ThreadLocal element not updated correctly when using UnconfinedTestDispatcher

[chuckjaz]

When using an UnconfinedTestDispatcher, the context element for a thread local is not restored when the coroutine is suspended.

Reproduction

playground

import kotlin.coroutines.*
import kotlinx.coroutines.*
import kotlinx.coroutines.channels.*
import kotlinx.coroutines.flow.*
import kotlinx.coroutines.test.*

val MyThreadLocal = ThreadLocal<String>().apply { set("initial") }

@OptIn(ExperimentalCoroutinesApi::class)
fun main() = runTest(UnconfinedTestDispatcher()) {
    val toResume = CompletableDeferred<Continuation<Unit>>()
    launch {
    	println("Initial value: ${MyThreadLocal.get()}")
    	withContext(MyThreadLocal.asContextElement(value = "inContext")) {
        	println("in withContext: ${MyThreadLocal.get()}")
        	suspendCoroutine { co ->
            	toResume.complete(co)
        	}
   		}
        println("after withContext: ${MyThreadLocal.get()}")
    }
    toResume.await().resume(Unit)
}

Expected: "after withContext: initial"

Received: "after withContext: inContext"

NOTE: Removing (UnconfinedTestDispatcher()) from the test case above produces the expected result.

This seems related to #2930 and #4121 as the repro steps are similar.

[dkhalanskyjb]

Duplicate of #4121; this is not a bug but documented behavior. Your main coroutine attempts to access a value access to which it hasn't declared. You can fix the issue by replacing

fun main() = runTest(UnconfinedTestDispatcher()) {

with

fun main() = runTest(UnconfinedTestDispatcher() + MyThreadLocal.asContextElement()) {

If you feel this is inadequate, please join #4121 and explain your use case!