如何在 swift actor 中支持带超时的异步回调

Question

我混合使用了 async/await（Actor）运行“遗留”代码（Process Pipes 和回调块）。我有一个类似的情况，我的演员的功能 returns 一个值，同时在完成某些工作后还存储一个延迟的回调（演员在等待工作完成时不应该阻塞，演员的目的只是为了支持需要线程安全的 ID 生成和内部更新）。理想情况下，我想将回调构建为符合人体工程学的“异步”变体我正在使用一个演员来保证线程安全。其中一个异步函数存储一个回调，一旦工作完成或一旦发生超时就会调用该回调。目前出现死锁以及“丑陋”的嵌套。

typealias Block = (String?) async -> Void // do I need the async keyword here necessarily?

actor Processor {
  let id = Int
  var callbacks = [Int:Block]()
  let process : Process // external process which communicates of stdin/stdout
  init(processPath: String) async throws {
    process = Process()
    process.launchPath = processPath
    process.standardInput = Pipe()
    process.standardOutput = Pipe()
    (process.standardOutput as! Pipe).fileHandleForReading.readabilityHandler = {[weak self] handler in
       // does not support async/await directly, wrapping in a task, perhaps better mechanism?
       Task {[weak self] in
          await self?.handleOutput(data: handler.availableData)
       }
    }
  }

  struct Result: Codable { 
     id: Int ,
     output: String
  }
  func handleOutput(data: Data) {
     if let decoded = try? JSONDecoder().decode(Result.self, from: data),
            let id = decoded.id ,
            let callback = pop(id: id) {
            await callback(decoded.output) // call the block, this time with real value vs when there was a timeout
        }
  }
  func work(text: String, completion: @escaping Block) async -> WorkArgs {
     id += 1 // increment the running id
     // store the callback
     callbacks[id] = completion
     // timeout if the result doesn't come back in a reasonable amount of time. 
     // I will perform a check after 1 second, and pop the callback if needed
     // problematic here..., how to wait while also not blocking this function
     Task { [weak self] in
       // invalidate if needed
       try await Task.sleep(nanoseconds: 1_000_000_000)
       // dead lock here:
       if let bl = await self?.pop(id: id) {
            print("invalidated the task \(id)")
            await bl(nil)
       }
     }
     // call out to the external process with this convention, it will run async and print a result with the same id
     let command = "\(id) \(text)\n"
     let d = command(using: .utf8)

     try! (process.standardInput as! Pipe).fileHandleForWriting.write(contentsOf: d)
     
  }
  // pop this callback
  func pop(id: Int) -> Block? {
        return callbacks.removeValue(forKey: id)
  }
}

struct WorkArgs {
  let id: Int
  let date: Date
}

actor IdGen {
    private var id : Int64 = 0
    func next() -> Int64 {
        id += 1
        return id
    }
}

actor CallbackActor {
var pendingCallbacks = [Int: (String) -> Void]()
    func push(_ key: Int, block: @escaping  (String) -> Void) {
        pendingCallbacks[key] = block
    }
    func pop(_ key: Int64) -> AutoCompleteBlock? {
        return pendingCallbacks.removeValue(forKey: key)
    }
}

Answer 1

在讨论超时问题之前，我们或许应该讨论一下如何将 Process 包装在 Swift 并发性中。

一种模式是对 stdout:

使用 AsyncSequence（即 AsyncStream）

actor ProcessWithStream {
    private let process = Process()
    private let stdin = Pipe()
    private let stdout = Pipe()
    private let stderr = Pipe()
    private var buffer = Data()

    init(url: URL) {
        process.standardInput = stdin
        process.standardOutput = stdout
        process.standardError = stderr
        process.executableURL = url
    }

    func start() throws {
        try process.run()
    }

    func terminate() {
        process.terminate()
    }

    func send(_ string: String) {
        guard let data = "\(string)\n".data(using: .utf8) else { return }
        stdin.fileHandleForWriting.write(data)
    }

    func stream() -> AsyncStream<Data> {
        AsyncStream(Data.self) { continuation in
            stdout.fileHandleForReading.readabilityHandler = { handler in
                continuation.yield(handler.availableData)
            }
            process.terminationHandler = { handler in
                continuation.finish()
            }
        }
    }
}

然后你可以for await那个流：

let process = ProcessWithStream(url: url)

override func viewDidLoad() {
    super.viewDidLoad()

    Task {
        try await startStream()
        print("done")
    }
}

@IBAction func didTapSend(_ sender: Any) {
    let string = textField.stringValue
    Task {
        await process.send(string)
    }
    textField.stringValue = ""
}

func startStream() async throws {
    try await process.start()
    let stream = await process.stream()

    for await data in stream {
        if let string = String(data: data, encoding: .utf8) {
            print(string, terminator: "")
        }
    }
}

这是一个简单的方法。它看起来不错（因为我正在打印没有终止符的响应）。

但是需要小心，因为 readabilityHandler 不会总是用某些特定输出的完整 Data 来调用。它可能会分解或拆分为对 reachabilityhandler.

的单独调用

另一种模式是使用 lines，它避免了 reachabilityHandler 可能被多次调用给定输出的问题：

actor ProcessWithLines {
    private let process = Process()
    private let stdin = Pipe()
    private let stdout = Pipe()
    private let stderr = Pipe()
    private var buffer = Data()
    private(set) var lines: AsyncLineSequence<FileHandle.AsyncBytes>?

    init(url: URL) {
        process.standardInput = stdin
        process.standardOutput = stdout
        process.standardError = stderr
        process.executableURL = url
    }

    func start() throws {
        lines = stdout.fileHandleForReading.bytes.lines
        try process.run()
    }

    func terminate() {
        process.terminate()
    }

    func send(_ string: String) {
        guard let data = "\(string)\n".data(using: .utf8) else { return }
        stdin.fileHandleForWriting.write(data)
    }
}

那么你可以这样做：

let process = ProcessWithLines(url: url)

override func viewDidLoad() {
    super.viewDidLoad()

    Task {
        try await startStream()
        print("done")
    }
}

@IBAction func didTapSend(_ sender: Any) {
    let string = textField.stringValue
    Task {
        await process.send(string)
    }
    textField.stringValue = ""
}

func startStream() async throws {
    try await process.start()
    guard let lines = await process.lines else { return }

    for try await line in lines {
        print(line)
    }
}

这避免了响应中断 mid-line。

您问的是：

How to support ... timeout in swift actor

模式是将请求包装在 Task 中，然后启动一个单独的任务，该任务将在 Task.sleep 间隔后取消之前的任务。

但在这种情况下，这将非常复杂，因为您必须将其与单独的 Process 协调，否则它仍将继续，而不知道 Task 已被取消。这在理论上会导致问题（例如，流程积压等）。

我建议将超时逻辑集成到 Process 调用的应用程序中，而不是试图让调用者处理它。它可以完成（例如，也许编写流程应用程序来捕获和处理 SIGINT，然后调用者可以在 Process 上调用 interrupt）。但它会很复杂，而且很可能很脆弱。

如何在 swift actor 中支持带超时的异步回调

How to support an async callback with timeout in swift actor

swift

async-await

actor

swift-concurrency