Hello, i have a question regarding the use of runB...

# coroutines

[Tower Guidev2]

Hello, i have a question regarding the use of runBlocking in production code and
what is causing my intermittent failure in the following code snippet

[Tower Guidev2]

private fun something(things: List<Thing>): List<OtherThing> {
val otherThings = mutableListOf<OtherThing>()

runBlocking(<http://Dispatchers.IO|Dispatchers.IO>) {
things.map {
async {
val whatEver = somethingElse(it)

if (whatEver) otherThings.add(process(it))
}
}.awaitAll()
}

return otherThings.take(someAmount.toInt())
}


fun process(thing: Thing): OtherThing {
return thing.copy(data = thing.data.replace(Regex(NON_LETTER_PATTERN), ""))
}

[Tower Guidev2]

when the code runs as above I see intermittent test failures in my asserts
when i refactor runBlocking(<http://Dispatchers.IO|Dispatchers.IO>) {} to runBlocking {}
i never see assert failures

[Tower Guidev2]

im sure i remember seeing a comment that xxx.map ( async {} ) has a subtle bug
however i cannot find that now
if i shouldnt be using runBlocking in production code
what are my alternatives?

[Sam]

I would guess that it could be an issue due to concurrent modification of the otherThings list

[Sam]

<http://Dispatchers.IO|Dispatchers.IO> has multiple threads so the async tasks could run in parallel, whereas with just runBlocking it would run in a single threaded event loop

[Tower Guidev2]

yes, i believe that is the root cause
maybe i could fix that by <http://Dispatchers.IO|Dispatchers.IO>.limitedParallelism(1)

[Tower Guidev2]

however i realy do not like the use of runBlocking

[Sam]

The advice against runBlocking is most important if this code is already running inside a coroutine. In that case, you should make something a suspend function, and replace runBlocking with coroutineScope { ... }.

[Sam]

If there's no existing coroutines, runBlocking may be an okay choice