Trying to understand thread safety in coroutines… ...

# coroutines

[allan.conda]

Trying to understand thread safety in coroutines… Will there be threading issues with this piece of code:

coroutineScope {
    var count = 0
    val mutableList = mutableListOf<Int>()
    launch {
        while(isActive) {
            mutableList.add(count++)
            delay(1000)
        }
    }
    launch {
        while(isActive) {
            delay(10000)
            mutableList.take(10).also(mutableList::removeAll)
        }
    }
}

[gildor]

There is a race condition on access mutableList, it’s not thread safe and there is a chance of ArrayIndexOutOfBoundsException or other errors

[gildor]

on practice it may have no problems, if this scope has only one thread and both launched coroutines run on it, but it’s too brittle to rely on this

[gildor]

so you should use the same global mutable state syncronoization techniques as with threads (thread-safe list implementations, locks, mutexes etc)

[allan.conda]

I see. I’m trying to have a batched flow actually but want to understand instead of copy pasting the gist here
https://github.com/Kotlin/kotlinx.coroutines/issues/1290#issuecomment-776239880
Won’t this be the same problem with channels? Perhaps it has thread-safety code for poll() and send() (poll is deprecated though)

[Zach Klippenstein (he/him) [MOD]]

I'm not sure which comment you linked to (mobile app doesn't handle the link great) so not sure which code you're referring to, but channels are definitely thread safe - cross-thread communication is one of their main purposes.

[gildor]

Additionally to Zach's message, Flow also can be used for safe cross-thread communication, it has more predictable and safe API, but it's not direct replacement in terms of how you use it