When the client reconnects, resubscribing will result in duplicate subscriptions

[xuzimianxzm]

🐛 Bug Report

🔬 How To Reproduce

Steps to reproduce the behavior:

When the client reconnects, I try to wait for the automatic disconnection to complete in the Disconnect event, and then try to subscribe again to the topic that was subscribed before the connection was lost. However, duplicate subscriptions will occur at this time, and the number of duplicate subscriptions to the same topic will double after each reconnection. Even if I have explicitly called the Client's unsubscribe topic method, it does not work.

Code sample

The following is the code for restoring topic subscription after automatic reconnection:

 if (sender is not IHiveMQClient client) return;

            logger.LogInformation("The connection has been disconnected. Try to reconnecting");

            while (!client.IsConnected())
            {
                logger.LogInformation("Wait mqtt client auto reconnecting...");
                Thread.Sleep(5000);
            }

            logger.LogInformation("The mqtt client has reconnected.");

            client.UnsubscribeAsync(client.Subscriptions)
                .ConfigureAwait(false)
                .GetAwaiter()
                .GetResult();

            var newsSubscribeResult = client.SubscribeAsync(_subscribeOptions!)
                .ConfigureAwait(false)
                .GetAwaiter()
                .GetResult();

When I consulted the HiveMQClient source code, I found that the subscription topics related information is stored in this variable, as follows:

Each time you subscribe, the topics and corresponding handlers that have completed the subscription will be put into this collection:

However, when canceling a subscription, there is no attempt to remove the unsubscribed topics and handlers. You can see that the Subscriptions collection of this HiveMQClient object does not have any call to remove elements:

Therefore, based on the above analysis, I took the following temporary corrective measures, that is, manually "Subscriptions.Clear();" to ensure that there will be no repeated subscriptions to Topics:

[pglombardo]

Hi @xuzimianxzm - thanks for letting us know. I'll take a closer look at this and respond back soon!