[Concurrency] Add API stubs for Task

stdlib/public/Concurrency/CMakeLists.txt

@@ -14,6 +14,7 @@ add_swift_target_library(swift_Concurrency ${SWIFT_STDLIB_LIBRARY_BUILD_TYPES} I
   Actor.swift
   PartialAsyncTask.swift
   Task.cpp
+  Task.swift
   TaskAlloc.cpp
   TaskStatus.cpp
   Mutex.cpp
@@ -36,6 +37,7 @@ add_swift_target_library(swift_Concurrency ${SWIFT_STDLIB_LIBRARY_BUILD_TYPES} I
   SWIFT_COMPILE_FLAGS
     ${SWIFT_STANDARD_LIBRARY_SWIFT_FLAGS}
     -parse-stdlib
+    -Xfrontend -enable-experimental-concurrency
   LINK_FLAGS "${SWIFT_RUNTIME_SWIFT_LINK_FLAGS}"
   INSTALL_IN_COMPONENT stdlib
 )

stdlib/public/Concurrency/PartialAsyncTask.swift

@@ -13,6 +13,7 @@
 import Swift
 @_implementationOnly import _SwiftConcurrencyShims
 
+/// A partial task is a unit of scheduleable work.
 public struct PartialAsyncTask {
   private var context: UnsafeMutablePointer<_SwiftContext>
 

stdlib/public/Concurrency/Task.swift

@@ -0,0 +1,264 @@
+////===----------------------------------------------------------------------===//
+////
+//// 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'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 {
+}
+
+// ==== Current Task -----------------------------------------------------------
+
+extension Task {
+  /// Returns the currently executing `Task`.
+  ///
+  /// As invoking this function is only possible from an asynchronous context
+  /// it is always able to return the current `Task` in which we are currently
+  /// running.
+  public static func current() async -> Task {
+    fatalError("\(#function) not implemented yet.") // TODO: needs a built-in function
+  }
+}
+
+// ==== Task Priority ----------------------------------------------------------
+
+extension Task {
+  /// 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 ("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 struct Priority: Comparable {
+    public static let `default`: Task.Priority = .init() // TODO: replace with actual values
+
+    // TODO: specifics of implementation are not decided yet
+    private let __value: Int = 0
+
+    public static func < (lhs: Self, rhs: Self) -> Bool {
+      lhs.__value < rhs.__value
+    }
+  }
+}
+
+// ==== Task Handle ------------------------------------------------------------
+
+extension Task {
+
+  /// A task handle refers to an in-flight `Task`,
+  /// allowing for potentially awaiting for its result or canceling it.
+  ///
+  /// It is not a programming error to drop a handle without awaiting or canceling 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 final class Handle<Success, Failure: Error> {
+    /// 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 the `get()` will throw a cancellation error.
+    public func get() async throws -> Success {
+      fatalError("\(#function) not implemented yet.")
+    }
+
+    /// 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() {
+      fatalError("\(#function) not implemented yet.")
+    }
+  }
+}
+
+// ==== Detached Tasks ---------------------------------------------------------
+
+extension Task {
+  /// Run given `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.
+  ///
+  /// Canceling a task must be performed explicitly via `handle.cancel()`.
+  ///
+  /// - Parameters:
+  ///   - priority: priority of the task TODO: reword and define more explicitly once we have priorities well-defined
+  ///   - operation:
+  /// - Returns: handle to the task, allowing to `await handle.get()` on the
+  ///     tasks result or `cancel` it.
+  ///
+  /// - 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.
+  public static func runDetached<T>(
+    priority: Priority = .default,
+    operation: () async -> T
+  ) -> Handle<T, Never> {
+    fatalError("\(#function) not implemented yet.")
+  }
+
+  /// 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.
+  ///
+  /// Canceling a task must be performed explicitly via `handle.cancel()`.
+  ///
+  /// - Parameters:
+  ///   - priority: priority of the task TODO: reword and define more explicitly once we have priorities well-defined
+  ///   - operation:
+  /// - 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.
+  ///
+  /// - 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.
+  public static func runDetached<T>(
+    priority: Priority = .default,
+    operation: () async throws -> T
+  ) -> Handle<T, Error> {
+    fatalError("\(#function) not implemented yet.")
+  }
+}
+
+// ==== UnsafeContinuation -----------------------------------------------------
+
+extension Task {
+  public struct UnsafeContinuation<T> {
+    /// Return a value into the continuation and make the task schedulable.
+    ///
+    /// The task will never run synchronously, even if the task does not
+    /// need to be resumed on a specific executor.
+    ///
+    /// This is appropriate when the caller is something "busy", like an event
+    /// loop, and doesn't want to be potentially delayed by arbitrary work.
+    public func resume(returning: T) {
+      fatalError("\(#function) not implemented yet.")
+    }
+  }
+
+  public struct UnsafeThrowingContinuation<T, E: Error> {
+    /// Return a value into the continuation and make the task schedulable.
+    ///
+    /// The task will never run synchronously, even if the task does not
+    /// need to be resumed on a specific executor.
+    ///
+    /// This is appropriate when the caller is something "busy", like an event
+    /// loop, and doesn't want to be potentially delayed by arbitrary work.
+    public func resume(returning: T) {
+      fatalError("\(#function) not implemented yet.")
+    }
+
+    /// Resume the continuation with an error and make the task schedulable.
+    ///
+    /// The task will never run synchronously, even if the task does not
+    /// need to be resumed on a specific executor.
+    ///
+    /// This is appropriate when the caller is something "busy", like an event
+    /// loop, and doesn't want to be potentially delayed by arbitrary work.
+    public func resume(throwing: E) {
+      fatalError("\(#function) not implemented yet.")
+    }
+  }
+
+  /// The operation functions must resume the continuation *exactly once*.
+  ///
+  /// The continuation will not begin executing until the operation function returns.
+  public static func withUnsafeContinuation<T>(
+    operation: (UnsafeContinuation<T>) -> Void
+  ) async -> T {
+    fatalError("\(#function) not implemented yet.")
+  }
+
+  /// The operation functions must resume the continuation *exactly once*.
+  ///
+  /// The continuation will not begin executing until the operation function returns.
+  public static func withUnsafeThrowingContinuation<T>(
+    operation: (UnsafeThrowingContinuation<T, Error>) -> Void
+  ) async throws -> T {
+    fatalError("\(#function) not implemented yet.")
+  }
+}

test/Concurrency/actor_isolation.swift

@@ -7,10 +7,12 @@ var mutableGlobal: String = "can't touch this" // expected-note 2{{var declared
 func globalFunc() { }
 func acceptClosure<T>(_: () -> T) { }
 func acceptEscapingClosure<T>(_: @escaping () -> T) { }
+func acceptEscapingClosure<T>(_: (String) -> ()) async -> T? { nil }
 
 func acceptAsyncClosure<T>(_: () async -> T) { }
 func acceptEscapingAsyncClosure<T>(_: @escaping () async -> T) { }
 
+
 // ----------------------------------------------------------------------
 // Actor state isolation restrictions
 // ----------------------------------------------------------------------

test/Concurrency/async_tasks.swift

@@ -0,0 +1,61 @@
+// RUN: %target-typecheck-verify-swift -enable-experimental-concurrency
+// REQUIRES: concurrency
+
+func someAsyncFunc() async -> String { "" }
+
+struct MyError: Error {}
+func someThrowingAsyncFunc() async throws -> String { throw MyError() }
+
+func test_unsafeContinuations() async {
+  // the closure should not allow async operations;
+  // after all: if you have async code, just call it directly, without the unsafe continuation
+  let _: String = Task.withUnsafeContinuation { continuation in // expected-error{{cannot convert value of type '(_) async -> ()' to expected argument type '(Task.UnsafeContinuation<String>) -> Void'}}
+    let s = await someAsyncFunc() // rdar://70610141 for getting a better error message here
+    continuation.resume(returning: s)
+  }
+
+  let _: String = await Task.withUnsafeContinuation { continuation in
+    continuation.resume(returning: "")
+  }
+}
+
+func test_unsafeThrowingContinuations() async {
+  let _: String = try await Task.withUnsafeThrowingContinuation { continuation in
+    continuation.resume(returning: "")
+  }
+
+  let _: String = try await Task.withUnsafeThrowingContinuation { continuation in
+    continuation.resume(throwing: MyError())
+  }
+
+  // TODO: Potentially could offer some warnings if we know that a continuation was resumed or escaped at all in a closure?
+}
+
+// ==== Detached Tasks ---------------------------------------------------------
+
+func test_detached() async throws {
+  let handle = Task.runDetached() {
+    await someAsyncFunc() // able to call async functions
+  }
+
+  let result: String = await try handle.get()
+  _ = result
+}
+
+func test_detached_throwing() async -> String {
+  let handle: Task.Handle<String, Error> = Task.runDetached() {
+    await try someThrowingAsyncFunc() // able to call async functions
+  }
+
+  do {
+    return await try handle.get()
+  } catch {
+    print("caught: \(error)")
+  }
+}
+
+// ==== Current Task -----------------------------------------------------------
+
+func test_current_task() async {
+    _ = await Task.current() // yay, we know "in" what task we're executing
+}