[server] Leader-only rebalance tasks should wait for tablet server acknowledgment before proceeding to the next migration

[swuferhong]

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Motivation

During rebalance, replica migration tasks (e.g., [0,1,2] → [3,4,5]) correctly wait for the task to complete before proceeding to the next migration, which avoids overloading the system. However, leader-only migration tasks (e.g., leader 0 → 1) are completed immediately after triggering the election, without waiting for the tablet server to acknowledge the change. This causes all leader migrations to fire simultaneously, which puts excessive pressure on tablet servers, especially for KV tables that need to rebuild state during leader transitions.

Solution

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Anything else?

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Willingness to contribute

• I'm willing to submit a PR!