ConflatedBroadcastChannel hangs in offer()

Hi there,

maybe there is a better way to achieve what I want, but with my current wrapper class around MutableMaps that allows coroutine waiting for key additions will at times get stuck and cause 100% cpu load. I’m using the recently added ConflatedBroadcastChannel class to inform all coroutines waiting for a key to arrive on a put operation.

Actually, I would only need a coroutine compatible wait/notify primitives… hints?

Anyway, when I pause the threads during the lockup, I will get a stacktrace that may look like this one:

<15> ForkJoinPool.commonPool-worker-3"@830,030,709,776 in group "main": RUNNING
getNext:121, LockFreeLinkedListNode {kotlinx.coroutines.experimental.internal}
takeFirstReceiveOrPeekClosed:920, AbstractSendChannel {kotlinx.coroutines.experimental.channels}
offerInternal:54, AbstractSendChannel {kotlinx.coroutines.experimental.channels}
offerInternal:39, ConflatedChannel {kotlinx.coroutines.experimental.channels}
offerInternal:268, ConflatedBroadcastChannel$Subscriber {kotlinx.coroutines.experimental.channels}
offerInternal:239, ConflatedBroadcastChannel {kotlinx.coroutines.experimental.channels}
offer:219, ConflatedBroadcastChannel {kotlinx.coroutines.experimental.channels}
put:11, AwaitableMap {com.example}

Any ideas what’s going on here?

package com.example

import kotlinx.coroutines.experimental.channels.ConflatedBroadcastChannel

class AwaitableMap<K : Any, V : Any>(val delegateMap: MutableMap<K, V>) : MutableMap<K, V> by delegateMap {

    val channel = ConflatedBroadcastChannel<K>()

    override fun put(key: K, value: V): V? {
        val putVal = delegateMap.put(key, value)
        channel.offer(key)
        return putVal
    }

    suspend fun await(key: K): V {
        return delegateMap[key] ?: channel.open().use {
            do {
                val fetched = delegateMap[key]
                if (fetched != null) {
                    return@use fetched
                }
                it.receive()
            } while (true)
            return@use null
        }!!
    }
}

[elizarov]

Fixed in develop branch

[elizarov]

It turned out to be a whole complex of problems that happen when you concurrently close/send/close channels. It was also independently discovered by @cy6erGn0m