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109 lines
3.9 KiB
Swift
109 lines
3.9 KiB
Swift
////===----------------------------------------------------------------------===//
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////
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//// This source file is part of the Swift.org open source project
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////
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//// Copyright (c) 2020 Apple Inc. and the Swift project authors
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//// Licensed under Apache License v2.0 with Runtime Library Exception
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////
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//// See https://swift.org/LICENSE.txt for license information
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//// See https://swift.org/CONTRIBUTORS.txt for the list of Swift project authors
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////
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////===----------------------------------------------------------------------===//
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import Swift
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@_implementationOnly import _SwiftConcurrencyShims
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// ==== Task Cancellation ------------------------------------------------------
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extension Task {
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/// Returns `true` if the task is cancelled, and should stop executing.
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///
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/// If no current `Task` is available, returns `false`, as outside of a task
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/// context no task cancellation may be observed.
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///
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/// - SeeAlso: `checkCancellation()`
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public static var isCancelled: Bool {
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Task.unsafeCurrent?.isCancelled ?? false
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}
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/// Returns `true` if the task is cancelled, and should stop executing.
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///
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/// - SeeAlso: `checkCancellation()`
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public var isCancelled: Bool {
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_taskIsCancelled(_task)
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}
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/// Check if the task is cancelled and throw an `CancellationError` if it was.
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///
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/// It is intentional that no information is passed to the task about why it
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/// was cancelled. A task may be cancelled for many reasons, and additional
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/// reasons may accrue / after the initial cancellation (for example, if the
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/// task fails to immediately exit, it may pass a deadline).
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///
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/// The goal of cancellation is to allow tasks to be cancelled in a
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/// lightweight way, not to be a secondary method of inter-task communication.
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///
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/// ### Suspension
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/// This function returns instantly and will never suspend.
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///
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/// - SeeAlso: `isCancelled()`
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public static func checkCancellation() throws {
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if Task.isCancelled {
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throw CancellationError()
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}
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}
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/// Execute an operation with cancellation handler which will immediately be
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/// invoked if the current task is cancelled.
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///
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/// This differs from the operation cooperatively checking for cancellation
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/// and reacting to it in that the cancellation handler is _always_ and
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/// _immediately_ invoked when the task is cancelled. For example, even if the
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/// operation is running code which never checks for cancellation, a cancellation
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/// handler still would run and give us a chance to run some cleanup code.
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///
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/// Does not check for cancellation, and always executes the passed `operation`.
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///
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/// This function returns instantly and will never suspend.
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public static func withCancellationHandler<T>(
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handler: @concurrent () -> (),
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operation: () async throws -> T
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) async rethrows -> T {
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let task = Builtin.getCurrentAsyncTask()
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guard !_taskIsCancelled(task) else {
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// If the current task is already cancelled, run the handler immediately.
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handler()
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return try await operation()
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}
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let record = _taskAddCancellationHandler(task: task, handler: handler)
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defer { _taskRemoveCancellationHandler(task: task, record: record) }
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return try await operation()
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}
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/// The default cancellation thrown when a task is cancelled.
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///
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/// This error is also thrown automatically by `Task.checkCancellation()`,
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/// if the current task has been cancelled.
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public struct CancellationError: Error {
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// no extra information, cancellation is intended to be light-weight
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public init() {}
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}
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}
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@_silgen_name("swift_task_addCancellationHandler")
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func _taskAddCancellationHandler(
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task: Builtin.NativeObject,
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handler: @concurrent () -> ()
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) -> UnsafeRawPointer /*CancellationNotificationStatusRecord*/
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@_silgen_name("swift_task_removeCancellationHandler")
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func _taskRemoveCancellationHandler(
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task: Builtin.NativeObject,
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record: UnsafeRawPointer /*CancellationNotificationStatusRecord*/
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)
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