In Kotlin Native, how to keep an object around in a separate thread, and mutate its state from any other thead without using C pointers?

Question

I'm exploring Kotlin Native and have a program with a bunch of Workers doing concurrent stuff (running on Windows, but this is a general question).

Now, I wanted to add simple logging. A component that simply logs strings by appending them as new lines to a file that is kept open in 'append' mode.

(Ideally, I'd just have a "global" function...

fun log(text:String) {...} ] 

...that I would be able to call from anywhere, including from "inside" other workers and that would just work. The implication here is that it's not trivial to do this because of Kotlin Native's rules regarding passing objects between threads (TLDR: you shouldn't pass mutable objects around. See: https://github.com/JetBrains/kotlin-native/blob/master/CONCURRENCY.md#object-transfer-and-freezing ). Also, my log function would ideally accept any frozen object. )

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What I've come up with are solutions using DetachedObjectGraph:

First, I create a detached logger object

val loggerGraph = DetachedObjectGraph { FileLogger("/foo/mylogfile.txt")}

and then use loggerGraph.asCPointer() ( asCPointer() ) to get a COpaquePointer to the detached graph:

val myPointer = loggerGraph.asCPointer()

Now I can pass this pointer into the workers ( via the producer lambda of the Worker's execute function ), and use it there. Or I can store the pointer in a @ThreadLocal global var.

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For the code that writes to the file, whenever I want to log a line, I have to create a DetachedObjectGraph object from the pointer again, and attach() it in order to get a reference to my fileLogger object:

val fileLogger = DetachedObjectGraph(myPointer).attach()

Now I can call a log function on the logger:

fileLogger.log("My log message")

This is what I've come up with looking at the APIs that are available (as of Kotlin 1.3.61) for concurrency in Kotlin Native, but I'm left wondering what a better approach would be ( using Kotlin, not resorting to C ). Clearly it's bad to create a DetachedObjectGraph object for every line written.

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One could pose this question in a more general way: How to keep a mutable resource open in a separate thread ( or worker ), and send messages to it.

Side comment: Having Coroutines that truly use threads would solve this problem, but the question is about how to solve this task with the APIs currently ( Kotlin 1.3.61 ) available.

Answer 1

You definitely shouldn't use DetachedObjectGraph in the way presented in the question. There's nothing to prevent you from trying to attach on multiple threads, or if you pass the same pointer, trying to attach to an invalid one after another thread as attached to it.

As Dominic mentioned, you can keep the DetachedObjectGraph in an AtomicReference. However, if you're going to keep DetachedObjectGraph in an AtomicReference, make sure the type is AtomicRef<DetachedObjectGraph?> and busy-loop while the DetachedObjectGraph is null. That will prevent the same DetachedObjectGraph from being used by multiple threads. Make sure to set it to null, and repopulate it, in an atomic way.

However, does FileLogger need to be mutable at all? If you're writing to a file, it doesn't seem so. Even if so, I'd isolate the mutable object to a separate worker and send log messages to it rather than doing a DetachedObjectGraph inside an AtomicRef.

In my experience, DetachedObjectGraph is super uncommon in production code. We don't use it anywhere at the moment.

To isolate mutable state to a Worker, something like this:

class MutableThing<T:Any>(private val worker:Worker = Worker.start(), producer:()->T){
    private val arStable = AtomicReference<StableRef<T>?>(null)
    init {
        worker.execute(TransferMode.SAFE, {Pair(arStable, producer).freeze()}){
            it.first.value = StableRef.create(it.second()).freeze()
        }
    }
    fun <R> access(block:(T)->R):R{
        return worker.execute(TransferMode.SAFE, {Pair(arStable, block).freeze()}){
            it.second(it.first.value!!.get())
        }.result
    }
}

object Log{
    private val fileLogger = MutableThing { FileLogger() }

    fun log(s:String){
        fileLogger.access { fl -> fl.log(s) }
    }
}

class FileLogger{
    fun log(s:String){}
}

The MutableThing uses StableRef internally. producer makes the mutable state you want to isolate. To log something, call Log.log, which will wind up calling the mutable FileLogger.

To see a basic example of MutableThing, run the following test:

@Test
fun goIso(){
    val mt = MutableThing { mutableListOf("a", "b")}
    val workers = Array(4){Worker.start()}
    val futures = mutableListOf<Future<*>>()
    repeat(1000) { rcount ->
        val future = workers[rcount % workers.size].execute(
            TransferMode.SAFE,
            { Pair(mt, rcount).freeze() }
        ) { pair ->
            pair.first.access {
                val element = "ttt ${pair.second}"
                println(element)
                it.add(element)
            }
        }
        futures.add(future)
    }

    futures.forEach { it.result }

    workers.forEach { it.requestTermination() }

    mt.access {
        println("size: ${it.size}")
    }
}

Answer 2

The approach you've taken is pretty much correct and the way it's supposed to be done.

The thing I would add is, instead of passing around a pointer around. You should pass around a frozen FileLogger, which will internally hold a reference to a AtomicRef<DetachedObjectGraph>, the the attaching and detaching should be done internally. Especially since DetachedObjectGraphs are invalid once attached.