Why doesn't this deadlock? ```val stream = MutableSharedFlow<Unit>() val unused = stream.buffe...

# coroutines

[Dominaezzz]

Why doesn't this deadlock?

val stream = MutableSharedFlow<Unit>()
val unused = stream.buffer(Channel.RENDEZVOUS).produceIn(GlobalScope)
stream.emit(Unit)

[mbonnin]

I think MutableSharedFlow will drop events if there are no subscribers

[mbonnin]

https://kotlin.github.io/kotlinx.coroutines/kotlinx-coroutines-core/kotlinx.coroutines.flow/-shared-flow/index.html

in the absence of subscribers emitter never suspends despite BufferOverflow.SUSPEND option and BufferOverflow.DROP_LATEST option does not have effect either

[Dominaezzz]

Yes but isn't the produceIn a subscriber?

[Dominaezzz]

If I call stream.emit(Unit) 20 times, in a row, it suspends after the 15th emit. Nvm, it just never suspends.

[mbonnin]

I got it to suspend at the 65th emission, which seems to indicate a default buffer size somewhere

[mbonnin]

stream.buffer(10)

suspends at the 11th emission so looks like Channel.RENDEZVOUS isn't honoured

[Dominaezzz]

Tried 200 emissions and it doesn't suspend.

[Dominaezzz]

That's odd. I'll try buffer(10)

[mbonnin]

I run this in a junit test:

@Test
  fun produceIn() {
    runBlocking {
      val stream = MutableSharedFlow<Unit>()
      val unused = stream.buffer(Channel.RENDEZVOUS).produceIn(GlobalScope)
      var i = 0
      while (true) {
        println("emitting $i")
        stream.emit(Unit)
        i++
      }
    }
  }

[mbonnin]

using coroutine 1.5.0

[Dominaezzz]

That stops at 75 for me.

[mbonnin]

Interesting 👀

[Dominaezzz]

Adding a delay before the loop makes it 65.

[mbonnin]

SharedFlow doesn't support RENDEZVOUS :

Application of flowOn, buffer with RENDEZVOUS capacity, or cancellable operators to a shared flow has no effect.

[mbonnin]

That might be part of the explanation

[Dominaezzz]

Nooooo!!!

[Dominaezzz]

Crap

[Dominaezzz]

I really don't want to use a channel for this. I guess I don't have a choice.

[mbonnin]

I guess having a RENDEZVOUS with multiple subscribers was too complex...

[Dominaezzz]

Thanks for helping find that.

[Dominaezzz]

There's still the issue of 65 emissions, I wonder where that comes from.

[Dominaezzz]

Ah, buffer(1) makes it 2. 🤦🏼

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

https://github.com/Kotlin/kotlinx.coroutines/blob/b231887c3214187cf7f09a74073f84383a487b71/kotlinx-coroutines-core/common/src/channels/Channel.kt#L750

[Dominaezzz]

https://github.com/Kotlin/kotlinx.coroutines/blob/65e1b84f742a4fff14c67a11a873a4977a0ba539/kotlinx-coroutines-core/common/src/flow/SharedFlow.kt#L693

[Dominaezzz]

Doesn't seem like a complexity issue, but I'm now wondering why they thought it'd be meaningless.

[Dominaezzz]

Must've not realised there would be an interesting default?

[Dominaezzz]

It's probable produceIn that's wrong I guess.

[Dominaezzz]

Opened these two.
https://github.com/Kotlin/kotlinx.coroutines/issues/2817
https://github.com/Kotlin/kotlinx.coroutines/issues/2818