Any way to avoid the warning with `suspend fun CoroutineScope.startThing(action: Deferred<String&...

# coroutines

[Daniel Pitts]

Any way to avoid the warning with suspend fun CoroutineScope.startThing(action: Deferred<String>): String 🧵

[Daniel Pitts]

package com.stochastictinkr

import kotlinx.coroutines.*

fun CoroutineScope.launchAction(action: String): String {
    launch {
        delay(1000)
        println("$action completed")
    }
    return "$action started"
}

suspend fun startJobSuspend(action: Deferred<String>): String {
    val string = action.await()
    return coroutineScope { launchAction(string) }
}

suspend fun CoroutineScope.startJobSuspendScoped(action: Deferred<String>): String {
    val string = action.await()
    return launchAction(string)
}

fun CoroutineScope.startJobDeferred(action: Deferred<String>): Deferred<String> {
    return async {
        launchAction(action.await())
    }
}

fun main() = runBlocking {
    println("About to start main")
    val main = launchAction("main")
    println(main)

    println("About to start suspend-action")    
    val suspend = startJobSuspend(async { "suspend-action" })
    println(suspend)
    
    println("About to start suspend-scoped-action")
    val suspendScoped = startJobSuspendScoped(async { "suspend-scoped-action" })
    println(suspendScoped)
    
    println("About to start deferred-action")
    val deferred = startJobDeferred(async { "deferred-action" })
    println(deferred.await())
}

[streetsofboston]

Why the need for a CoroutineScope receiver for the suspend version of startThing?

[Daniel Pitts]

So that I can call the non-suspend version which needs it for launch

[streetsofboston]

Within a suspend function, you can call the function coroutineScope { /* this is a CoroutineScope*/... }, in which the CoroutineScope of the calling function will be provided.

[Daniel Pitts]

Hmm, that's not what the docs say. coroutineScope will suspend until all the child jobs are complete.

[streetsofboston]

Correct. You want the suspend fun to resume before the Deferred 'await' call has finished?

[Daniel Pitts]

I want the non-suspend startThing to return the string immediately.

[streetsofboston]

(the code snippet is a bit hard to follow with all things called startThing 🙂

[Daniel Pitts]

Yeah, let me rewrite it.

[streetsofboston]

Still, your fun CoroutineScope.startThing(action: String): String will return immediately.
A call coroutineScope { } calling that function will only resume until that launch has finished.

[Daniel Pitts]

I've edited the snippet above, and added example main.

[streetsofboston]

You also want startJobSuspendScoped to possibly resume before the launchAction has finished?

[Daniel Pitts]

I want it to resume before the launch within launchAction has finished..

[Daniel Pitts]

The actual work is basically an event loop, not just a delay then return.

[streetsofboston]

And you have the startJobSuspendScoped marked as suspend only so that you can call the await on the action ?

[Daniel Pitts]

Thats the main reason, yeah.

[Daniel Pitts]

Hmm, even startJobDeferred doesn't do what I want. Only startJobSuspendScoped does what I expect.

[streetsofboston]

And this is the output you want to see?:

suspend-scoped-action started
suspend-scoped-action completed

[Daniel Pitts]

Yes.

[streetsofboston]

Your startJobSuspendScoped needs to suspend to await the action string, but should launch the action separately. This means they have separate lifecycles.

suspend fun CoroutineScope.startJobSuspendScoped(action: Deferred<String>): String {
  val string = action.await()
  return launchAction(string)
}

will accomplish this, but you get the warning about it being both suspend and having a CoroutineScope.

[Daniel Pitts]

Yes.

[Daniel Pitts]

That warning is what prompted this thread.

[streetsofboston]

This is not an error, but a warning, which you can choose to ignore, but it may be a bit unclear to the caller on how this function should behave. That is why the warning is shown.
For more clear, although more verbose code, you should separate those two concerns (lifecycle differences).

[streetsofboston]

val result = launchAction(async { "suspend-scoped-action" }.await())
println(result)

[streetsofboston]

Here, the await is called in the calling function and then the launchAction is done asynchronously.

[Daniel Pitts]

Yeah, though the real use-case has one more layer of indirection that makes that impossible.

[streetsofboston]

Ugh.... 🙂

[streetsofboston]

Then I'd do the one that generates that warnign, but suppress that warning with a proper comment to explain it.

[Daniel Pitts]

The real use-case receives a host-name, and then asynchronously resolves it.

[streetsofboston]

Or wrap launcAction inside an object that has its own CoroutineScope

[Daniel Pitts]

So I have basically 2 versions of connect: CoroutineScope.connect(existingConnection: Connection) and CoroutineScope.connect(hostName: String)

[Daniel Pitts]

Here's the real code:

fun CoroutineScope.telnet(
    readWriteCloser: ReadWriteCloser,
    virtualTerminal: NetworkVirtualTerminal,
    options: suspend OptionsConfiguration.() -> Unit
): Telnet {
    val (reader, writer, closer) = readWriteCloser
    val output = TelnetOutput(writer)
    val optionNegotiator = OptionNegotiator(output)
    launch {
        optionNegotiator.options.options()
        TelnetInputProcessor(reader, optionNegotiator, virtualTerminal).processInput()
    }
    return Telnet(optionNegotiator, output, closer)
}

[streetsofboston]

class Action(private val scope: CoroutineScope = CoroutineScope(Job())) {    
    fun launch(action: String): String {
        scope.launch {
            delay(1000)
            println("$action completed")
        }
        return "$action started"
    }
}

suspend fun startJobSuspendUsingAction(action: Deferred<String>): String {
    val string = action.await()
    return Action().launch(string)
}

[Daniel Pitts]

Hmm, that gave me an idea:

suspend fun telnet(
    coroutineScope: CoroutineScope = CoroutineScope(Job()),
    remoteHost: String,
    remotePort: Int = 23,
    socketConfiguration: suspend SocketChannel.() -> Unit = {},
    virtualTerminal: NetworkVirtualTerminal,
    options: suspend OptionsConfiguration.() -> Unit = {},
): Telnet {
    SelectorProvider.provider()
    val socket = SocketChannel.open().apply {
        configureBlocking(false)
        socketConfiguration()
        awaitConnect(inetSocketAddress(remoteHost, remotePort))
    }
    return coroutineScope.telnet(virtualTerminal, socket, options)
}