Presence.endSync throws NullPointerException when processing a message

[davyskiba]

A client has reported a NullPointerException thrown in ConnectionManager.onMessage(). Looking through the source, I think it is thrown in this line in Presence.endSync method.

Looks like we're trying to get an entry that is no longer present in the map. Since the client is operating with an unstable network connection, I think it might be caused by concurrent modification, caused by concurrent calls to endSync in response to messages arriving in a batch after restoring connectivity. 🤔

Originally reported here in ably-asset-tracking-android.

[sync-by-unito]

➤ Automation for Jira commented:

The link to the corresponding Jira issue is https://ably.atlassian.net/browse/SDK-2957

[davyskiba]

After extensive investigation, I failed to reproduce the issue and implemented a bandaid fix in #873.

The implemented fixe has two downsides worth noting:

• this is a bandaid fix. No root cause/reproduction scenario was found. The underlying issue might still cause problems
• if this issue would be about to occur, listeners won't be notified about a member leaving the channel presence