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398 lines
14 KiB
Swift
398 lines
14 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: (inout Task.Group<TaskResult>) async throws -> BodyResult
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) async throws -> BodyResult {
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let task = Builtin.getCurrentAsyncTask()
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let _group = _taskGroupCreate(task: task)
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var group: Task.Group<TaskResult>! = Task.Group(task: task, group: _group)
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// Run the withGroup body.
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do {
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let result = try await body(&group)
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await group._tearDown()
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group = nil
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_taskGroupDestroy(task: task, group: _group)
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return result
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} catch {
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group.cancelAll()
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await group._tearDown()
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group = nil
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_taskGroupDestroy(task: task, group: _group)
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throw error
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}
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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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public struct Group<TaskResult>: AsyncSequence {
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public typealias AsyncIterator = GroupIterator
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public typealias Element = TaskResult
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private let _task: Builtin.NativeObject
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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 _group: Builtin.RawPointer
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/// No public initializers
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init(task: Builtin.NativeObject, group: Builtin.RawPointer) {
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// TODO: this feels slightly off, any other way to avoid the task being too eagerly released?
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_swiftRetain(task) // to avoid the task being destroyed when the group is destroyed
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self._task = task
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self._group = group
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}
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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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/// - Returns:
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/// - `true` if the operation was added to the group successfully,
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/// `false` otherwise (e.g. because the group `isCancelled`)
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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: @concurrent @escaping () async throws -> TaskResult
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) async -> Bool {
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let canAdd = _taskGroupAddPendingTask(group: _group)
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guard canAdd else {
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// the group is cancelled and is not accepting any new work
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return false
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}
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// Set up the job flags for a new task.
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var flags = Task.JobFlags()
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flags.kind = .task
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flags.priority = priorityOverride ?? getJobFlags(_task).priority
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flags.isFuture = true
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flags.isChildTask = true
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flags.isGroupChildTask = true
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// Create the asynchronous task future.
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let (childTask, _) = Builtin.createAsyncTaskGroupFuture(
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flags.bits, _task, _group, operation)
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// Attach it to the group's task record in the current task.
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_taskGroupAttachChild(group: _group, child: childTask)
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// Enqueue the resulting job.
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_enqueueJobGlobal(Builtin.convertTaskToJob(childTask))
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return true
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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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/// It is also possible to use `for await` to collect results of a task groups:
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///
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/// for await try value in group {
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/// collected += value
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/// }
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///
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/// ### Thread-safety
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/// Please note that the `group` object MUST NOT escape into another task.
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/// The `group.next()` MUST be awaited from the task that had originally
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/// created the group. It is not allowed to escape the group reference.
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///
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/// Note also that this is generally prevented by Swift's type-system,
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/// as the `add` operation is `mutating`, and those may not be performed
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/// from concurrent execution contexts, such as child tasks.
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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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#if NDEBUG
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let callingTask = Builtin.getCurrentAsyncTask() // can't inline into the assert sadly
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assert(unsafeBitCast(callingTask, to: size_t.self) ==
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unsafeBitCast(_task, to: size_t.self),
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"""
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group.next() invoked from task other than the task which created the group! \
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This means the group must have illegally escaped the withGroup{} scope.
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""")
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#endif
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return try await _taskGroupWaitNext(waitingTask: _task, group: _group)
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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(_group)
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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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/// This function may be called even from within child (or any other) tasks,
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/// and will reliably cause the group to become cancelled.
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///
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/// - SeeAlso: `Task.isCancelled`
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/// - SeeAlso: `TaskGroup.isCancelled`
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public func cancelAll() {
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_taskGroupCancelAll(task: _task, group: _group)
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}
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/// Returns `true` if the group was cancelled, e.g. by `cancelAll`.
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///
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/// If the task currently running this group was cancelled, the group will
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/// also be implicitly cancelled, which will be reflected in the return
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/// value of this function as well.
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///
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/// - Returns: `true` if the group (or its parent task) was cancelled,
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/// `false` otherwise.
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public var isCancelled: Bool {
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return _taskIsCancelled(_task) ||
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_taskGroupIsCancelled(task: _task, group: _group)
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}
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}
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}
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/// ==== TaskGroup: AsyncSequence.AsyncIterator --------------------------------
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extension Task.Group {
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public func makeAsyncIterator() -> GroupIterator {
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return GroupIterator(group: self)
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}
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/// Allows iterating over results of tasks added to the group.
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///
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/// The order of elements returned by this iterator is the same as manually
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/// invoking the `group.next()` function in a loop, meaning that results
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/// are returned in *completion order*.
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///
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/// - SeeAlso: `TaskGroup.next()`
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public struct GroupIterator: AsyncIteratorProtocol {
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public typealias Element = TaskResult
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@usableFromInline
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var group: Task.Group<TaskResult>
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// no public constructors
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init(group: Task.Group<TaskResult>) {
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self.group = group
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}
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/// - SeeAlso: `TaskGroup.next()` for a detailed discussion its semantics.
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public mutating func next() async throws -> Element? {
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try await group.next()
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}
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public mutating func cancel() {
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group.cancelAll()
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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 exits, and initiates an orderly
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/// teardown of the group.
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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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/// Once this function returns the underlying task group has been freed,
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/// and must not be touched ever again.
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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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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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_ object: Builtin.NativeObject
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)
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@_silgen_name("swift_release")
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func _swiftRelease(
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_ object: Builtin.NativeObject
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)
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@_silgen_name("swift_taskGroup_create")
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func _taskGroupCreate(
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task: Builtin.NativeObject
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) -> Builtin.RawPointer
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/// Attach task group child to the group group to the task.
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@_silgen_name("swift_taskGroup_attachChild")
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func _taskGroupAttachChild(
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group: Builtin.RawPointer,
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child: Builtin.NativeObject
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) -> UnsafeRawPointer /*ChildTaskStatusRecord*/
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@_silgen_name("swift_taskGroup_destroy")
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func _taskGroupDestroy(
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task: Builtin.NativeObject,
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group: __owned Builtin.RawPointer
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)
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@_silgen_name("swift_taskGroup_addPending")
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func _taskGroupAddPendingTask(
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group: Builtin.RawPointer
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) -> Bool
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@_silgen_name("swift_taskGroup_cancelAll")
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func _taskGroupCancelAll(
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task: Builtin.NativeObject,
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group: Builtin.RawPointer
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)
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/// Checks ONLY if the group was specifically cancelled.
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/// The task itself being cancelled must be checked separately.
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@_silgen_name("swift_taskGroup_isCancelled")
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func _taskGroupIsCancelled(
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task: Builtin.NativeObject,
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group: Builtin.RawPointer
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) -> Bool
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@_silgen_name("swift_taskGroup_wait_next_throwing")
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func _taskGroupWaitNext<T>(
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waitingTask: Builtin.NativeObject,
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group: Builtin.RawPointer
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) async throws -> T?
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enum PollStatus: 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_taskGroup_isEmpty")
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func _taskGroupIsEmpty(
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_ group: Builtin.RawPointer
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) -> Bool
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