swift-mirror/stdlib/public/Concurrency/Task.swift

////===----------------------------------------------------------------------===//
////
//// This source file is part of the Swift.org open source project
////
//// Copyright (c) 2020 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 Swift
@_implementationOnly import _SwiftConcurrencyShims

// ==== Task -------------------------------------------------------------------

/// An asynchronous task (just "Task" hereafter) is the analogue of a thread for
/// asynchronous functions. All asynchronous functions run as part of some task.
///
/// A task can only be interacted with by code running "in" the task,
/// by invoking the appropriate context sensitive static functions which operate
/// on the "current" task. Because all such functions are `async` they can only
/// be invoked as part of an existing task, and therefore are guaranteed to be
/// effective.
///
/// A task's execution can be seen as a series of periods where the task was
/// running. Each such period ends at a suspension point or -- finally -- the
/// completion of the task.
///
/// These partial periods towards the task's completion are `PartialAsyncTask`.
/// Partial tasks are generally not interacted with by end-users directly,
/// unless implementing a scheduler.
public struct Task {
  internal let _task: Builtin.NativeObject

  // May only be created by the standard library.
  internal init(_ task: Builtin.NativeObject) {
    self._task = task
  }
}

// ==== Current Task -----------------------------------------------------------

extension Task {

  /// Returns 'current' `Task` instance, representing the task from within which
  /// this function was called.
  ///
  /// All functions available on the Task
  // TODO: once we can have async properties land make this computed property
  public static func current() async -> Task {
    return Task.unsafeCurrent!.task // !-safe, we are guaranteed to have a task available within an async function
  }

}

// ==== Task Priority ----------------------------------------------------------

extension Task {

  /// Returns the `current` task's priority.
  ///
  /// If no current `Task` is available, returns `Priority.default`.
  ///
  /// - SeeAlso: `Task.Priority`
  /// - SeeAlso: `Task.priority`
  public static var currentPriority: Priority {
    Task.unsafeCurrent?.priority ?? Priority.default
  }

  /// Returns the `current` task's priority.
  ///
  /// If no current `Task` is available, returns `Priority.default`.
  ///
  /// - SeeAlso: `Task.Priority`
  /// - SeeAlso: `Task.currentPriority`
  public var priority: Priority {
    getJobFlags(_task).priority
  }

  /// Task priority may inform decisions an `Executor` makes about how and when
  /// to schedule tasks submitted to it.
  ///
  /// ### Priority scheduling
  /// An executor MAY utilize priority information to attempt running higher
  /// priority tasks first, and then continuing to serve lower priority tasks.
  ///
  /// The exact semantics of how priority is treated are left up to each
  /// platform and `Executor` implementation.
  ///
  /// ### Priority inheritance
  /// Child tasks automatically inherit their parent task's priority.
  ///
  /// Detached tasks (created by `Task.runDetached`) DO NOT inherit task priority,
  /// as they are "detached" from their parent tasks after all.
  ///
  /// ### Priority elevation
  /// In some situations the priority of a task must be elevated (or "escalated", "raised"):
  ///
  /// - if a `Task` running on behalf of an actor, and a new higher-priority
  ///   task is enqueued to the actor, its current task must be temporarily
  ///   elevated to the priority of the enqueued task, in order to allow the new
  ///   task to be processed at--effectively-- the priority it was enqueued with.
  ///   - this DOES NOT affect `Task.currentPriority()`.
  /// - if a task is created with a `Task.Handle`, and a higher-priority task
  ///   calls the `await handle.get()` function the priority of this task must be
  ///   permanently increased until the task completes.
  ///   - this DOES affect `Task.currentPriority()`.
  ///
  /// TODO: Define the details of task priority; It is likely to be a concept
  ///       similar to Darwin Dispatch's QoS; bearing in mind that priority is not as
  ///       much of a thing on other platforms (i.e. server side Linux systems).
  public enum Priority: Int, Comparable {
    // Values must be same as defined by the internal `JobPriority`.
    case userInteractive = 0x21
    case userInitiated   = 0x19
    case `default`       = 0x15
    case utility         = 0x11
    case background      = 0x09
    case unspecified     = 0x00

    public static func < (lhs: Priority, rhs: Priority) -> Bool {
      lhs.rawValue < rhs.rawValue
    }
  }
}

// ==== Task Handle ------------------------------------------------------------

extension Task {
  /// A task handle refers to an in-flight `Task`,
  /// allowing for potentially awaiting for its result or Cancelling it.
  ///
  /// It is not a programming error to drop a handle without awaiting or cancelling it,
  /// i.e. the task will run regardless of the handle still being present or not.
  /// Dropping a handle however means losing the ability to await on the task's result
  /// and losing the ability to cancel it.
  public struct Handle<Success, Failure: Error> {
    internal let _task: Builtin.NativeObject

    internal init(_ task: Builtin.NativeObject) {
      self._task = task
    }

    /// Returns the `Task` that this handle refers to.
    public var task: Task {
      Task(_task)
    }

    /// Wait for the task to complete, returning (or throwing) its result.
    ///
    /// ### Priority
    /// If the task has not completed yet, its priority will be elevated to the
    /// priority of the current task. Note that this may not be as effective as
    /// creating the task with the "right" priority to in the first place.
    ///
    /// ### Cancellation
    /// If the awaited on task gets cancelled externally the `get()` will throw
    /// a cancellation error.
    ///
    /// If the task gets cancelled internally, e.g. by checking for cancellation
    /// and throwing a specific error or using `checkCancellation` the error
    /// thrown out of the task will be re-thrown here.
    public func get() async throws -> Success {
      return try await _taskFutureGetThrowing(_task)
    }

    /// Wait for the task to complete, returning its `Result`.
    ///
    /// ### Priority
    /// If the task has not completed yet, its priority will be elevated to the
    /// priority of the current task. Note that this may not be as effective as
    /// creating the task with the "right" priority to in the first place.
    ///
    /// ### Cancellation
    /// If the awaited on task gets cancelled externally the `get()` will throw
    /// a cancellation error.
    ///
    /// If the task gets cancelled internally, e.g. by checking for cancellation
    /// and throwing a specific error or using `checkCancellation` the error
    /// thrown out of the task will be re-thrown here.
    public func getResult() async -> Result<Success, Failure> {
      do {
        return .success(try await get())
      } catch {
        return .failure(error as! Failure) // as!-safe, guaranteed to be Failure
      }
    }

    /// Attempt to cancel the task.
    ///
    /// Whether this function has any effect is task-dependent.
    ///
    /// For a task to respect cancellation it must cooperatively check for it
    /// while running. Many tasks will check for cancellation before beginning
    /// their "actual work", however this is not a requirement nor is it guaranteed
    /// how and when tasks check for cancellation in general.
    public func cancel() {
      Builtin.cancelAsyncTask(_task)
    }
  }
}

extension Task.Handle where Failure == Never {

  /// Wait for the task to complete, returning its result.
  ///
  /// ### Priority
  /// If the task has not completed yet, its priority will be elevated to the
  /// priority of the current task. Note that this may not be as effective as
  /// creating the task with the "right" priority to in the first place.
  ///
  /// ### Cancellation
  /// The task this handle refers to may check for cancellation, however
  /// since it is not-throwing it would have to handle it using some other
  /// way than throwing a `CancellationError`, e.g. it could provide a neutral
  /// value of the `Success` type, or encode that cancellation has occurred in
  /// that type itself.
  public func get() async -> Success {
    return await _taskFutureGet(_task)
  }
  
}

extension Task.Handle: Hashable {
  public func hash(into hasher: inout Hasher) {
    UnsafeRawPointer(Builtin.bridgeToRawPointer(_task)).hash(into: &hasher)
  }
}

extension Task.Handle: Equatable {
  public static func ==(lhs: Self, rhs: Self) -> Bool {
    UnsafeRawPointer(Builtin.bridgeToRawPointer(lhs._task)) ==
      UnsafeRawPointer(Builtin.bridgeToRawPointer(rhs._task))
  }
}

// ==== Conformances -----------------------------------------------------------

extension Task: Hashable {
  public func hash(into hasher: inout Hasher) {
    UnsafeRawPointer(Builtin.bridgeToRawPointer(_task)).hash(into: &hasher)
  }
}

extension Task: Equatable {
  public static func ==(lhs: Self, rhs: Self) -> Bool {
    UnsafeRawPointer(Builtin.bridgeToRawPointer(lhs._task)) ==
      UnsafeRawPointer(Builtin.bridgeToRawPointer(rhs._task))
  }
}

// ==== Job Flags --------------------------------------------------------------

extension Task {
  /// Flags for schedulable jobs.
  ///
  /// This is a port of the C++ FlagSet.
  struct JobFlags {
    /// Kinds of schedulable jobs.
    enum Kind: Int {
      case task = 0
    };

    /// The actual bit representation of these flags.
    var bits: Int = 0

    /// The kind of job described by these flags.
    var kind: Kind {
      get {
        Kind(rawValue: bits & 0xFF)!
      }

      set {
        bits = (bits & ~0xFF) | newValue.rawValue
      }
    }

    /// Whether this is an asynchronous task.
    var isAsyncTask: Bool { kind == .task }

    /// The priority given to the job.
    var priority: Priority {
      get {
        Priority(rawValue: (bits & 0xFF00) >> 8)!
      }

      set {
        bits = (bits & ~0xFF00) | (newValue.rawValue << 8)
      }
    }

    /// Whether this is a child task.
    var isChildTask: Bool {
      get {
        (bits & (1 << 24)) != 0
      }

      set {
        if newValue {
          bits = bits | 1 << 24
        } else {
          bits = (bits & ~(1 << 24))
        }
      }
    }

    /// Whether this is a future.
    var isFuture: Bool {
      get {
        (bits & (1 << 25)) != 0
      }

      set {
        if newValue {
          bits = bits | 1 << 25
        } else {
          bits = (bits & ~(1 << 25))
        }
      }
    }

    /// Whether this is a group child.
    var isGroupChildTask: Bool {
      get {
        (bits & (1 << 26)) != 0
      }

      set {
        if newValue {
          bits = bits | 1 << 26
        } else {
          bits = (bits & ~(1 << 26))
        }
      }
    }

    /// Whether this (or its parents) have task local values.
    var hasLocalValues: Bool {
      get {
        (bits & (1 << 27)) != 0
      }

      set {
        if newValue {
          bits = bits | 1 << 27
        } else {
          bits = (bits & ~(1 << 27))
        }
      }
    }

  }
}

// ==== Detached Tasks ---------------------------------------------------------

extension Task {
  /// Run given throwing `operation` as part of a new top-level task.
  ///
  /// Creating detached tasks should, generally, be avoided in favor of using
  /// `async` functions, `async let` declarations and `await` expressions - as
  /// those benefit from structured, bounded concurrency which is easier to reason
  /// about, as well as automatically inheriting the parent tasks priority,
  /// task-local storage, deadlines, as well as being cancelled automatically
  /// when their parent task is cancelled. Detached tasks do not get any of those
  /// benefits, and thus should only be used when an operation is impossible to
  /// be modelled with child tasks.
  ///
  /// ### Cancellation
  /// A detached task always runs to completion unless it is explicitly cancelled.
  /// Specifically, dropping a detached tasks `Task.Handle` does _not_ automatically
  /// cancel given task.
  ///
  /// Cancelling a task must be performed explicitly via `handle.cancel()`.
  ///
  /// - Note: it is generally preferable to use child tasks rather than detached
  ///   tasks. Child tasks automatically carry priorities, task-local state,
  ///   deadlines and have other benefits resulting from the structured
  ///   concurrency concepts that they model. Consider using detached tasks only
  ///   when strictly necessary and impossible to model operations otherwise.
  ///
  /// - Parameters:
  ///   - priority: priority of the task
  ///   - executor: the executor on which the detached closure should start
  ///               executing on.
  ///   - operation: the operation to execute
  /// - Returns: handle to the task, allowing to `await handle.get()` on the
  ///     tasks result or `cancel` it. If the operation fails the handle will
  ///     throw the error the operation has thrown when awaited on.
  @discardableResult
  public static func runDetached<T>(
    priority: Priority = .default,
    operation: @concurrent @escaping () async -> T
    // TODO: Allow inheriting task-locals?
  ) -> Handle<T, Never> {
    // Set up the job flags for a new task.
    var flags = JobFlags()
    flags.kind = .task
    flags.priority = priority
    flags.isFuture = true

    // Create the asynchronous task future.
    let (task, _) = Builtin.createAsyncTaskFuture(flags.bits, nil, operation)

    // Enqueue the resulting job.
    _enqueueJobGlobal(Builtin.convertTaskToJob(task))

    return Handle<T, Never>(task)
  }

  /// Run given throwing `operation` as part of a new top-level task.
  ///
  /// Creating detached tasks should, generally, be avoided in favor of using
  /// `async` functions, `async let` declarations and `await` expressions - as
  /// those benefit from structured, bounded concurrency which is easier to reason
  /// about, as well as automatically inheriting the parent tasks priority,
  /// task-local storage, deadlines, as well as being cancelled automatically
  /// when their parent task is cancelled. Detached tasks do not get any of those
  /// benefits, and thus should only be used when an operation is impossible to
  /// be modelled with child tasks.
  ///
  /// ### Cancellation
  /// A detached task always runs to completion unless it is explicitly cancelled.
  /// Specifically, dropping a detached tasks `Task.Handle` does _not_ automatically
  /// cancel given task.
  ///
  /// Cancelling a task must be performed explicitly via `handle.cancel()`.
  ///
  /// - Note: it is generally preferable to use child tasks rather than detached
  ///   tasks. Child tasks automatically carry priorities, task-local state,
  ///   deadlines and have other benefits resulting from the structured
  ///   concurrency concepts that they model. Consider using detached tasks only
  ///   when strictly necessary and impossible to model operations otherwise.
  ///
  /// - Parameters:
  ///   - priority: priority of the task
  ///   - executor: the executor on which the detached closure should start
  ///               executing on.
  ///   - operation: the operation to execute
  /// - Returns: handle to the task, allowing to `await handle.get()` on the
  ///     tasks result or `cancel` it. If the operation fails the handle will
  ///     throw the error the operation has thrown when awaited on.
  @discardableResult
  public static func runDetached<T, Failure>(
    priority: Priority = .default,
    operation: @concurrent @escaping () async throws -> T
  ) -> Handle<T, Failure> {
    // Set up the job flags for a new task.
    var flags = JobFlags()
    flags.kind = .task
    flags.priority = priority
    flags.isFuture = true

    // Create the asynchronous task future.
    let (task, _) = Builtin.createAsyncTaskFuture(flags.bits, nil, operation)

    // Enqueue the resulting job.
    _enqueueJobGlobal(Builtin.convertTaskToJob(task))

    return Handle<T, Failure>(task)
  }
}

// ==== Async Handler ----------------------------------------------------------

public func _runAsyncHandler(operation: @escaping () async -> ()) {
  typealias ConcurrentFunctionType = @concurrent () async -> ()
  Task.runDetached(
    operation: unsafeBitCast(operation, to: ConcurrentFunctionType.self)
  )
}

// ==== UnsafeCurrentTask ------------------------------------------------------

extension Task {
  /// If available, returns the 'current' task, representing the async context
  /// from which this function was called.
  ///
  /// This computed property can be called from 'async' as well as synchronous
  /// functions. Within synchronous functions it may return a `nil` value,
  /// which means that the function was not called within any asynchronous task
  /// at all, otherwise the returned task is equal to the task of the nearest
  /// asynchronous function present in this functions call stack.
  ///
  /// The returned value must not be accessed from tasks other than the current one.
  public static var unsafeCurrent: UnsafeCurrentTask? {
    guard let _task = _getCurrentAsyncTask() else {
      return nil
    }
    // FIXME: This retain seems pretty wrong, however if we don't we WILL crash
    //        with "destroying a task that never completed" in the task's destroy.
    //        How do we solve this properly?
    _swiftRetain(_task)
    return UnsafeCurrentTask(_task)
  }

}

/// An *unsafe* 'current' task handle.
///
/// An `UnsafeCurrentTask` should not be stored for "later" access.
///
/// Storing an `UnsafeCurrentTask` has no implication on the task's actual lifecycle.
///
/// The sub-set of APIs of `UnsafeCurrentTask` which also exist on `Task` are
/// generally safe to be invoked from any task/thread.
///
/// All other APIs must not, be called 'from' any other task than the one
/// represented by this handle itself. Doing so may result in undefined behavior,
/// and most certainly will break invariants in other places of the program
/// actively running on this task.
public struct UnsafeCurrentTask {
  internal let _task: Builtin.NativeObject

  // May only be created by the standard library.
  internal init(_ task: Builtin.NativeObject) {
    self._task = task
  }

  /// Returns `Task` representing the same asynchronous context as this 'UnsafeCurrentTask'.
  ///
  /// Operations on `Task` (unlike `UnsafeCurrentTask`) are safe to be called
  /// from any other task (or thread).
  public var task: Task {
    Task(_task)
  }

  /// Returns `true` if the task is cancelled, and should stop executing.
  ///
  /// - SeeAlso: `checkCancellation()`
  public var isCancelled: Bool {
    _taskIsCancelled(_task)
  }

  /// Returns the `current` task's priority.
  ///
  /// If no current `Task` is available, returns `Priority.default`.
  ///
  /// - SeeAlso: `Task.Priority`
  /// - SeeAlso: `Task.currentPriority`
  public var priority: Task.Priority {
    getJobFlags(_task).priority
  }

}

extension UnsafeCurrentTask: Hashable {
  public func hash(into hasher: inout Hasher) {
    UnsafeRawPointer(Builtin.bridgeToRawPointer(_task)).hash(into: &hasher)
  }
}

extension UnsafeCurrentTask: Equatable {
  public static func ==(lhs: Self, rhs: Self) -> Bool {
    UnsafeRawPointer(Builtin.bridgeToRawPointer(lhs._task)) ==
      UnsafeRawPointer(Builtin.bridgeToRawPointer(rhs._task))
  }
}

// ==== Internal ---------------------------------------------------------------

@_silgen_name("swift_task_getCurrent")
func _getCurrentAsyncTask() -> Builtin.NativeObject?

@_silgen_name("swift_task_getJobFlags")
func getJobFlags(_ task: Builtin.NativeObject) -> Task.JobFlags

@_silgen_name("swift_task_enqueueGlobal")
@usableFromInline
func _enqueueJobGlobal(_ task: Builtin.Job)

@available(*, deprecated)
@_silgen_name("swift_task_runAndBlockThread")
public func runAsyncAndBlock(_ asyncFun: @escaping () async -> ())

@_silgen_name("swift_task_asyncMainDrainQueue")
public func _asyncMainDrainQueue() -> Never

public func _runAsyncMain(_ asyncFun: @escaping () async throws -> ()) {
#if os(Windows)
  Task.runDetached {
    do {
      try await asyncFun()
      exit(0)
    } catch {
      _errorInMain(error)
    }
  }
#else
  @MainActor @concurrent
  func _doMain(_ asyncFun: @escaping () async throws -> ()) async {
    do {
      try await asyncFun()
    } catch {
      _errorInMain(error)
    }
  }

  Task.runDetached {
    await _doMain(asyncFun)
    exit(0)
  }
#endif
  _asyncMainDrainQueue()
}

// FIXME: both of these ought to take their arguments _owned so that
// we can do a move out of the future in the common case where it's
// unreferenced
@_silgen_name("swift_task_future_wait")
public func _taskFutureGet<T>(_ task: Builtin.NativeObject) async -> T

@_silgen_name("swift_task_future_wait_throwing")
public func _taskFutureGetThrowing<T>(_ task: Builtin.NativeObject) async throws -> T

public func _runChildTask<T>(
  operation: @concurrent @escaping () async throws -> T
) async -> Builtin.NativeObject {
  let currentTask = Builtin.getCurrentAsyncTask()

  // Set up the job flags for a new task.
  var flags = Task.JobFlags()
  flags.kind = .task
  flags.priority = getJobFlags(currentTask).priority
  flags.isFuture = true
  flags.isChildTask = true

  // Create the asynchronous task future.
  let (task, _) = Builtin.createAsyncTaskFuture(
      flags.bits, currentTask, operation)

  // Enqueue the resulting job.
  _enqueueJobGlobal(Builtin.convertTaskToJob(task))

  return task
}

@_silgen_name("swift_task_cancel")
func _taskCancel(_ task: Builtin.NativeObject)

@_silgen_name("swift_task_isCancelled")
func _taskIsCancelled(_ task: Builtin.NativeObject) -> Bool

#if _runtime(_ObjC)

/// Intrinsic used by SILGen to launch a task for bridging a Swift async method
/// which was called through its ObjC-exported completion-handler-based API.
@_alwaysEmitIntoClient
@usableFromInline
internal func _runTaskForBridgedAsyncMethod(_ body: @escaping () async -> Void) {
  // TODO: We can probably do better than Task.runDetached
  // if we're already running on behalf of a task,
  // if the receiver of the method invocation is itself an Actor, or in other
  // situations.
  Task.runDetached { await body() }
}

#endif