Kotlinx.coroutines: Failed to receive all elements when using produce with a buffered channel

[elizarov]

Run the following code

fun main(args: Array<String>) = runBlocking<Unit> {
    val producer = produce<Int>(coroutineContext, capacity = 1) {
        for(i in 1..2) {
            send(i)
            println("Sent: $i")
        }
    }

    for (x in producer) {
        println("Received: $x")
    }
}

The output is:

Sent: 1
Sent: 2
Received: 1

while expected output is:

Sent: 1
Sent: 2
Received: 1
Received: 2

The problem shows up with any buffered chennel (ArrayChannel and LinkedListChannel) and does not show up with RendezvousChannel (capacity = 0). It only affects produce builder. Replacing produce with an "manual" code via launch works as expected:

    val producer = Channel<Int>(1)
    launch(coroutineContext) {
        for(i in 1..2) {
            producer.send(i)
            println("Sent: $i")
        }
        producer.close()
    }

[jcornaz]

I don't know if it helps, but I've notice that if we don't specify a context it succeed with ArrayChannel but still fails with LinkedListChannel and ConflatedChannel.

This code:

fun main(args: Array<String>) = runBlocking {
  val producer = produce(capacity = 1) {
    for(i in 1..2) {
      send(i)
      println("Sent: $i")
    }
  }

  for (x in producer) {
    println("Received: $x")
  }
}

produce the following output:

Sent: 1
Sent: 2
Received: 1
Received: 2

But this code:

fun main(args: Array<String>) = runBlocking {
  val producer = produce(capacity = Channel.UNLIMITED) { // same problem with Channel.CONFLATED
    for(i in 1..2) {
      send(i)
      println("Sent: $i")
    }
  }

  for (x in producer) {
    println("Received: $x")
  }
}

produce:

Sent: 1
Sent: 2
Received: 2

[maxd]

@jcornaz Try to run your first example several times. Sometimes it will return wrong result:

Sent: 1
Received: 1
Sent: 2

Thus I think ArrayChannel have the same problem.

[jcornaz]

@maxd Indeed, you're right! Haven't seen that.

However, I was already convinced of a problem when using ArrayChannel (as demonstrated by @elizarov). I just hoped that knowing the difference of behavior depending on the context could help to debug it. It seems that at the end it doesn't really help. sorry.

[elizarov]

The problem is actually in produce. It cancels(sic!) the underlying channel (instead of closing) when producing coroutine returns. That's the bug. will be fixed soon.

[ansman]

@elizarov Is the only workaround not to use produce for now?