dongnuo123 opened a new pull request, #22457:
URL: https://github.com/apache/kafka/pull/22457

   When group.consumer.migration.policy=disabled, a new classic-protocol member 
is still admitted into an existing consumer group. Because the policy permits 
neither an online upgrade nor an online downgrade, the group can never converge 
back to a single protocol — it stays permanently mixed.
   
   This is inconsistent with the other direction. With disabled, a new 
consumer-protocol member joining a classic group is refused, because the 
upgrade it would require is turned off. A new classic member joining a consumer 
group should be refused for the same reason: the downgrade it would imply is 
also turned off. Under disabled the intent is that no migration happens and 
groups stay single-protocol, so a new classic member joining a consumer group 
should be rejected, mirroring the consumer-into-classic case that is already 
rejected today.
   
   This applies to disabled only. Under upgrade, classic members already in the 
group during an in-progress upgrade must still be able to rejoin after a 
transient fence, and under downgrade, new classic members are the mechanism of 
the downgrade rollout, so neither policy should reject them.
   
   This patch rejects the new classic member if it tries to join a consumer 
group. Members that are already in the group may still rejoin.
   


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