[TaskExecutors] Task initializer and withTaskExecutor parameter changes (#70783)

Author: ktoso

stdlib/public/Concurrency/CMakeLists.txt

@@ -107,6 +107,7 @@ set(SWIFT_RUNTIME_CONCURRENCY_SWIFT_SOURCES
   AsyncThrowingMapSequence.swift
   AsyncThrowingPrefixWhileSequence.swift
   GlobalActor.swift
+  GlobalConcurrentExecutor.swift
   MainActor.swift
   PartialAsyncTask.swift
   SourceCompatibilityShims.swift

stdlib/public/Concurrency/Executor.swift

@@ -113,8 +113,8 @@ public protocol SerialExecutor: Executor {
 /// requirements.
 ///
 /// By setting a task executor preference, either with a
-/// ``_withTaskExecutor(_:operation:)``, creating a task with a preference
-/// (`Task(_on:)`, or `group.addTask(on:)`), the task and all of its child
+/// ``_withTaskExecutorPreference(_:operation:)``, creating a task with a preference
+/// (`Task(_executorPreference:)`, or `group.addTask(executorPreference:)`), the task and all of its child
 /// tasks (unless a new preference is set) will be preferring to execute on
 /// the provided task executor.
 ///
@@ -375,9 +375,8 @@ internal func _task_serialExecutor_getExecutorRef<E>(_ executor: E) -> Builtin.E
 @_unavailableInEmbedded
 @available(SwiftStdlib 9999, *)
 @_silgen_name("_task_executor_getTaskExecutorRef")
-internal func _task_executor_getTaskExecutorRef<E>(_ executor: E) -> Builtin.Executor
-    where E: _TaskExecutor {
-  return executor.asUnownedTaskExecutor().executor
+internal func _task_executor_getTaskExecutorRef(_ taskExecutor: any _TaskExecutor) -> Builtin.Executor {
+  return taskExecutor.asUnownedTaskExecutor().executor
 }
 
 // Used by the concurrency runtime

stdlib/public/Concurrency/GlobalConcurrentExecutor.swift

@@ -0,0 +1,66 @@
+//===----------------------------------------------------------------------===//
+//
+// 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
+
+// None of _TaskExecutor APIs are available in task-to-thread concurrency model.
+#if !SWIFT_STDLIB_TASK_TO_THREAD_MODEL_CONCURRENCY
+
+/// The global concurrent executor that is used by default for Swift Concurrency
+/// tasks.
+///
+/// The executor's implementation is platform dependent.
+/// By default it uses a fixed size pool of threads and should not be used for
+/// blocking operations which do not guarantee forward progress as doing so may
+/// prevent other tasks from being executed and render the system unresponsive.
+///
+/// You may pass this executor explicitly to a ``Task`` initializer as a task
+/// executor preference, in order to ensure and document that task be executed
+/// on the global executor, instead e.g. inheriting the enclosing actor's
+/// executor. Refer to ``_withTaskExecutorPreference(_:operation:)`` for a
+/// detailed discussion of task executor preferences.
+///
+/// Customizing the global concurrent executor is currently not supported.
+@available(SwiftStdlib 9999, *)
+nonisolated(unsafe)
+public var globalConcurrentExecutor: any _TaskExecutor {
+  get {
+    _DefaultGlobalConcurrentExecutor.shared
+  }
+  // TODO: introduce a set {} once we are ready to allow customizing the
+  //       default global executor. This should be done the same for main actor
+}
+
+/// A task executor which enqueues all work on the default global concurrent
+/// thread pool that is used as the default executor for Swift concurrency
+/// tasks.
+@available(SwiftStdlib 9999, *)
+internal final class _DefaultGlobalConcurrentExecutor: _TaskExecutor {
+  public static let shared: _DefaultGlobalConcurrentExecutor = .init()
+
+  private init() {}
+
+  public func enqueue(_ job: consuming ExecutorJob) {
+    _enqueueJobGlobal(job.context)
+  }
+
+  public func asUnownedTaskExecutor() {
+    // The "default global concurrent executor" is simply the "undefined" one.
+    // We represent it as the `(0, 0)` ExecutorRef and it is handled properly
+    // by the runtime, without having to call through to the
+    // `_DefaultGlobalConcurrentExecutor` declared in Swift.
+    UnownedTaskExecutor(_getUndefinedTaskExecutor())
+  }
+}
+
+#endif

stdlib/public/Concurrency/PartialAsyncTask.swift

@@ -121,7 +121,7 @@ public struct UnownedJob: Sendable {
   @available(SwiftStdlib 9999, *)
   @_alwaysEmitIntoClient
   @inlinable
-  public func runSynchronously(isolated serialExecutor: UnownedSerialExecutor,
+  public func runSynchronously(isolatedTo serialExecutor: UnownedSerialExecutor,
                                taskExecutor: UnownedTaskExecutor) {
     _swiftJobRunOnTaskExecutor(self, serialExecutor, taskExecutor)
   }