Question

我正在尝试运行一些MailboxProcessor测试，看起来邮箱Scan（）失败了“System.Exception：邮箱的多个等待阅读器延续”。这种情况发生在Async.Start和Async.StartImmediate等（Async.RunSynchronously也不起作用，因为只有一个处理器，初始客户后没有客户）。

这是演示代码，这是交互式的：

#if INTERACTIVE
#r "../packages/FSharp.Data.2.0.4/lib/net40/FSharp.Data.dll"
#endif
open System
open FSharp.Data
let random = new Random()
let data = FreebaseData.GetDataContext()
let customerNames = data.Commons.Computers.``Computer Scientists``
let nameAmount = customerNames |> Seq.length
// ----

type Customer() =
    let person = customerNames |> Seq.nth (random.Next nameAmount)
    member x.Id = Guid.NewGuid()
    member x.Name = person.Name
    member x.RequiredTime = random.Next(10000)

type Barber(name) =
    member x.Name = name

type ``Possible actions notified to barber`` = 
| CustomerWalksIn of Customer

let availableCustomers = new MailboxProcessor<``Possible actions notified to barber``>(fun c -> async { () })

let createBarber name = 
    Console.WriteLine("Barber " + name + " takes inital nap...")
    let rec cutSomeHairs () = 
        async{
            do! availableCustomers.Scan(function 
                | CustomerWalksIn customer ->
                    async {
                        Console.WriteLine("Barber " + name + " is awake and started cutting " + customer.Name + "'s hair.")
                        // exception also happen with Threading.Thread.Sleep()
                        do! Async.Sleep customer.RequiredTime
                        Console.WriteLine("Barber " + name + " finnished cutting " + customer.Name + "'s hair. Going to sleep now...")
                    } |> Some)
            do! cutSomeHairs ()
            }
    cutSomeHairs() |> Async.StartImmediate

availableCustomers.Post(new Customer() |> CustomerWalksIn)
availableCustomers.Post(new Customer() |> CustomerWalksIn)
availableCustomers.Post(new Customer() |> CustomerWalksIn)
availableCustomers.Post(new Customer() |> CustomerWalksIn)
availableCustomers.Post(new Customer() |> CustomerWalksIn)

createBarber "Tuomas";
createBarber "Seppo";

availableCustomers.Post(new Customer() |> CustomerWalksIn)
availableCustomers.Post(new Customer() |> CustomerWalksIn)
availableCustomers.Post(new Customer() |> CustomerWalksIn)
availableCustomers.Post(new Customer() |> CustomerWalksIn)
availableCustomers.Post(new Customer() |> CustomerWalksIn)
availableCustomers.Post(new Customer() |> CustomerWalksIn)
availableCustomers.Post(new Customer() |> CustomerWalksIn)
availableCustomers.Post(new Customer() |> CustomerWalksIn)
availableCustomers.Post(new Customer() |> CustomerWalksIn)

...运行一段时间后我得到的堆栈跟踪是：

System.Exception: multiple waiting reader continuations for mailbox
   at <StartupCode$FSharp-Core>.$Control.-ctor@2136-3.Invoke(AsyncParams`1 _arg1)
   at <StartupCode$FSharp-Core>.$Control.loop@435-40(Trampoline this, FSharpFunc`2 action)
   at Microsoft.FSharp.Control.Trampoline.ExecuteAction(FSharpFunc`2 firstAction)
   at Microsoft.FSharp.Control.TrampolineHolder.Protect(FSharpFunc`2 firstAction)
   at <StartupCode$FSharp-Core>.$Control.Sleep@1508-1.Invoke(Object state)
   at System.Threading.TimerQueueTimer.CallCallbackInContext(Object state)
   at System.Threading.ExecutionContext.RunInternal(ExecutionContext executionContext, ContextCallback callback, Object state, Boolean preserveSyncCtx)
   at System.Threading.ExecutionContext.Run(ExecutionContext executionContext, ContextCallback callback, Object state, Boolean preserveSyncCtx)
   at System.Threading.TimerQueueTimer.CallCallback()
   at System.Threading.TimerQueueTimer.Fire()
   at System.Threading.TimerQueue.FireNextTimers()
   at System.Threading.TimerQueue.AppDomainTimerCallback()
Stopped due to error

或没有线程的相同：

System.Exception: multiple waiting reader continuations for mailbox
   at <StartupCode$FSharp-Core>.$Control.-ctor@2136-3.Invoke(AsyncParams`1 _arg1)
   at <StartupCode$FSharp-Core>.$Control.loop@435-40(Trampoline this, FSharpFunc`2 action)
   at Microsoft.FSharp.Control.Trampoline.ExecuteAction(FSharpFunc`2 firstAction)
   at Microsoft.FSharp.Control.TrampolineHolder.Protect(FSharpFunc`2 firstAction)
   at <StartupCode$FSharp-Core>.$Control.-ctor@511.Invoke(Object state)
Stopped due to error

Answer 1

Receive的{{1}}和Scan方法只能从代理正文中调用。引用MSDN documentation：

此方法适用于代理的正文。对于每个代理程序，最多只有一个并发读取器可能处于活动状态，因此对Receive，TryReceive，Scan或TryScan的并发调用不能超过一个。扫描仪功能的主体在执行期间被锁定，但锁定在执行异步工作流程之前被释放。

因此，您需要以不同方式构建代码。我没有详细的答案，但听起来我implementing blocking queue using agents上的文章可以帮到这里。

Answer 2

正如Tomas已经指出的那样，MailboxProcessor直接只允许一个读者，而在异步系统中解决此问题的一种方法是编写自己的队列或邮箱类型。然而，Tomas指出的文章没有谈到的一件事是实现新的通信原语的另一种方法是使用Async.FromContinuations而不是MailboxProcessor和{ {1}}。

使用AsyncReplyChannel的主要优点是您可以更直接地访问异步机制，而不必在Async.FromContinuations和MailboxProcessor强加的限制范围内工作。主要缺点是您需要自己安全地保护队列或邮箱线程。

作为一个具体的例子，Anton Tayanovskyy的博客文章Making Async 5x Faster包含使用AsyncReplyChannel实现的多个读写器同步通道的实现。

F＃3.0：System.Exception：邮箱的多个等待读取器延续

2 个答案: