I’m trying to set up a minimal example that gives ...

# coroutines

[Viktor Qvarfordt]

I’m trying to set up a minimal example that gives race conditions. Any suggestions? The best I’ve got is this https://pl.kotl.in/4yU_BS_qv but it has a quirk that I don’t understand. (Question embedded in the code snippet).

[bdawg.io]

Because GlobalScope is not the same as the scope provided by runBlocking.

Your runBlocking parent scope only has the main thread, so only one coroutine will run at a time accessing the counter from the same thread, so there's no shared mutable state going on.

GlobalScope, on the other-hand, has multiple threads available to it, which means multiple of your `massiveRun`'s job will run in parallel, causing overwriting of mutable state

[bdawg.io]

You would want to do

runBlocking {
    launch(Dispatchers.Default) {
        // paste the function body
    }.join()
}

[Viktor Qvarfordt]

Excellent explanation! Thank you.

[bdawg.io]

Immediately joining on a job you created with launch is generally a code smell that can be replaced with

runBlocking {
    withContext(Dispatchers.Default) {
        // paste the function body
    }
}

[bdawg.io]

happy to help! 🎉

[Viktor Qvarfordt]

Awesome, that’s a great tip! Based on your answer I also found this in the documentation (for anyone reading this that wants more): https://kotlinlang.org/docs/reference/coroutines/coroutine-context-and-dispatchers.html

[louiscad]

Why not just use suspend fun main(): Unit = coroutineScope { … }? It uses Dispatchers.Default.

[bdawg.io]

That's good to know @louiscad! I would say it's a bit harder to know when reading the code though without background knowledge

[louiscad]

Here's a little part of my background @bdawg.io: The first time I noticed it was during my first English talk (I'm French native, we are renowned for our foreign languages skill 😅), when my demo using 2 concurrent Thread.sleep failed to show any difference with `delay`… It was the first I used it as it was new in 1.3RC. I natively thought it'd be like runBlocking. Found the answer and fixed it during Q&A, fortunately.