The IActorRef of a child dies or restarts could not be retrieve by mailbox.Sender() in parent

[ingted]

======================================== Version of Akka.NET? 1.5.0-Alpha Which Akka.NET Modules? Akka or Akka.FSharp

The IActorRef of a child dies or restarts could not be retrieve by mailbox.Sender() in parent

======================================== Reproduce

Example Code File Location

#if INTERACTIVE
#r @"nuget: Akka"
#r @"nuget: Akka.FSharp"
#endif

open System
open Akka.Actor
open Akka.Configuration
open Akka.FSharp

type CustomException() =
    inherit Exception()

type Message =
    | Echo of string
    | Crash

let system = System.create "system" (Configuration.defaultConfig())
    // create parent actor to watch over jobs delegated to it's child
let parent = 
    spawnOpt system "parent" 
        <| fun parentMailbox ->
            // define child actor
            let child = 
                    spawn parentMailbox "child" <| fun childMailbox ->
                        childMailbox.Defer (fun () -> printfn "Child stopping")
                        printfn "Child started"
                        let rec childLoop() = 
                            actor {
                                let! msg = childMailbox.Receive()
                                match msg with
                                | Echo info -> 
                                    // respond to original sender
                                    let response = "Child " + (childMailbox.Self.Path.ToStringWithAddress()) + " received: "  + info
                                    childMailbox.Sender() <! response
                                | Crash -> 
                                    // log crash request and crash
                                    //printfn "Child %A received crash order" (childMailbox.Self.Path)
                                    raise (CustomException())
                                return! childLoop()
                            }
                        childLoop()
            // define parent behavior
            let rec parentLoop() =
                actor {
                    let! (msg: Message) = parentMailbox.Receive()
                    child.Forward(msg)  // forward all messages through

                    //###########################################
                    //###### Notice this sender()

                    let sender = parentMailbox.Sender()
                    if sender <> null then
                        printfn "sender: %A" sender

                    //###########################################
                    return! parentLoop()
                }
            parentLoop()
        // define supervision strategy
        <| [ SpawnOption.SupervisorStrategy (
                // restart on Custom Exception, default behavior on all other exception types
                Strategy.OneForOne(fun e ->
                match e with
                | :? CustomException -> Directive.Restart 
                | _ -> SupervisorStrategy.DefaultDecider.Decide(e)
                //Directive.Restart
                )
        )  ]

parent <! Crash

=> See "Notice this sender()"

Expected behavior

The parentMailbox.Sender() should give us the IActorRef of child

Actual behavior

The parentMailbox.Sender() is null

Environment

Windows Server 2019 + .NET 7.0

[ingted]

According to fault-tolerance

If the strategy is declared inside the supervising actor (as opposed to within a companion object) its decider has access to all internal state of the actor in a thread-safe fashion, including obtaining a reference to the currently failed child (available as the Sender of the failure message).

[ingted]

Workaround:

derive the SupervisorStrategy class and put the error handler inside it...

type SS () =
    inherit SupervisorStrategy ()   
    override u.Decider :IDecider = 
        //Unchecked.defaultof<IDecider>
        SupervisorStrategy.DefaultDecider
    override u.Handle(child:IActorRef, exn:Exception) =
        //Unchecked.defaultof<Directive>
        Directive.Restart
    override u.ProcessFailure(
        context:IActorContext, 
        restart:bool, 
        child:IActorRef, 
        cause:Exception, 
        stats:ChildRestartStats, 
        children:IReadOnlyCollection<ChildRestartStats>) =
            printfn "====> %A" child
    override u.HandleChildTerminated(
        actorContext:IActorContext,  
        child:IActorRef, 
        children:IEnumerable<IInternalActorRef>) = 
            printfn "====> %A" child
    override u.ToSurrogate(system:ActorSystem) =
        Unchecked.defaultof<ISurrogate>

<| [ SpawnOption.SupervisorStrategy (SS ()) ]

[ingted]

It seems like we can override the method to send the IActorRef back to parent's mailbox... But this is not convinient like the Sender() method... (illustrated in the document)