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The `try await` ordering is both easier to read and indicates the order of operations better, because the suspension point occurs first and then one can observe a thrown error.
299 lines
11 KiB
Swift
299 lines
11 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 Group -------------------------------------------------------------
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extension Task {
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/// Starts a new task group which provides a scope in which a dynamic number of
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/// tasks may be spawned.
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///
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/// Tasks added to the group by `group.add()` will automatically be awaited on
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/// when the scope exits. If the group exits by throwing, all added tasks will
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/// be cancelled and their results discarded.
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///
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/// ### Implicit awaiting
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/// When results of tasks added to the group need to be collected, one can
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/// gather their results using the following pattern:
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///
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/// while let result = await group.next() {
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/// // some accumulation logic (e.g. sum += result)
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/// }
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///
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/// ### Thrown errors
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/// When tasks are added to the group using the `group.add` function, they may
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/// immediately begin executing. Even if their results are not collected explicitly
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/// and such task throws, and was not yet cancelled, it may result in the `withGroup`
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/// throwing.
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///
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/// ### Cancellation
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/// If an error is thrown out of the task group, all of its remaining tasks
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/// will be cancelled and the `withGroup` call will rethrow that error.
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///
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/// Individual tasks throwing results in their corresponding `try group.next()`
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/// call throwing, giving a chance to handle individual errors or letting the
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/// error be rethrown by the group.
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///
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/// Postcondition:
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/// Once `withGroup` returns it is guaranteed that the `group` is *empty*.
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///
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/// This is achieved in the following way:
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/// - if the body returns normally:
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/// - the group will await any not yet complete tasks,
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/// - if any of those tasks throws, the remaining tasks will be cancelled,
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/// - once the `withGroup` returns the group is guaranteed to be empty.
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/// - if the body throws:
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/// - all tasks remaining in the group will be automatically cancelled.
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// TODO: Do we have to add a different group type to accommodate throwing
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// tasks without forcing users to use Result? I can't think of how that
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// could be propagated out of the callback body reasonably, unless we
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// commit to doing multi-statement closure typechecking.
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public static func withGroup<TaskResult, BodyResult>(
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resultType: TaskResult.Type,
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returning returnType: BodyResult.Type = BodyResult.self,
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body: @escaping ((inout Task.Group<TaskResult>) async throws -> BodyResult)
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) async throws -> BodyResult {
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let parent = Builtin.getCurrentAsyncTask()
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// Set up the job flags for a new task.
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var groupFlags = JobFlags()
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groupFlags.kind = .task
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groupFlags.priority = getJobFlags(parent).priority
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groupFlags.isChildTask = true
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groupFlags.isTaskGroup = true
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groupFlags.isFuture = true
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let (groupTask, _) =
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Builtin.createAsyncTaskFuture(groupFlags.bits, parent) { () async throws -> BodyResult in
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let task = Builtin.getCurrentAsyncTask()
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var group = Task.Group<TaskResult>(task: task)
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// This defer handles both return/throw cases in the following way:
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// - body throws: all tasks must be cancelled immediately as we re-throw
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// - body returns: as good measure, cancel tasks after draining
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// (although there should be none left by that time)
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defer { group.cancelAll() }
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let result = try await body(&group)
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/// Drain all remaining tasks by awaiting on them;
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/// Their failures are ignored;
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await group._tearDown()
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return result
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}
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// Enqueue the resulting job.
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_enqueueJobGlobal(Builtin.convertTaskToJob(groupTask))
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return try await Handle<BodyResult>(task: groupTask).get()
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}
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/// A task group serves as storage for dynamically started tasks.
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///
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/// Its intended use is with the `Task.withGroup` function.
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/* @unmoveable */
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public struct Group<TaskResult> {
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/// Group task into which child tasks offer their results,
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/// and the `next()` function polls those results from.
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private let task: Builtin.NativeObject
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/// No public initializers
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init(task: Builtin.NativeObject) {
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self.task = task
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_swiftRetain(task)
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}
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// Swift will statically prevent this type from being copied or moved.
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// For now, that implies that it cannot be used with generics.
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/// Add a child task to the group.
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///
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/// ### Error handling
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/// Operations are allowed to `throw`, in which case the `try await next()`
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/// invocation corresponding to the failed task will re-throw the given task.
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///
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/// The `add` function will never (re-)throw errors from the `operation`.
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/// Instead, the corresponding `next()` call will throw the error when necessary.
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///
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/// - Parameters:
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/// - overridingPriority: override priority of the operation task
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/// - operation: operation to execute and add to the group
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@discardableResult
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public mutating func add(
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overridingPriority priorityOverride: Priority? = nil,
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operation: @escaping () async throws -> TaskResult
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) async -> Task.Handle<TaskResult> {
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// Increment the number of pending tasks immediately;
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// We don't need to know which specific task is pending, just that pending
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// ones exist, so that next() can know if to wait or return nil.
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_taskGroupAddPendingTask(self.task)
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let childTask =
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await _runGroupChildTask(overridingPriority: priorityOverride, operation: operation)
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return Handle<TaskResult>(task: childTask)
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}
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/// Wait for the a child task that was added to the group to complete,
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/// and return (or rethrow) the value it completed with. If no tasks are
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/// pending in the task group this function returns `nil`, allowing the
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/// following convenient expressions to be written for awaiting for one
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/// or all tasks to complete:
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///
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/// Await on a single completion:
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///
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/// if let first = try await group.next() {
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/// return first
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/// }
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///
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/// Wait and collect all group child task completions:
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///
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/// while let first = try await group.next() {
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/// collected += value
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/// }
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/// return collected
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///
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/// Awaiting on an empty group results in the immediate return of a `nil`
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/// value, without the group task having to suspend.
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///
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/// ### Ordering
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/// Order of values returned by next() is *completion order*, and not
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/// submission order. I.e. if tasks are added to the group one after another:
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///
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/// await group.add { 1 }
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/// await group.add { 2 }
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///
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/// print(await group.next())
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/// /// Prints "1" OR "2"
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///
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/// ### Errors
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/// If an operation added to the group throws, that error will be rethrown
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/// by the next() call corresponding to that operation's completion.
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///
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/// It is possible to directly rethrow such error out of a `withGroup` body
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/// function's body, causing all remaining tasks to be implicitly cancelled.
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public mutating func next() async throws -> TaskResult? {
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let rawResult = await _taskGroupWaitNext(on: self.task)
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if rawResult.hadErrorResult {
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// Throw the result on error.
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let error = unsafeBitCast(rawResult.storage, to: Error.self)
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throw error
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}
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guard let storage = rawResult.storage else {
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// The group was empty, return nil
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return nil
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}
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// Take the value on success.
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let storagePtr =
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storage.bindMemory(to: TaskResult.self, capacity: 1)
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let value = UnsafeMutablePointer<TaskResult>(mutating: storagePtr).pointee
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return value
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}
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/// Query whether the group has any remaining tasks.
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///
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/// Task groups are always empty upon entry to the `withGroup` body, and
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/// become empty again when `withGroup` returns (either by awaiting on all
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/// pending tasks or cancelling them).
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///
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/// - Returns: `true` if the group has no pending tasks, `false` otherwise.
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public var isEmpty: Bool {
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_taskGroupIsEmpty(self.task)
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}
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/// Cancel all the remaining tasks in the group.
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///
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/// A cancelled group will not will NOT accept new tasks being added into it.
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///
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/// Any results, including errors thrown by tasks affected by this
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/// cancellation, are silently discarded.
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///
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/// - SeeAlso: `Task.addCancellationHandler`
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/// - SeeAlso: `Task.checkCancelled`
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/// - SeeAlso: `Task.isCancelled`
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public mutating func cancelAll() {
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_taskCancel(self.task) // TODO: do we also have to go over all child tasks and cancel there?
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}
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}
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}
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/// ==== -----------------------------------------------------------------------
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extension Task.Group {
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/// Invoked after a withGroup's body returns, and initiates an orderly
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/// teardown of the task.group. No new tasks are accepted by the group // TODO: don't accept new tasks once body returned to avoid infinitely waiting on accident
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///
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/// This function waits until all pending tasks have been processed before
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/// returning.
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///
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/// Failures thrown by remaining tasks are ignored
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///
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/// Child tasks are NOT cancelled by this function implicitly.
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/// If tasks should be cancelled before returning this must be done by an
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/// explicit `group.cancelAll()` call within the `withGroup`'s function body.
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mutating func _tearDown() async {
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// Drain any not next() awaited tasks if the group wasn't cancelled
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// If any of these tasks were to throw
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//
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// Failures of tasks are ignored.
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while !self.isEmpty {
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_ = try? await self.next()
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// TODO: Should a failure cause a cancellation of the task group?
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// This looks very much like supervision trees,
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// where one may have various decisions depending on use cases...
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continue // keep awaiting on all pending tasks
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}
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}
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}
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/// ==== -----------------------------------------------------------------------
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@_silgen_name("swift_retain")
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func _swiftRetain(
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_ task: Builtin.NativeObject
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)
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@_silgen_name("swift_task_group_add_pending")
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func _taskGroupAddPendingTask(
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_ groupTask: Builtin.NativeObject
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)
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@_silgen_name("swift_task_group_offer")
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func taskGroupOffer(
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group: Builtin.NativeObject,
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completedTask: Builtin.NativeObject
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)
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@_silgen_name("swift_task_group_wait_next")
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func _taskGroupWaitNext(
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on groupTask: Builtin.NativeObject
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) async -> (hadErrorResult: Bool, storage: UnsafeRawPointer?)
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enum GroupPollStatus: Int {
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case empty = 0
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case waiting = 1
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case success = 2
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case error = 3
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}
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@_silgen_name("swift_task_group_is_empty")
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func _taskGroupIsEmpty(
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_ groupTask: Builtin.NativeObject
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) -> Bool
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