Unintuitive dispatching behavior in some rare cases with Android's Dispatchers.Main.immediate

[dkhalanskyjb]

See #3924 (comment)

This code produces 1, 2, 3:

@Test
fun foo() {
    GlobalScope.launch(Dispatchers.IO) {
        withContext(Dispatchers.Main.immediate) {
            println("1")
            launch(Dispatchers.Main) {
                println("2")
            }
            withContext(Dispatchers.Main) {
                println("3")
            }
        }
    }
}

This code, however, produces 1, 3, 2:

@Test
fun foo() {
    GlobalScope.launch(Dispatchers.Main) { // <-- note the different dispatcher
        withContext(Dispatchers.Main.immediate) {
            println("1")
            launch(Dispatchers.Main) {
                println("2")
            }
            withContext(Dispatchers.Main) {
                println("3")
            }
        }
    }
}

This behavior may be surprising. For example, consider this pseudo-code:

fun updateUi() = scope.launch(Dispatchers.Main) { }

suspend fun readUiElement(): Int = withContext(Dispatchers.Main) { 1 }

fun update() = viewModelScope.launch {
  updateUi()
  cache = readUiElement()
}

Depending on whether update is called from the main thread, the block in updateUi will happen either earlier or later than readUiElement, even though visually, it seems like updateUi should happen earlier.

Notably, all the other implementations of Dispatchers.Main that we provide don't suffer from the same problem but instead consistently output 1, 2, 3 in both scenarios.

[dkhalanskyjb]

Fixed in 7f32340