After reading about how `produce` can lose elements, I wonder whether `Flow::produceIn` has the same...

# server

[Marcus Ilgner]

After reading about how produce can lose elements, I wonder whether Flow::produceIn has the same potential issue? Does anyone know? I want to use select to pull elements out of a flow but without the chance of losing any... (i.e. if that SelectClause gets cancelled, it should just receive that element on the next attempt).

[Jakub Zalas]

Oh.. I don't know the answer, but do you know any particular scenarios when produce might lose elements?

[Marcus Ilgner]

I haven't delved into the implementation yet to fully understand how it happens but it has to do with cancellation of the receive suspend function. There's a block titled "Undelivered elements" in the documentation here: https://kotlinlang.org/api/kotlinx.coroutines/kotlinx-coroutines-core/kotlinx.coroutines.channels/produce.html

[Emil Kantis]

If you use a Channel with 0 capacity (RENDEZVOUS channel), this would not be an issue I believe?

[Marcus Ilgner]

that's the big question... although from how I understand it right now, it can occur even then, as long as one doesn't provide an onUndeliveredElement callback that somehow handles the element. How I understand it to work, it's like this: the onReceive SelectClause waits for elements to appear in the flow. Once that happens, they are (a) collected and (b) sent to the channel. But if the select statement happens to succeed on another SelectClause after (a) and before (b), the onReceive clause would be cancelled and the element could become lost.