databendlabs / openraft / 16498629934
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drmingdrmer 
Improve: Ensure uniform membership config during config changes

This commit improves the handling of membership configuration changes to
ensure that the system reliably transitions to a uniform configuration
in the second step of a joint membership change.

### Problem:

During a membership config change, there are two steps:

1. Transition to a joint configuration containing both `Cold` (current
   config) and `Cnew` (new config).
2. Transition to a uniform configuration containing only `Cnew`.

Previously, the second step attempted to apply the same change on the
current membership state. If multiple membership change requests were
processed in parallel, this approach could result in the system being
left in a joint configuration. For example:

- Initial config: `{a, b, c}`.
- Task 1: `AddVoterIds(x)`. After the first step:
  `[{a, b, c}, {a, b, c, x}]`.

- Task 2: `RemoveVoters(x)`. After the first step:
  `[{a, b, c, x}, {a, b, c}]` (applied on the last config `{a, b, c, x}`).

- Task 1 proceeds to the second step, re-applies `AddVoterIds(x)`, and
  the result is `[{a, b, c}, {a, b, c, x}]`.

- The system remains in a joint configuration, which is unintuitive and
  contradicts standard Raft expectations.

### Solution:

The second step now applies an **empty change request**
(`AddVoterIds({})`) to the last configuration in the current joint
config. This ensures that the system always transitions to a uniform
configuration in the second step.

### Impact:

- No behavior changes occur if only one membership change request is in
  progress.

- If multiple requests are processed concurrently, the application must
  still verify the result, and the new behavior ensures the system
  transitions to a uniform state.

This fix prevents the system from being left in an unintended joint
configuration, improving consistency and adherence to Raft principles.

Thanks to @tvsfx for providing feedback on this issue and offering a
detailed explanation of the solution.

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