Allow MQTT QoS 0 subscribers to reconnect (backport #10244) #10252
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The solution in #10203 has the following issues: 1. Bindings can be left ofter in Mnesia table rabbit_durable_queue. One solution to 1. would be to first delete the old queue via `rabbit_amqqueue:internal_delete(Q, User, missing_owner)` and subsequently declare the new queue via `rabbit_amqqueue:internal_declare(Q, false)` However, even then, it suffers from: 2. Race conditions between `rabbit_amqqueue:on_node_down/1` and `rabbit_mqtt_qos0_queue:declare/2`: `rabbit_amqqueue:on_node_down/1` could first read the queue records that need to be deleted, thereafter `rabbit_mqtt_qos0_queue:declare/2` could re-create the queue owned by the new connection PID, and `rabbit_amqqueue:on_node_down/1` could subsequently delete the re-created queue. Unfortunately, `rabbit_amqqueue:on_node_down/1` does not delete transient queues in one isolated transaction. Instead it first reads queues and subsequenlty deletes queues in batches making it prone to race conditions. Ideally, this commit deletes all rabbit_mqtt_qos0_queue queues of the node that has crashed including their bindings. However, doing so in one transaction is risky as there may be millions of such queues and the current code path applies the same logic on all live nodes resulting in conflicting transactions and therefore a long database operation. Hence, this commit uses the simplest approach which should still be safe: Do not remove rabbit_mqtt_qos0_queue queues if a node crashes. Other live nodes will continue to route to these dead queues. That should be okay, given that the rabbit_mqtt_qos0_queue clients auto confirm. Continuing routing however has the effect of counting as routing result for AMQP 0.9.1 `mandatory` property. If an MQTT client re-connects to a live node with the same client ID, the new node will delete and then re-create the queue. Once the crashed node comes back online, it will clean up its leftover queues and bindings. (cherry picked from commit 78b4fcc) # Conflicts: # deps/rabbitmq_mqtt/src/rabbit_mqtt_qos0_queue.erl
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This is an automatic backport of pull request #10244 done by Mergify.
Cherry-pick of 78b4fcc has failed:
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