`ConcurrentModificationException` when `unsubscribe` then `detach` channel presence listener

[QuintinWillison]

A customer is experiencing a crash with ably-java version 1.2.10, therefore my understanding is that this is pure JVM (i.e. a server type application - I think it's some kind of a bot), not Android. They have described the scenario to us:

When the UserA encounters a new user join to chatroom he was in , it registers the user presence listener.

presenceChannel.presence.subscribe(member -> {

When UserA no longer needs to track the users presence (not a member of any active chat rooms), the listener is removed:

presenceChannel.presence.unsubscribe();
try {
    presenceChannel.detach();
} catch (AblyException e) {
    throw new BotException(e.toString());
}

The error is encountered mostly when a user goes offline....
The following is the exception stack:

2022-01-25T15:37:29,129|INFO|SomeClient|WebSocketConnectReadThread-132|{"l":"DEBUG","msg":"receive presence event, action: leave, data: {\"userId\":\"SomeUserIdValue\",\"status\":\"Online\",\"customStatus\":\"Available From SomeNamedTool\"}","fnc":"SomeClient.registerUserPresence"} 
2022-01-25T15:37:29,131|DEBUG|UserPresenceManager|WebSocketConnectReadThread-132|updateUserPresence: userId=PATESTFICA138, source=MESSAGING, state=OFFLINE 
(ERROR): io.ably.lib.transport.WebSocketTransport: Unexpected exception processing received binary message 

io.ably.lib.types.AblyException: java.util.ConcurrentModificationException 
    at io.ably.lib.types.AblyException.fromThrowable(AblyException.java:54) 
    at io.ably.lib.transport.ConnectionManager.onMessage(ConnectionManager.java:1080) 
    at io.ably.lib.transport.WebSocketTransport$WsClient.onMessage(WebSocketTransport.java:161) 
    at org.java_websocket.client.WebSocketClient.onWebsocketMessage(WebSocketClient.java:505) 
    at org.java_websocket.drafts.Draft_6455.processFrameBinary(Draft_6455.java:835) 
    at org.java_websocket.drafts.Draft_6455.processFrame(Draft_6455.java:794) 
    at org.java_websocket.WebSocketImpl.decodeFrames(WebSocketImpl.java:381) 
    at org.java_websocket.WebSocketImpl.decode(WebSocketImpl.java:218) 
    at org.java_websocket.client.WebSocketClient.run(WebSocketClient.java:425) 
    at java.base/java.lang.Thread.run(Thread.java:829) 

Caused by: java.util.ConcurrentModificationException 
    at java.base/java.util.ArrayList$Itr.checkForComodification(ArrayList.java:1043) 
    at java.base/java.util.ArrayList$Itr.next(ArrayList.java:997) 
    at io.ably.lib.realtime.Presence$Multicaster.onPresenceMessage(Presence.java:359) 
    at io.ably.lib.realtime.Presence.broadcastPresence(Presence.java:345) 
    at io.ably.lib.realtime.Presence.setPresence(Presence.java:334) 
    at io.ably.lib.realtime.ChannelBase.onPresence(ChannelBase.java:769) 
    at io.ably.lib.realtime.ChannelBase.onChannelMessage(ChannelBase.java:1158) 
    at io.ably.lib.realtime.AblyRealtime$InternalChannels.onMessage(AblyRealtime.java:187) 
    at io.ably.lib.transport.ConnectionManager.onChannelMessage(ConnectionManager.java:1090) 
    at io.ably.lib.transport.ConnectionManager.onMessage(ConnectionManager.java:1075) 
    ... 8 more

(some details redacted above)

Hubspot (internal) threads conversing with this customer are here and, then, here.

┆Issue is synchronized with this Jira Bug by Unito