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java.lang.ClassCastException: to CoroutineStackFrame at kotlinx.coroutines.internal.ScopeCoroutine.getCallerFrame(Scopes.kt:19) #2386

willbuck opened this issue Nov 13, 2020 · 4 comments


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For library authors wishing to implement a custom kotlin.coroutines.Continuation class, casting to an internal class like CoroutineStackFrame is problematic.

See, for example, this issue filed against micronaut:


The error comes on this line, where micronaut has implemented Continuation with the CompletableFutureContinuation class

I think the solve is to have ScopeCoroutine optionally cast uCont to CoroutineStackFrame, as callerFrame is nullable anyway (and it also seems like what other implementations of CoroutineStackFrame, like SelectBuilderImpl and StackFrameContinuation are doing).

I can work on adding a test to reproduce this issue and create a PR since I'm already in the weeds on it, will update once I do!

willbuck added a commit to willbuck/kotlinx.coroutines that referenced this issue Nov 14, 2020
@elizarov elizarov added the bug label Nov 14, 2020
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qwwdfsad commented Nov 16, 2020

Thanks for the report! Filed

This is indeed bug on our side worth fixing, but while we are here, I would strongly suggest implementing CoroutineStackFrame in CompletableFutureContinuation in the following way:

class CompletableFutureContinuation(private val continuation: Continuation<Any>) : Continuation<Any>, CoroutineStackFrame {

    override val callerFrame: CoroutineStackFrame?
            get() = continuation as? CoroutineStackFrame

    override fun getStackTraceElement(): StackTraceElement? = null

This change will improve debugging experience of Micronaut+coroutines usage, including IDE debugging and runtime exceptions recovery machinery

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willbuck commented Nov 17, 2020

@qwwdfsad thanks for the feedback! As far as we could tell, CoroutineStackFrame is an internal class, and thus would not be accessible outside of the kotlinx.coroutines module for Micronaut to extend it. Is that mistaken?

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qwwdfsad commented Nov 17, 2020

CoroutineStackFrame is actually a public interface from kotlin stdlib, in the package kotlin.coroutines.jvm.internal.

While it may seem dangerous to implement it because it is internal, we are not going to change it because a lot of binary code already depend on it. The worst thing we can do is to "deprecate it" with a replacement to the same very interface but in another package

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@qwwdfsad alright then, sounds like something to PR against micronaut :) thanks for the clarification!

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