Read nil from a new-joined slave node in Redis cluster.

[lemonlinger]

We enable ReadOnly=true for the Redis cluster client. When a new slave node joins the cluster, the client can quickly detect the new slave node by updating the cluster states. Once a new node is detected, requests are immediately forwarded to that node. If the new slave node is still in the master-slave replication process and is waiting for the master node to send the RDB file, the result of the client's read operation will be nil.

Expected Behavior

A new slave node should not be added to the cluster state before completing master-slave replication.

Current Behavior

When a slave node joins the cluster, it is immediately added to the cluster state to receive client requests.

Possible Solution

To check if a node is in replication state when loading cluster state. Send the command info replication to the slave node and add it into cluster state only when master_link_status is up.

Steps to Reproduce

1. Setup a Redis cluster with at least one slave node for each shard.
2. Fill a bulk of random data into the cluster. This is for extending the replication duration intentionally.
3. Run a Redis cluster client (ReadOnly=true is enabled) to keep reading existing keys from the cluster.
4. Add a slave node into the cluster.
5. Check if the client reads nil values.

Context (Environment)

Detailed Description

Possible Implementation

[ndyakov]

@lemonlinger thank you for reporting this, I will look into it asap.