ThreadContextElement failure breaks cancellation

[cy6erGn0m]

An exception in updateThreadContext breaks job cancellation sequence so the following example hangs forever

package org.jetbrains.tests.cy.element.failure

import kotlinx.coroutines.*
import java.lang.IllegalStateException
import kotlin.coroutines.*

class MyElement : ThreadContextElement<Unit> {
    private object Key : CoroutineContext.Key<MyElement>

    override val key: CoroutineContext.Key<*> get() = Key

    override fun restoreThreadContext(context: CoroutineContext, oldState: Unit) {
    }

    override fun updateThreadContext(context: CoroutineContext) {
        throw IllegalStateException("Prevent a coroutine from starting right here for some reason")
    }
}

fun main() {
    runBlocking {
        GlobalScope.launch(MyElement()) {
            println("OK")
        }.join()
    }
}

I'd expect it to cancel job and resume join (since it is running not under MyElement so it is possible to resume it) however it remains Active

[qwwdfsad]

There is not much we can do.
We can't resume coroutine in a semi-updated context. If we can't resume it (and cancellation implies resume with exception), we can't complete it, so any join will hang.

All solutions may lead to unexpected behaviour:

1. Resuming job in a semi-updated context breaks the contract of ThreadContextElement
2. Transferring job to the final state breaks atomicity and cancellation contract
3. Catching and reporting exceptions in every context element and then resuming coroutine will effectively disable "restore" and may leave context element in an inconsistent state.