Integrate event loops of runBlocking and UNCONFINED

[elizarov]

The problem is that the code that uses runBlocking { ... } to provide integration for blocking Java APIs (like blocking IO) can deadlock when being run inside another runBlocking { ... } (see ktorio/ktor#672 for a real-life problem in Ktor that is causes). To avoid that, a nested runBlocking shall use the same event queue as the outer runBlocking. The same is true for Dispatchers.UNCONFINED. There should be at most one active event queue per each thread for the purposes of runBlocking and UNCONFINED, which would allow to mix them in a deadlock-free way.

One test that shows the problem is:

fun main() = runBlocking { // outer event loop
    // Produce string "OK"
    val ch = produce { send("OK") }
    // try receive this string in a blocking way:
    println(runBlocking { ch.receive() }) // it hangs here !!!
}

[elizarov]

Note, that UNCONFINED dispatcher shall still keep its existing behavior when used from inside runBlocking { ... } in that a top-level execution of an unconfined task shall wait until all unconfined task in the queue are executed before exiting. Use case:

Consider a regular function with an injected coroutine context

fun launchFoo(ctx: CoroutineContext) = launch(ctx) { ... }

We are writing a test for it like this:

fun testFoo() {
    launchFoo(Dispatchers.UNCONFINED) 
    // ^^^ foo's job completes before executing the next line
    assert { fooDidEverything }
}

Wrapping this test function into a runBlocking { .... } should not invalidate the test:

fun testFoo() = runBlocking<Unit> {
    launchFoo(Dispatchers.UNCONFINED) 
    // ^^^ foo's job completes before executing the next line
    assert { fooDidEverything }
}