[native-mt] Leaks of objects crossing a thread boundary on iOS

[EuV]

Kotlin: 1.4.10
Coroutines: 1.3.9-native-mt-2

MainScope().launch {
    withContext(Dispatchers.Default) { // Leaks on iOS Simulator
    // withContext(Dispatchers.Main) { // Doesn't leak on iOS Simulator if GC.collect() is called
        mutableListOf<Int>().apply {
            repeat(3_000_000) {
                this += it
            }
        }
    }
    GC.collect()
}

MainScope().launch(Dispatchers.Default) {
    withContext(Dispatchers.Main) {
        // Also leaks
    }
}

MainScope().launch(Dispatchers.Default) {
    withContext(Dispatchers.Default) {
        // Does not
    }
}

[mvicsokolova]

Most probably, leaks in these scenarios were caused by the leaking Worker created by DefaultDispatcher (see #2491). After introducing public shutdown method to DefaultDispatcher to request termination on the leaking worker (#2652), I could not reproduce any object leaks.