Question: Should variable in coroutine body be synchronized with AtomicReference or Channel if only one inner coroutine changes it?

Having simple variable in coroutine block and only one producer coroutine changes the state of the variable and another coroutine consumes it, should this variable be synchronized somehow with AtomicReference or Channel to be visible in reader coroutine or reader coroutine automatically fetches new data from the memory?

fun main() = runBlocking<Unit>(Dispatchers.Default) {
    var state: State? = null

    launch {
        data0Flow.collect { newState ->
            state = newState
        }
    }

    launch {
        data1Flow.collect { someData ->
            // Will the state be visible immediately when first coroutine updated it ?
            if (state != null) {
                // do something
            }
        }
    }
}

[fvasco]

should this variable be synchronized somehow

Yes, absolutely.

Local variables are not thread safe.

[fvasco]

read coroutine can cache the state and it will not have fresh values

Yes, on JVM you can get worse than you expect (see JVM Memory Model).

But changing state within the same coroutine (launched by launch, async, flow, …) is safe, correct ?

Yes, this is my experience.