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//===----------------------------------------------------------------------===//
//
// This source file is part of the Swift.org open source project
//
// Copyright (c) 2014 - 2018 Apple Inc. and the Swift project authors
// Licensed under Apache License v2.0 with Runtime Library Exception
//
// See https://swift.org/LICENSE.txt for license information
// See https://swift.org/CONTRIBUTORS.txt for the list of Swift project authors
//
//===----------------------------------------------------------------------===//
import Foundation
/// Abstraction layer so we can store a heterogeneous collection of tasks in an
/// array.
private protocol AnyTask: Sendable {
func waitForCompletion() async
func cancel()
}
extension Task: AnyTask {
func waitForCompletion() async {
_ = try? await value
}
}
/// A type that is able to track dependencies between tasks.
package protocol DependencyTracker: Sendable {
/// Which tasks need to finish before a task described by `self` may start executing.
/// `pendingTasks` is sorted in the order in which the tasks were enqueued to `AsyncQueue`.
func dependencies(in pendingTasks: [PendingTask<Self>]) -> [PendingTask<Self>]
}
/// A dependency tracker where each task depends on every other, i.e. a serial
/// queue.
package struct Serial: DependencyTracker {
package func dependencies(in pendingTasks: [PendingTask<Self>]) -> [PendingTask<Self>] {
if let lastTask = pendingTasks.last {
return [lastTask]
}
return []
}
}
package struct PendingTask<TaskMetadata: Sendable>: Sendable {
/// The task that is pending.
fileprivate let task: any AnyTask
package let metadata: TaskMetadata
/// A unique value used to identify the task. This allows tasks to get
/// removed from `pendingTasks` again after they finished executing.
fileprivate let id: UUID
}
/// A list of pending tasks that can be sent across actor boundaries and is guarded by a lock.
///
/// - Note: Unchecked sendable because the tasks are being protected by a lock.
private class PendingTasks<TaskMetadata: Sendable>: @unchecked Sendable {
/// Lock guarding `pendingTasks`.
private let lock = NSLock()
/// Pending tasks that have not finished execution yet.
///
/// - Important: This must only be accessed while `lock` has been acquired.
private var tasks: [PendingTask<TaskMetadata>] = []
init() {
self.lock.name = "AsyncQueue"
}
/// Capture a lock and execute the closure, which may modify the pending tasks.
func withLock<T>(_ body: (_ pendingTasks: inout [PendingTask<TaskMetadata>]) throws -> T) rethrows -> T {
try lock.withLock {
try body(&tasks)
}
}
}
/// A queue that allows the execution of asynchronous blocks of code.
package final class AsyncQueue<TaskMetadata: DependencyTracker>: Sendable {
private let pendingTasks: PendingTasks<TaskMetadata> = PendingTasks()
package init() {}
/// Schedule a new closure to be executed on the queue.
///
/// If this is a serial queue, all previously added tasks are guaranteed to
/// finished executing before this closure gets executed.
///
/// If this is a barrier, all previously scheduled tasks are guaranteed to
/// finish execution before the barrier is executed and all tasks that are
/// added later will wait until the barrier finishes execution.
@discardableResult
package func async<Success: Sendable>(
priority: TaskPriority? = nil,
metadata: TaskMetadata,
@_inheritActorContext operation: @escaping @Sendable () async -> Success
) -> Task<Success, Never> {
let throwingTask = asyncThrowing(priority: priority, metadata: metadata, operation: operation)
return Task(priority: priority) {
do {
return try await throwingTask.valuePropagatingCancellation
} catch {
// We know this can never happen because `operation` does not throw.
preconditionFailure("Executing a task threw an error even though the operation did not throw")
}
}
}
/// Same as ``AsyncQueue/async(priority:barrier:operation:)`` but allows the
/// operation to throw.
///
/// - Important: The caller is responsible for handling any errors thrown from
/// the operation by awaiting the result of the returned task.
package func asyncThrowing<Success: Sendable>(
priority: TaskPriority? = nil,
metadata: TaskMetadata,
@_inheritActorContext operation: @escaping @Sendable () async throws -> Success
) -> Task<Success, any Error> {
let id = UUID()
return pendingTasks.withLock { tasks in
// Build the list of tasks that need to finished execution before this one
// can be executed
let dependencies = metadata.dependencies(in: tasks)
// Schedule the task.
let task = Task(priority: priority) { [pendingTasks] in
// IMPORTANT: The only throwing call in here must be the call to
// operation. Otherwise the assumption that the task will never throw
// if `operation` does not throw, which we are making in `async` does
// not hold anymore.
for dependency in dependencies {
await dependency.task.waitForCompletion()
}
let result = try await operation()
pendingTasks.withLock { tasks in
tasks.removeAll(where: { $0.id == id })
}
return result
}
tasks.append(PendingTask(task: task, metadata: metadata, id: id))
return task
}
}
}
/// Convenience overloads for serial queues.
extension AsyncQueue where TaskMetadata == Serial {
/// Same as ``async(priority:operation:)`` but specialized for serial queues
/// that don't specify any metadata.
@discardableResult
package func async<Success: Sendable>(
priority: TaskPriority? = nil,
@_inheritActorContext operation: @escaping @Sendable () async -> Success
) -> Task<Success, Never> {
return self.async(priority: priority, metadata: Serial(), operation: operation)
}
/// Same as ``asyncThrowing(priority:metadata:operation:)`` but specialized
/// for serial queues that don't specify any metadata.
package func asyncThrowing<Success: Sendable>(
priority: TaskPriority? = nil,
@_inheritActorContext operation: @escaping @Sendable () async throws -> Success
) -> Task<Success, any Error> {
return self.asyncThrowing(priority: priority, metadata: Serial(), operation: operation)
}
}
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