Hey guys I ve got a question about parent child coroutines w

Question

Hey guys I ve got a question about parent child coroutines what do I do if I want to start a coroutine that might not complete should I set its parent to the caller or not ``` producer function following conventional coroutine parent child ing as per Romans Coroutines part II at kotlinconf 2018 fun CoroutineScope longRunningProducer RecieveChannel lt Info gt then a user fun someUser println started runBlocking val selfScope CoroutineScope = this runBlocking val producer RecieveChannel lt Info gt = selfScope longRunningProducer for item in producer if businessLogic item bail break else println almost done println done ``` if `longRunningProducer` ~never returns~ never stops producing values on its returned channel which is reasonable and dispatches a child coroutine that runs forever then your output will be ``` started almost done ``` because the `nestedEventLoop` wont close until its children are done What I d like to have happen is for an abandoned `longRunningProducer` coroutine to simply get garbage collected but I guess that means I gotta be careful with `weakReferences` and such yeah

gildor · Answer

What is definition of abandoned producer You can just add some time out logic or something similar In general your coroutine cannot be garbage collected until you subscribed so you can of course use some weakReferences but better to handle lifecycle of such coroutine properly and cancel not required coroutines to let them be garbage collected

groostav · Answer

< gildor> abandoned producer as in a producer that is still open but will never `yield` another element I could enforce some kind of timeout policy but in my case its not unreasonable to have a producer go for hours without producing anything Think of a cron job or scheduled task kinda thing

groostav · Answer

the requirement of completion is that ``` runBlocking launch delay 999 999 999 println done ``` means `done` will never get printed because by attaching a child coroutine that is effectively abandoned to a parent scope `runBlocking` wont exit until the child job is finished I dont want that But I do want the fact that if you cancel the parent the child is also cancelled

groostav · Answer

that also begs the question how does one listen for cancellation

groostav · Answer

I could use a kind ve `launchDaemon` to get what I want ``` Test fun `one can create daemon coroutines with onjoin` = runBlocking lt Unit gt runBlocking launchDaemon println I m a daemon doLongRunningPossiblyAbandonedProcess println daemon is done launch println I m a prime job delay 10 println prime job is done println last line in outer job println outer job returned private suspend fun CoroutineScope launchDaemon job suspend CoroutineScope gt Unit val runningJob = GlobalScope launch block = job val parentJob = this coroutineContext Job GlobalScope launch val cancel = select lt Boolean gt runningJob onJoin false parentJob onJoin true if cancel runningJob cancel ```

groostav · Answer

but I think its also pretty clear I m not understanding something more reading tomorrow

gildor · Answer

Not sure now that understand your goal

gildor · Answer

and code of launchDaemon looks strange

gildor · Answer

gt that also begs the question how does one listen for cancellation `Job invokeOnCompletion `

gildor · Answer

gt means `done` will never get printed Yes as I said what is your target strategy in this case I suggested to use withTimeout because if some process is abandoned it means something went wrong or if it s fine you need some strategy how detach from it by timeout or by some event that clean ups coroutines for both cases it can be easily achieved but not clear what is your case gt `runBlocking` wont exit until the child job is finished Of course but what do you want Also probably runBlocking is probably bad choice because not clear here is it just an example or you really want to use it gt I do want the fact that if you cancel the parent the child is also cancelled This is how Job and nested coroutines work