[native-mt] runBlocking in Unit test with 1.3.9-native-mt or 1.3.9-native-mt-2 never exits

[werner77]

Consider the very simple unit test:

@Test
    fun coroutines() {
        runBlocking {
            withContext(Dispatchers.Main) {
                println("1")
                withContext(Dispatchers.Main) {
                    println("2")
                }
                println("3")
            }
            println("4")
        }
        println("5")
    }

This test runs and finishes fine on JVM or native with the main (non-mt) version, but with the mt version it hangs indefinitely.

[ntherning]

I have seen this deadlock too on iOS. I believe the problem with runBlocking() is that it is blocking the main thread without letting iOS process any blocks (coroutines) enqueued on the native GDC main queue. On iOS Dispatchers.Main uses this queue to asynchronously run coroutines.

I have experimented with making my own version of runBlocking() for my tests which uses NSRunLoop.mainRunLoop.runUntilDate(...) to give GDC a chance to process the queued blocks. It works, coroutines are run, but my understanding of coroutine internals is not deep enough to make it reliable (e.g. waiting for all descendant jobs to complete).

I see that there's a concept of an EventLoop in the coroutines internals which I don't fully understand. Perhaps a custom EventLoop impl for the iOS main thread could make runBlocking() pump the main thread run loop as needed and prevent the deadlocking?

[qwwdfsad]

native-mt is no longer supported and it is recommended to use regular releases of coroutines along with new memory model