I m trying to understand coroutines and I have a question wh

Question

I m trying to understand coroutines and I have a question why does `c2` method hang here

Dico · Answer

Because the scope provided by `runBlocking` has a dispatcher that wraps a single threaded event loop

Dico · Answer

The `async` call is waiting on the `producer ` invocation to become available which in turn is waiting on the thread to become available that is used by `async`

Michal Fudala · Answer

< Dico> ok so what would be the simplest solution to fix it `runBlocking newFixedThreadPoolContext 10 a `

Dico · Answer

You could use any multithreaded dispatcher runBlocking won t work with a dispatcher that isn t an event loop

Michal Fudala · Answer

< Dico> could you give some example of a multihreaded event loop dispatcher

Dico · Answer

Um I think it s internal api Read the docs

gildor · Answer

No runBlocking has nothing with it This code hangs because you start coroutine in async Async never returns because waits all child coroutines work finish but because producer s channel is never closed it will never return this is how parent child relationships work with structured concurrency

gildor · Answer

If you really want to create global coroutine and manage lifecycle manually do not do this `async producer await ` which starts new coroutine async and async itself creates another one and waits for child coroutine work result to avoid leaks If you really need channels that can be consumed globally just use GlobalScope ```async GlobalScope producer await ```

Dico · Answer

Right the task inside `produce` can t be started because it s been scheduled on this particular dispatcher with one thread Thus `async` needs to suspend first `async` can only complete when `produce` started and invoked `close `

gildor · Answer

No dispatcher is not involved

Dico · Answer

However that has everything to do with `runBlocking` because this wouldn t block otherwise on default dispatcher for example

gildor · Answer

problem with StructuredConcurrency

gildor · Answer

nono see no dispatching problems there

Dico · Answer

Can you explain once more

gildor · Answer

even if you replace runBlocking with any other coroutine builder you will get the same

gildor · Answer

async producer await

gildor · Answer

awiat will never return

Dico · Answer

Well why

gildor · Answer

because `producer ` is a new coroutine that never finishes because nobody consumes value from channel

gildor · Answer

this is how coroutines scopes work

gildor · Answer

`async ` is scope

Dico · Answer

Ah it needs to be consumed That makes sense

gildor · Answer

so every child coroutine will be attached to this scope

gildor · Answer

and this producer coroutine is attached and async cannot return value before all child coroutines are finished otherwise coroutine may leak

Dico · Answer

I get that yeah but i didn t know it isn t considered completed unless all values were consumed

gildor · Answer

produce is just a coroutine with channel and coroutine cannot finish until channel is open

Dico · Answer

What kind of channel is used by default Rendezvous

Dico · Answer

Because it does invoke `close ` on the channel

gildor · Answer

yes

gildor · Answer

this this is Rendezvous

Dico · Answer

Right so `send` will suspend and thus `close ` is not reached unless the value is consumed

gildor · Answer

`close ` is useless in general because it will be closed when produce block return

Dico · Answer

Fair enough

gildor · Answer

yes send suspend and coroutine is working forewer

Dico · Answer

If it were another kind of channel though it might work right

Dico · Answer

Like unlimited conflated

gildor · Answer

I m not sure that such pattern is good in general when you create some produce coroutine in some other not related coroutines even if you fix it with GlobalScope there is a chance that nobody consume it and you will leak coroutine if you have 1 it s fine what if create millions of them

Dico · Answer

I agree

gildor · Answer

depends on what you consider as work

Dico · Answer

You mean it s good that `await ` waits on the channel to prevent you making ones unnecessarily

gildor · Answer

I don t think that it will work in any case

gildor · Answer

because if send will not suspend channel will be closed

Dico · Answer

I m not able to test right now

gildor · Answer

gt You mean it s good that `await ` waits on the channel to prevent you making ones unnecessarily Yes this is child parent relationships that doesn t allow to start async background task in nowhere At least you should be explicit and use GlobalScope

gildor · Answer

Just concider `async produce ` as `async launch ` so you don t want to start launch coroutine and do not wait for result