Relax feature flag compat check during join cluster #9729
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... with older RabbitMQ versions which don't know about Khepri. [Why] When an older node wants to join a cluster, it calls `node_info/0` and `cluster_status_from_mnesia/0` directly using RPC calls. If it does that against a node already using Khepri, t will get an error telling it that Mnesia is not running. The error is reported to the end user, making it difficult to understand the problem: both nodes are simply incompatible. It's better to leave the final decision to the Feature flags subsystem, but for that, `rabbit_mnesia` on the newer Khepri-based node still needs to return something the older version can accept. [How] `cluster_status_from_mnesia/0` and `node_info/0` are modified to verify if Khepri is enabled and if it is, return a value based on Khepri's status as if it was from Mnesia. This will let the remote older node to continue all its checks and eventually refuse to join because the Feature flags subsystem will indicate they are incompatible.
…stency` is false [Why] `CheckNodesConsistency` is set to false when the `check_cluster_consistency()` is called as part of a node joining a cluster. And the generic compatibility check was already executed by `rabbit_db_cluster`. There is no need to run it again. This is even counter-productive with the improvement to `rabbit_feature_flags:check_node_compatibility/2` that follows.
... that considers the local node as if it was reset. [Why] When a node joins a cluster, we check its compatibility with the cluster, reset the node, copy the feature flags states from the remote cluster and add that node to the cluster. However, the compatibility check is performed with the current feature flags states, even though they are about to be reset. Therefore, a node with an enabled feature flag that is unsupported by the cluster will refuse to join. It's incorrect because after the reset and the states copy, it could have join the cluster just fine. [How] We introduce a new variant of `check_node_compatibility/2` that takes an argument to indicate if the local node should be considered as a virgin node (i.e. like after a reset). This way, the joining node will always be able to join, regardless of its initial feature flags states, as long as it doesn't require a feature flag that is unsupported by the cluster. This also removes the need to use `$RABBITMQ_FEATURE_FLAGS` environment variable to force a new node to leave stable feature flags disabled to allow it to join a cluster running an older version. References #9677.
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mkuratczyk
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rabbitmq/rabbitmq-server#9729 has been merged. Starting with 4.1, there's no need to disable the new FFs when starting a new node.
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Why
When a node joins a cluster, we check its compatibility with the cluster, reset the node, copy the feature flags states from the remote cluster and add that node to the cluster.
However, the compatibility check is performed with the current feature flags states, even though they are about to be reset. Therefore, a node with an enabled feature flag that is unsupported by the cluster will refuse to join. It's incorrect because after the reset and the states copy, it could have join the cluster just fine.
How
We introduce a new variant of
check_node_compatibility/2that takes an argument to indicate if the local node should be considered as a virgin node (i.e. like after a reset).This way, the joining node will always be able to join, regardless of its initial feature flags states, as long as it doesn't require a feature flag that is unsupported by the cluster.
This also removes the need to use
$RABBITMQ_FEATURE_FLAGSenvironment variable to force a new node to leave stable feature flags disabled to allow it to join a cluster running an older version.References #9677.