Consolidate and document order between cancellation handlers and continuation resume

[qwwdfsad]

Reproducer from #407 will fail with exception in coroutines machinery

@Test
fun testCancelProduce() = runTest {
    val source = Channel<Int>()
    val produced = produce<Int>(ctx, onCompletion = source.consumes()) {
        source.receive() // <- when exception is thrown from here, source is NOT empty for Unconfined
    }
  
    produced.cancel()
}

Behaviour will be different for Unconfined and coroutineContext as ctx.

It's because we do the following:

cont.cancel(e) 
->  dispatch() { cont.resumeWithException(e) }
->  for (handler in handler) handler.invoke(e)

We have data-race with small window between dispatched continuation and handlers invocation. Same race is 100% reproducible with Unconfined.

[PaulWoitaschek]

So this is fixed but not released yet?

[qwwdfsad]

Yes

[PaulWoitaschek]

Can this please be released soon? About 0.7% of my sessions are crashing with this exception.

[qwwdfsad]

Presumably it will be released next week