException] is considered to be _cancelled normally_.
 * If a cancellation cause is a different exception type, then the job is considered to have _failed_.
 * When a job has _failed_, then its parent gets cancelled with the exception of the same type,
 * thus ensuring transparency in delegating parts of the job to its children.
 *
 * Note, that [cancel] function on a job only accepts [CancellationException] as a cancellation cause, thus
 * calling [cancel] always results in a normal cancellation of a job, which does not lead to cancellation
 * of its parent. This way, a parent can [cancel] its own children (cancelling all their children recursively, too)
 * without cancelling itself.
 *
 * ### Concurrency and synchronization
 *
 * All functions on this interface and on all interfaces derived from it are **thread-safe** and can
 * be safely invoked from concurrent coroutines without external synchronization.
 *
 * ### Not stable for inheritance
 *
 * **`Job` interface and all its derived interfaces are not stable for inheritance in 3rd party libraries**,
 * as new methods might be added to this interface in the future, but is stable for use.
 */
public interface Job : CoroutineContext.Element {
    /**
     * Key for [Job] instance in the coroutine context.
     */
    public companion object Key : CoroutineContext.Key<Job>

    // ------------ state query ------------

    /**
     * Returns `true` when this job is active -- it was already started and has not completed nor was cancelled yet.
     * The job that is waiting for its [children] to complete is still considered to be active if it
     * was not cancelled nor failed.
     *
     * See [Job] documentation for more details on job states.
     */
    public val isActive: Boolean

    /**
     * Returns `true` when this job has completed for any reason. A job that was cancelled or failed
     * and has finished its execution is also considered complete. Job becomes complete only after
     * all its [children] complete.
     *
     * See [Job] documentation for more details on job states.
     */
    public val isCompleted: Boolean

    /**
     * Returns `true` if this job was cancelled for any reason, either by explicit invocation of [cancel] or
     * because it had failed or its child or parent was cancelled.
     * In the general case, it does not imply that the
     * job has already [completed][isCompleted], because it may still be finishing whatever it was doing and
     * waiting for its [children] to complete.
     *
     * See [Job] documentation for more details on cancellation and failures.
     */
    public val isCancelled: Boolean

    /**
     * Returns [CancellationException] that signals the completion of this job. This function is
     * used by [cancellable][suspendCancellableCoroutine] suspending functions. They throw exception
     * returned by this function when they suspend in the context of this job and this job becomes _complete_.
     *
     * This function returns the original [cancel] cause of this job if that `cause` was an instance of
     * [CancellationException]. Otherwise (if this job was cancelled with a cause of a different type, or
     * was cancelled without a cause, or had completed normally), an instance of [CancellationException] is
     * returned. The [CancellationException.cause] of the resulting [CancellationException] references
     * the original cancellation cause that was passed to [cancel] function.
     *
     * This function throws [IllegalStateException] when invoked on a job that is still active.
     *
     * @suppress **This an internal API and should not be used from general code.**
     */
    @InternalCoroutinesApi
    public fun getCancellationException(): CancellationException

    // ------------ state update ------------

    /**
     * Starts coroutine related to this job (if any) if it was not started yet.
     * The result is `true` if this invocation actually started coroutine or `false`
     * if it was already started or completed.
     */
    public fun start(): Boolean


    /**
     * Cancels this job with an optional cancellation [cause].
     * A cause can be used to specify an error message or to provide other details on
     * the cancellation reason for debugging purposes.
     * See [Job] documentation for full explanation of cancellation machinery.
     */
    public fun cancel(cause: CancellationException? = null)

    /**
     * @suppress This method implements old version of JVM ABI. Use [cancel].
     */
    @Deprecated(level = DeprecationLevel.HIDDEN, message = "Since 1.2.0, binary compatibility with versions <= 1.1.x")
    public fun cancel(): Unit = cancel(null)

    /**
     * @suppress This method has bad semantics when cause is not a [CancellationException]. Use [cancel].
     */
    @Deprecated(level = DeprecationLevel.HIDDEN, message = "Since 1.2.0, binary compatibility with versions <= 1.1.x")
    public fun cancel(cause: Throwable? = null): Boolean

    // ------------ parent-child ------------

    /**
     * Returns a sequence of this job's children.
     *
     * A job becomes a child of this job when it is constructed with this job in its
     * [CoroutineContext] or using an explicit `parent` parameter.
     *
     * A parent-child relation has the following effect:
     *
     * * Cancellation of parent with [cancel] or its exceptional completion (failure)
     *   immediately cancels all its children.
     * * Parent cannot complete until all its children are complete. Parent waits for all its children to
     *   complete in _completing_ or _cancelling_ state.
     * * Uncaught exception in a child, by default, cancels parent. This applies even to
     *   children created with [async][CoroutineScope.async] and other future-like
     *   coroutine builders, even though their exceptions are caught and are encapsulated in their result.
     *   This default behavior can be overridden with [SupervisorJob].
     */
    public val children: Sequence<Job>

    /**
     * Attaches child job so that this job becomes its parent and
     * returns a handle that should be used to detach it.
     *
     * A parent-child relation has the following effect:
     * * Cancellation of parent with [cancel] or its exceptional completion (failure)
     *   immediately cancels all its children.
     * * Parent cannot complete until all its children are complete. Parent waits for all its children to
     *   complete in _completing_ or _cancelling_ states.
     *
     * **A child must store the resulting [ChildHandle] and [dispose][DisposableHandle.dispose] the attachment
     * to its parent on its own completion.**
     *
     * Coroutine builders and job factory functions that accept `parent` [CoroutineContext] parameter
     * lookup a [Job] instance in the parent context and use this function to attach themselves as a child.
     * They also store a reference to the resulting [ChildHandle] and dispose a handle when they complete.
     *
     * @suppress This is an internal API. This method is too error prone for public API.
     */
    // ChildJob and ChildHandle are made internal on purpose to further deter 3rd-party impl of Job
    @InternalCoroutinesApi
    public fun attachChild(child: ChildJob): ChildHandle

    // ------------ state waiting ------------

    /**
     * Suspends the coroutine until this job is complete. This invocation resumes normally (without exception)
     * when the job is complete for any reason and the [Job] of the invoking coroutine is still [active][isActive].
     * This function also [starts][Job.start] the corresponding coroutine if the [Job] was still in _new_ state.
     *
     * Note that the job becomes complete only when all its children are complete.
     *
     * This suspending function is cancellable and **always** checks for a cancellation of the invoking coroutine's Job.
     * If the [Job] of the invoking coroutine is cancelled or completed when this
     * suspending function is invoked or while it is suspended, this function
     * throws [CancellationException].
     *
     * In particular, it means that a parent coroutine invoking `join` on a child coroutine throws
     * [CancellationException] if the child had failed, since a failure of a child coroutine cancels parent by default,
     * unless the child was launched from within [supervisorScope].
     *
     * This function can be used in [select] invocation with [onJoin] clause.
     * Use [isCompleted] to check for a completion of this job without waiting.
     *
     * There is [cancelAndJoin] function that combines an invocation of [cancel] and `join`.
     */
    public suspend fun join()

    /**
     * Clause for [select] expression of [join] suspending function that selects when the job is complete.
     * This clause never fails, even if the job completes exceptionally.
     */
    public val onJoin: SelectClause0

    // ------------ low-level state-notification ------------

    /**
     * Registers handler that is **synchronously** invoked once on completion of this job.
     * When the job is already complete, then the handler is immediately invoked
     * with the job's exception or cancellation cause or `null`. Otherwise, the handler will be invoked once when this
     * job is complete.
     *
     * The meaning of `cause` that is passed to the handler:
     * * Cause is `null` when the job has completed normally.
     * * Cause is an instance of [CancellationException] when the job was cancelled _normally_.
     *   **It should not be treated as an error**. In particular, it should not be reported to error logs.
     * * Otherwise, the job had _failed_.
     *
     * The resulting [DisposableHandle] can be used to [dispose][DisposableHandle.dispose] the
     * registration of this handler and release its memory if its invocation is no longer needed.
     * There is no need to dispose the handler after completion of this job. The references to
     * all the handlers are released when this job completes.
     *
     * Installed [handler] should not throw any exceptions. If it does, they will get caught,
     * wrapped into [CompletionHandlerException], and rethrown, potentially causing crash of unrelated code.
     *
     * **Note**: Implementation of `CompletionHandler` must be fast, non-blocking, and thread-safe.
     * This handler can be invoked concurrently with the surrounding code.
     * There is no guarantee on the execution context in which the [handler] is invoked.
     */
    public fun invokeOnCompletion(handler: CompletionHandler): DisposableHandle

    /**
     * Registers handler that is **synchronously** invoked once on cancellation or completion of this job.
     * when the job was already cancelled and is completed its execution, then the handler is immediately invoked
     * with the job's cancellation cause or `null` unless [invokeImmediately] is set to false.
     * Otherwise, handler will be invoked once when this job is cancelled or is complete.
     *
     * The meaning of `cause` that is passed to the handler:
     * * Cause is `null` when the job has completed normally.
     * * Cause is an instance of [CancellationException] when the job was cancelled _normally_.
     *   **It should not be treated as an error**. In particular, it should not be reported to error logs.
     * * Otherwise, the job had _failed_.
     *
     * Invocation of this handler on a transition to a _cancelling_ state
     * is controlled by [onCancelling] boolean parameter.
     * The handler is invoked when the job becomes _cancelling_ if [onCancelling] parameter is set to `true`.
     *
     * The resulting [DisposableHandle] can be used to [dispose][DisposableHandle.dispose] the
     * registration of this handler and release its memory if its invocation is no longer needed.
     * There is no need to dispose the handler after completion of this job. The references to
     * all the handlers are released when this job completes.
     *
     * Installed [handler] should not throw any exceptions. If it does, they will get caught,
     * wrapped into [CompletionHandlerException], and rethrown, potentially causing crash of unrelated code.
     *
     * **Note**: This function is a part of internal machinery that supports parent-child hierarchies
     * and allows for implementation of suspending functions that wait on the Job's state.
     * This function should not be used in general application code.
     * Implementation of `CompletionHandler` must be fast, non-blocking, and thread-safe.
     * This handler can be invoked concurrently with the surrounding code.
     * There is no guarantee on the execution context in which the [handler] is invoked.
     *
     * @param onCancelling when `true`, then the [handler] is invoked as soon as this job transitions to _cancelling_ state;
     *        when `false` then the [handler] is invoked only when it transitions to _completed_ state.
     * @param invokeImmediately when `true` and this job is already in the desired state (depending on [onCancelling]),
     *        then the [handler] is immediately and synchronously invoked and no-op [DisposableHandle] is returned;
     *        when `false` then no-op [DisposableHandle] is returned, but the [handler] is not invoked.
     * @param handler the handler.
     *
     * @suppress **This an internal API and should not be used from general code.**
     */
    @InternalCoroutinesApi
    public fun invokeOnCompletion(
        onCancelling: Boolean = false,
        invokeImmediately: Boolean = true,
        handler: CompletionHandler): DisposableHandle

    // ------------ unstable internal API ------------

    /**
     * @suppress **Error**: Operator '+' on two Job objects is meaningless.
     * Job is a coroutine context element and `+` is a set-sum operator for coroutine contexts.
     * The job to the right of `+` just replaces the job the left of `+`.
     */
    @Suppress("DeprecatedCallableAddReplaceWith")
    @Deprecated(message = "Operator '+' on two Job objects is meaningless. " +
        "Job is a coroutine context element and `+` is a set-sum operator for coroutine contexts. " +
        "The job to the right of `+` just replaces the job the left of `+`.",
        level = DeprecationLevel.ERROR)
    public operator fun plus(other: Job): Job = other
}

/**
 * Creates a job object in an active state.
 * A failure of any child of this job immediately causes this job to fail, too, and cancels the rest of its children.
 *
 * To handle children failure independently of each other use [SupervisorJob].
 *
 * If [parent] job is specified, then this job becomes a child job of its parent and
 * is cancelled when its parent fails or is cancelled. All this job's children are cancelled in this case, too.
 * The invocation of [cancel][Job.cancel] with exception (other than [CancellationException]) on this job also cancels parent.
 *
 * Conceptually, the resulting job works in the same way as the job created by the `launch { body }` invocation
 * (see [launch]), but without any code in the body. It is active until cancelled or completed. Invocation of
 * [CompletableJob.complete] or [CompletableJob.completeExceptionally] corresponds to the successful or
 * failed completion of the body of the coroutine.
 *
 * @param parent an optional parent job.
 */
@Suppress("FunctionName")
public fun Job(parent: Job? = null): CompletableJob = JobImpl(parent)

/** @suppress Binary compatibility only */
@Suppress("FunctionName")
@Deprecated(level = DeprecationLevel.HIDDEN, message = "Since 1.2.0, binary compatibility with versions <= 1.1.x")
@JvmName("Job")
public fun Job0(parent: Job? = null): Job = Job(parent)

/**
 * A handle to an allocated object that can be disposed to make it eligible for garbage collection.
 */
public fun interface DisposableHandle {
    /**
     * Disposes the corresponding object, making it eligible for garbage collection.
     * Repeated invocation of this function has no effect.
     */
    public fun dispose()
}

// -------------------- Parent-child communication --------------------

/**
 * A reference that parent receives from its child so that it can report its cancellation.
 *
 * @suppress **This is unstable API and it is subject to change.**
 */
@InternalCoroutinesApi
@Deprecated(level = DeprecationLevel.ERROR, message = "This is internal API and may be removed in the future releases")
public interface ChildJob : Job {
    /**
     * Parent is cancelling its child by invoking this method.
     * Child finds the cancellation cause using [ParentJob.getChildJobCancellationCause].
     * This method does nothing is the child is already being cancelled.
     *
     * @suppress **This is unstable API and it is subject to change.**
     */
    @InternalCoroutinesApi
    public fun parentCancelled(parentJob: ParentJob)
}

/**
 * A reference that child receives from its parent when it is being cancelled by the parent.
 *
 * @suppress **This is unstable API and it is subject to change.**
 */
@InternalCoroutinesApi
@Deprecated(level = DeprecationLevel.ERROR, message = "This is internal API and may be removed in the future releases")
public interface ParentJob : Job {
    /**
     * Child job is using this method to learn its cancellation cause when the parent cancels it with [ChildJob.parentCancelled].
     * This method is invoked only if the child was not already being cancelled.
     *
     * Note that [CancellationException] is the method's return type: if child is cancelled by its parent,
     * then the original exception is **already** handled by either the parent or the original source of failure.
     *
     * @suppress **This is unstable API and it is subject to change.**
     */
    @InternalCoroutinesApi
    public fun getChildJobCancellationCause(): CancellationException
}

/**
 * A handle that child keep onto its parent so that it is able to report its cancellation.
 *
 * @suppress **This is unstable API and it is subject to change.**
 */
@InternalCoroutinesApi
@Deprecated(level = DeprecationLevel.ERROR, message = "This is internal API and may be removed in the future releases")
public interface ChildHandle : DisposableHandle {

    /**
     * Returns the parent of the current parent-child relationship.
     * @suppress **This is unstable API and it is subject to change.**
     */
    @InternalCoroutinesApi
    public val parent: Job?

    /**
     * Child is cancelling its parent by invoking this method.
     * This method is invoked by the child twice. The first time child report its root cause as soon as possible,
     * so that all its siblings and the parent can start cancelling their work asap. The second time
     * child invokes this method when it had aggregated and determined its final cancellation cause.
     *
     * @suppress **This is unstable API and it is subject to change.**
     */
    @InternalCoroutinesApi
    public fun childCancelled(cause: Throwable): Boolean
}

// -------------------- Job extensions --------------------

/**
 * Disposes a specified [handle] when this job is complete.
 *
 * This is a shortcut for the following code with slightly more efficient implementation (one fewer object created).
 * ```
 * invokeOnCompletion { handle.dispose() }
 * ```
 */
internal fun Job.disposeOnCompletion(handle: DisposableHandle): DisposableHandle =
    invokeOnCompletion(handler = DisposeOnCompletion(handle).asHandler)

/**
 * Cancels the job and suspends the invoking coroutine until the cancelled job is complete.
 *
 * This suspending function is cancellable and **always** checks for a cancellation of the invoking coroutine's Job.
 * If the [Job] of the invoking coroutine is cancelled or completed when this
 * suspending function is invoked or while it is suspended, this function
 * throws [CancellationException].
 *
 * In particular, it means that a parent coroutine invoking `cancelAndJoin` on a child coroutine throws
 * [CancellationException] if the child had failed, since a failure of a child coroutine cancels parent by default,
 * unless the child was launched from within [supervisorScope].
 *
 * This is a shortcut for the invocation of [cancel][Job.cancel] followed by [join][Job.join].
 */
public suspend fun Job.cancelAndJoin() {
    cancel()
    return join()
}

/**
 * Cancels all [children][Job.children] jobs of this coroutine using [Job.cancel] for all of them
 * with an optional cancellation [cause].
 * Unlike [Job.cancel] on this job as a whole, the state of this job itself is not affected.
 */
public fun Job.cancelChildren(cause: CancellationException? = null) {
    children.forEach { it.cancel(cause) }
}

/**
 * @suppress This method implements old version of JVM ABI. Use [cancel].
 */
@Deprecated(level = DeprecationLevel.HIDDEN, message = "Since 1.2.0, binary compatibility with versions <= 1.1.x")
public fun Job.cancelChildren(): Unit = cancelChildren(null)

/**
 * @suppress This method has bad semantics when cause is not a [CancellationException]. Use [Job.cancelChildren].
 */
@Deprecated(level = DeprecationLevel.HIDDEN, message = "Since 1.2.0, binary compatibility with versions <= 1.1.x")
public fun Job.cancelChildren(cause: Throwable? = null) {
    children.forEach { (it as? JobSupport)?.cancelInternal(cause.orCancellation(this)) }
}

// -------------------- CoroutineContext extensions --------------------

/**
 * Returns `true` when the [Job] of the coroutine in this context is still active
 * (has not completed and was not cancelled yet).
 *
 * Check this property in long-running computation loops to support cancellation
 * when [CoroutineScope.isActive] is not available:
 *
 * ```
 * while (coroutineContext.isActive) {
 *     // do some computation
 * }
 * ```
 *
 * The `coroutineContext.isActive` expression is a shortcut for `coroutineContext[Job]?.isActive == true`.
 * See [Job.isActive].
 */
public val CoroutineContext.isActive: Boolean
    get() = this[Job]?.isActive == true

/**
 * Cancels [Job] of this context with an optional cancellation cause.
 * See [Job.cancel] for details.
 */
public fun CoroutineContext.cancel(cause: CancellationException? = null) {
    this[Job]?.cancel(cause)
}

/**
 * @suppress This method implements old version of JVM ABI. Use [CoroutineContext.cancel].
 */
@Deprecated(level = DeprecationLevel.HIDDEN, message = "Since 1.2.0, binary compatibility with versions <= 1.1.x")
public fun CoroutineContext.cancel(): Unit = cancel(null)

/**
 * Ensures that current job is [active][Job.isActive].
 * If the job is no longer active, throws [CancellationException].
 * If the job was cancelled, thrown exception contains the original cancellation cause.
 *
 * This method is a drop-in replacement for the following code, but with more precise exception:
 * ```
 * if (!job.isActive) {
 *     throw CancellationException()
 * }
 * ```
 */
public fun Job.ensureActive(): Unit {
    if (!isActive) throw getCancellationException()
}

/**
 * Ensures that job in the current context is [active][Job.isActive].
 *
 * If the job is no longer active, throws [CancellationException].
 * If the job was cancelled, thrown exception contains the original cancellation cause.
 * This function does not do anything if there is no [Job] in the context, since such a coroutine cannot be cancelled.
 *
 * This method is a drop-in replacement for the following code, but with more precise exception:
 * ```
 * if (!isActive) {
 *     throw CancellationException()
 * }
 * ```
 */
public fun CoroutineContext.ensureActive() {
    get(Job)?.ensureActive()
}

/**
 * Cancels current job, including all its children with a specified diagnostic error [message].
 * A [cause] can be specified to provide additional details on a cancellation reason for debugging purposes.
 */
public fun Job.cancel(message: String, cause: Throwable? = null): Unit = cancel(CancellationException(message, cause))

/**
 * @suppress This method has bad semantics when cause is not a [CancellationException]. Use [CoroutineContext.cancel].
 */
@Deprecated(level = DeprecationLevel.HIDDEN, message = "Since 1.2.0, binary compatibility with versions <= 1.1.x")
public fun CoroutineContext.cancel(cause: Throwable? = null): Boolean {
    val job = this[Job] as? JobSupport ?: return false
    job.cancelInternal(cause.orCancellation(job))
    return true
}

/**
 * Cancels all children of the [Job] in this context, without touching the state of this job itself
 * with an optional cancellation cause. See [Job.cancel].
 * It does not do anything if there is no job in the context or it has no children.
 */
public fun CoroutineContext.cancelChildren(cause: CancellationException? = null) {
    this[Job]?.children?.forEach { it.cancel(cause) }
}

/**
 * @suppress This method implements old version of JVM ABI. Use [CoroutineContext.cancelChildren].
 */
@Deprecated(level = DeprecationLevel.HIDDEN, message = "Since 1.2.0, binary compatibility with versions <= 1.1.x")
public fun CoroutineContext.cancelChildren(): Unit = cancelChildren(null)

/**
 * Retrieves the current [Job] instance from the given [CoroutineContext] or
 * throws [IllegalStateException] if no job is present in the context.
 *
 * This method is a short-cut for `coroutineContext[Job]!!` and should be used only when it is known in advance that
 * the context does have instance of the job in it.
 */
public val CoroutineContext.job: Job get() = get(Job) ?: error("Current context doesn't contain Job in it: $this")

/**
 * @suppress This method has bad semantics when cause is not a [CancellationException]. Use [CoroutineContext.cancelChildren].
 */
@Deprecated(level = DeprecationLevel.HIDDEN, message = "Since 1.2.0, binary compatibility with versions <= 1.1.x")
public fun CoroutineContext.cancelChildren(cause: Throwable? = null) {
    val job = this[Job] ?: return
    job.children.forEach { (it as? JobSupport)?.cancelInternal(cause.orCancellation(job)) }
}

private fun Throwable?.orCancellation(job: Job): Throwable = this ?: JobCancellationException("Job was cancelled", null, job)

/**
 * No-op implementation of [DisposableHandle].
 * @suppress **This an internal API and should not be used from general code.**
 */
@InternalCoroutinesApi
public object NonDisposableHandle : DisposableHandle, ChildHandle {

    override val parent: Job? get() = null

    /**
     * Does not do anything.
     * @suppress
     */
    override fun dispose() {}

    /**
     * Returns `false`.
     * @suppress
     */
    override fun childCancelled(cause: Throwable): Boolean = false

    /**
     * Returns "NonDisposableHandle" string.
     * @suppress
     */
    override fun toString(): String = "NonDisposableHandle"
}