feat: Implement AsynceEventLoop and MultiThreadedEventLoopGroup using Swift Concurrency and the AsyncEventLoopExecutor. Forms a key foundation for several vapor repositories to use NIO without using NIOPosix. This enables a large amount of wasm compilation for packages that currently consume NIOPosix.

Author: scottmarchant

Sources/NIOAsyncRuntime/AsyncEventLoop.swift

@@ -0,0 +1,355 @@
+//===----------------------------------------------------------------------===//
+//
+// Copyright (c) 2025 PassiveLogic, Inc.
+// Licensed under Apache License v2.0
+//
+// See LICENSE.txt for license information
+//
+// SPDX-License-Identifier: Apache-2.0
+//
+//===----------------------------------------------------------------------===//
+
+import Atomics
+import NIOCore
+
+import struct Foundation.UUID
+
+#if canImport(Dispatch)
+  import Dispatch
+#endif
+
+// MARK: - AsyncEventLoop -
+
+/// A single‑threaded `EventLoop` implemented solely with Swift Concurrency.
+@available(macOS 13, *)
+public final class AsyncEventLoop: EventLoop, @unchecked Sendable {
+  public enum AsynceEventLoopError: Error {
+    case cancellationFailure
+  }
+
+  private let _id = UUID()  // unique identifier
+  private let executor: AsyncEventLoopExecutor
+  private var cachedSucceededVoidFuture: EventLoopFuture<Void>?
+  private enum ShutdownState: UInt8 {
+    case running = 0
+    case closing = 1
+    case closed = 2
+  }
+  private let shutdownState = ManagedAtomic<UInt8>(ShutdownState.running.rawValue)
+
+  public init(manualTimeModeForTesting: Bool = false) {
+    self.executor = AsyncEventLoopExecutor(loopID: _id, manualTimeMode: manualTimeModeForTesting)
+  }
+
+  // MARK: - EventLoop basics -
+
+  public var inEventLoop: Bool {
+    _CurrentEventLoopKey.id == _id
+  }
+
+  private func isAcceptingNewTasks() -> Bool {
+    shutdownState.load(ordering: .acquiring) == ShutdownState.running.rawValue
+  }
+
+  private func isFullyShutdown() -> Bool {
+    shutdownState.load(ordering: .acquiring) == ShutdownState.closed.rawValue
+  }
+
+  @_disfavoredOverload
+  public func execute(_ task: @escaping @Sendable () -> Void) {
+    guard self.isAcceptingNewTasks() || self._canAcceptExecuteDuringShutdown else { return }
+    executor.enqueue(task)
+  }
+
+  private var _canAcceptExecuteDuringShutdown: Bool {
+    self.inEventLoop
+      || MultiThreadedEventLoopGroup._GroupContextKey.isFromMultiThreadedEventLoopGroup
+  }
+
+  // MARK: - Promises / Futures -
+
+  public func makeSucceededFuture<T: Sendable>(_ value: T) -> EventLoopFuture<T> {
+    if T.self == Void.self {
+      return self.makeSucceededVoidFuture() as! EventLoopFuture<T>
+    }
+    let p = makePromise(of: T.self)
+    p.succeed(value)
+    return p.futureResult
+  }
+
+  public func makeFailedFuture<T>(_ error: Error) -> EventLoopFuture<T> {
+    let p = makePromise(of: T.self)
+    p.fail(error)
+    return p.futureResult
+  }
+
+  public func makeSucceededVoidFuture() -> EventLoopFuture<Void> {
+    if self.inEventLoop {
+      if let cached = self.cachedSucceededVoidFuture {
+        return cached
+      }
+      let future = self.makeSucceededVoidFutureUncached()
+      self.cachedSucceededVoidFuture = future
+      return future
+    } else {
+      return self.makeSucceededVoidFutureUncached()
+    }
+  }
+
+  private func makeSucceededVoidFutureUncached() -> EventLoopFuture<Void> {
+    let promise = self.makePromise(of: Void.self)
+    promise.succeed(())
+    return promise.futureResult
+  }
+
+  // MARK: - Submitting work -
+  @preconcurrency
+  public func submit<T>(_ task: @escaping @Sendable () throws -> T) -> EventLoopFuture<T> {
+    self.submit { () throws -> _UncheckedSendable<T> in
+      _UncheckedSendable(try task())
+    }.map { $0.value }
+  }
+
+  public func submit<T: Sendable>(_ task: @escaping @Sendable () throws -> T) -> EventLoopFuture<T>
+  {
+    guard self.isAcceptingNewTasks() else {
+      return self.makeFailedFuture(EventLoopError.shutdown)
+    }
+    let promise = makePromise(of: T.self)
+    executor.enqueue {
+      do {
+        let value = try task()
+        promise.succeed(value)
+      } catch { promise.fail(error) }
+    }
+    return promise.futureResult
+  }
+
+  public func flatSubmit<T: Sendable>(_ task: @escaping @Sendable () -> EventLoopFuture<T>)
+    -> EventLoopFuture<T>
+  {
+    guard self.isAcceptingNewTasks() else {
+      return self.makeFailedFuture(EventLoopError.shutdown)
+    }
+    let promise = makePromise(of: T.self)
+    executor.enqueue {
+      let future = task()
+      future.cascade(to: promise)
+    }
+    return promise.futureResult
+  }
+
+  // MARK: - Scheduling -
+
+  /// NOTE:
+  ///
+  /// Timing for execute vs submit vs schedule:
+  ///
+  /// Tasks scheduled via `execute` or `submit` are appended to the back of the event loop's task queue
+  /// and are executed serially in FIFO order. Scheduled tasks (e.g., via `schedule(deadline:)`) are
+  /// placed in a timing wheel and, when their deadline arrives, are enqueued at the back of the main
+  /// queue after any already-pending work. This means that if the event loop is backed up, a scheduled
+  /// task may execute slightly after its scheduled time, as it must wait for previously enqueued tasks
+  /// to finish. Scheduled tasks never preempt or jump ahead of already-queued immediate work.
+  @preconcurrency
+  public func scheduleTask<T>(
+    deadline: NIODeadline,
+    _ task: @escaping @Sendable () throws -> T
+  ) -> Scheduled<T> {
+    let scheduled: Scheduled<_UncheckedSendable<T>> = self._scheduleTask(
+      deadline: deadline,
+      task: { try _UncheckedSendable(task()) }
+    )
+    return self._unsafelyRewrapScheduled(scheduled)
+  }
+
+  public func scheduleTask<T: Sendable>(
+    deadline: NIODeadline,
+    _ task: @escaping @Sendable () throws -> T
+  ) -> Scheduled<T> {
+    self._scheduleTask(deadline: deadline, task: task)
+  }
+
+  @preconcurrency
+  public func scheduleTask<T>(
+    in delay: TimeAmount,
+    _ task: @escaping @Sendable () throws -> T
+  ) -> Scheduled<T> {
+    let scheduled: Scheduled<_UncheckedSendable<T>> = self._scheduleTask(
+      in: delay,
+      task: { try _UncheckedSendable(task()) }
+    )
+    return self._unsafelyRewrapScheduled(scheduled)
+  }
+
+  public func scheduleTask<T: Sendable>(
+    in delay: TimeAmount,
+    _ task: @escaping @Sendable () throws -> T
+  ) -> Scheduled<T> {
+    self._scheduleTask(in: delay, task: task)
+  }
+
+  private func _scheduleTask<T: Sendable>(
+    deadline: NIODeadline,
+    task: @escaping @Sendable () throws -> T
+  ) -> Scheduled<T> {
+    let promise = makePromise(of: T.self)
+    guard self.isAcceptingNewTasks() else {
+      promise.fail(EventLoopError._shutdown)
+      return Scheduled(promise: promise) {}
+    }
+
+    let jobID = executor.schedule(
+      at: deadline,
+      job: {
+        do {
+          promise.succeed(try task())
+        } catch {
+          promise.fail(error)
+        }
+      },
+      failFn: { error in
+        promise.fail(error)
+      }
+    )
+
+    return Scheduled(promise: promise) { [weak self] in
+      // NOTE: Documented cancellation procedure indicates
+      // cancellation is not guaranteed. As such, and to match existing Promise API's,
+      // using a Task here to avoid pushing async up the software stack.
+      self?.executor.cancelScheduledJob(withID: jobID)
+
+      // NOTE: NIO Core already fails the promise before calling the cancellation closure,
+      // so we do NOT try to fail the promise. Also cancellation is not guaranteed, so we
+      // allow cancellation to silently fail rather than re-negotiating to a throwing API.
+    }
+  }
+
+  private func _scheduleTask<T: Sendable>(
+    in delay: TimeAmount,
+    task: @escaping @Sendable () throws -> T
+  ) -> Scheduled<T> {
+    // NOTE: This is very similar to the `scheduleTask(deadline:)` implementation. However
+    // due to the nonisolated context here, we keep the implementations separate until they
+    // reach isolating mechanisms within the executor.
+
+    let promise = makePromise(of: T.self)
+    guard self.isAcceptingNewTasks() else {
+      promise.fail(EventLoopError._shutdown)
+      return Scheduled(promise: promise) {}
+    }
+
+    let jobID = executor.schedule(
+      after: delay,
+      job: {
+        do {
+          promise.succeed(try task())
+        } catch {
+          promise.fail(error)
+        }
+      },
+      failFn: { error in
+        promise.fail(error)
+      }
+    )
+
+    return Scheduled(promise: promise) { [weak self] in
+      // NOTE: Documented cancellation procedure indicates
+      // cancellation is not guaranteed. As such, and to match existing Promise API's,
+      // using a Task here to avoid pushing async up the software stack.
+      self?.executor.cancelScheduledJob(withID: jobID)
+
+      // NOTE: NIO Core already fails the promise before calling the cancellation closure,
+      // so we do NOT try to fail the promise. Also cancellation is not guaranteed, so we
+      // allow cancellation to silently fail rather than re-negotiating to a throwing API.
+    }
+  }
+
+  func closeGracefully() async {
+    let previous = shutdownState.exchange(ShutdownState.closing.rawValue, ordering: .acquiring)
+    guard ShutdownState(rawValue: previous) != .closed else { return }
+    self.cachedSucceededVoidFuture = nil
+    await executor.clearQueue()
+    shutdownState.store(ShutdownState.closed.rawValue, ordering: .releasing)
+  }
+
+  public func next() -> EventLoop {
+    self
+  }
+  public func any() -> EventLoop {
+    self
+  }
+
+  /// Moves time forward by specified increment, and runs event loop, causing
+  /// all pending events either from enqueing or scheduling requirements to run.
+  func advanceTime(by increment: TimeAmount) async throws {
+    try await executor.advanceTime(by: increment)
+  }
+
+  func advanceTime(to deadline: NIODeadline) async throws {
+    try await executor.advanceTime(to: deadline)
+  }
+
+  func run() async {
+    await executor.run()
+  }
+
+  #if canImport(Dispatch)
+    public func shutdownGracefully(
+      queue: DispatchQueue, _ callback: @escaping @Sendable (Error?) -> Void
+    ) {
+      if MultiThreadedEventLoopGroup._GroupContextKey.isFromMultiThreadedEventLoopGroup {
+        Task {
+          await closeGracefully()
+          queue.async { callback(nil) }
+        }
+      } else {
+        // Bypassing the group shutdown and calling an event loop
+        // shutdown directly is considered api-misuse
+        callback(EventLoopError.unsupportedOperation)
+      }
+    }
+  #endif
+
+  public func syncShutdownGracefully() throws {
+    // The test AsyncEventLoopTests.testIllegalCloseOfEventLoopFails requires
+    // this implementation to throw an error, because uses should call shutdown on
+    // MultiThreadedEventLoopGroup instead of calling it directly on the loop.
+    throw EventLoopError.unsupportedOperation
+  }
+
+  public func shutdownGracefully() async throws {
+    await self.closeGracefully()
+  }
+
+  #if !canImport(Dispatch)
+    public func _preconditionSafeToSyncShutdown(file: StaticString, line: UInt) {
+      assertionFailure("Synchronous shutdown API's are not currently supported by AsyncEventLoop")
+    }
+  #endif
+
+  @preconcurrency
+  private func _unsafelyRewrapScheduled<T>(
+    _ scheduled: Scheduled<_UncheckedSendable<T>>
+  ) -> Scheduled<T> {
+    let promise = self.makePromise(of: T.self)
+    scheduled.futureResult.whenComplete { result in
+      switch result {
+      case .success(let boxed):
+        promise.assumeIsolatedUnsafeUnchecked().succeed(boxed.value)
+      case .failure(let error):
+        promise.fail(error)
+      }
+    }
+    return Scheduled(promise: promise) {
+      scheduled.cancel()
+    }
+  }
+
+  /// This is a shim used to support older protocol-required API's without compiler warnings, and provide more modern
+  /// concurrency-ready overloads.
+  private struct _UncheckedSendable<T>: @unchecked Sendable {
+    let value: T
+    init(_ value: T) { self.value = value }
+  }
+}