feat: Supports user-defined playback points

[panlei-coder]

让braft支持自定义设置节点重启和正常安装快照时的日志回放点。（pikiwidb层的代码修改见pr：https://github.com/OpenAtomFoundation/pikiwidb/pull/319）
（1）pikiwidb可以在重载的on_snapshot_load接口中根据当前是否是节点重启来设置日志的回放点，因此需要提交设置回放点的接口set_self_playback_point，并在后续的FSMCaller::do_snapshot_load接口中判断之前自定义设置的回放点是否比快照中保存的snapshot_index新，如果没有snapshot_index新，则以snapshot_index作为回放点。

void FSMCaller::do_snapshot_load(LoadSnapshotClosure* done) {
    SnapshotMeta meta;
    int ret = reader->load_meta(&meta);
    // ... 
    ret = _fsm->on_snapshot_load(reader);
    // ...

    if (meta.old_peers_size() == 0) {
        // Joint stage is not supposed to be noticeable by end users.
        Configuration conf;
        for (int i = 0; i < meta.peers_size(); ++i) {
            conf.add_peer(meta.peers(i));
        }
        _fsm->on_configuration_committed(conf, meta.last_included_index());
    }

    **// If the log playback point is not customized or is configured but smaller than that
    // in the snapshot(Consider booting when the node is normally shut down), 
    // the log playback point in the snapshot prevails.
    if (last_applied_index() < meta.last_included_index()) {
        _last_applied_index.store(meta.last_included_index(),
                              butil::memory_order_release);
        _last_applied_term = meta.last_included_term();
    }**
    
    done->Run();
}

（2）在生成快照时考虑直接将快照中的last_applied_index和last_applied_term设置为min(all column family flushed index)(这里在那时假设时以一个RocksDB一个Raft)，需要提供根据log_index来获取对应term的接口。

[AlexStocks]

@CodiumAI-Agent /review

[CodiumAI-Agent]

PR Review 🔍

⏱️ Estimated effort to review [1-5]	3, because the PR involves changes across multiple files and functions, impacting core functionality related to log playback points in a distributed system. Understanding the implications of these changes requires a good grasp of the system's snapshot and log management mechanisms.
🧪 Relevant tests	No
⚡ Possible issues	Possible Bug: The method set_self_playback_point in NodeImpl does not handle the case where self_playback_point is less than or equal to _fsm_caller->last_applied_index() or greater than _log_manager->last_log_index(). This could potentially lead to setting an invalid playback point without any feedback or error.
🔒 Security concerns	No

Code feedback:

---

relevant file	src/braft/node.cpp
suggestion	Consider adding error handling or feedback mechanism in NodeImpl::set_self_playback_point when the playback point is not set due to invalid conditions. This could be crucial for debugging and ensuring system stability. [important]
relevant line	if (self_playback_point <= _fsm_caller->last_applied_index() ||

---

relevant file	src/braft/fsm_caller.cpp
suggestion	Ensure thread safety for _last_applied_term updates in FSMCaller::set_self_playback_point. Since _last_applied_term is potentially accessed from multiple threads, consider using atomic operations or locks to prevent race conditions. [important]
relevant line	_last_applied_term = _log_manager->get_term(self_playback_point);

---

relevant file	src/braft/fsm_caller.cpp
suggestion	Optimize the memory order used in _last_applied_index.store within FSMCaller::set_self_playback_point. Using butil::memory_order_relaxed might lead to issues in a multi-threaded environment where the order of writes is crucial. Consider using a stronger ordering like butil::memory_order_release. [medium]
relevant line	_last_applied_index.store(self_playback_point,

---

relevant file	src/braft/node.cpp
suggestion	Add logging in NodeImpl::set_self_playback_point to provide visibility when playback points are set or when conditions prevent setting them. This can aid in monitoring and troubleshooting the system's behavior in production. [medium]
relevant line	if (self_playplayback_point <= _fsm_caller->last_applied_index() ||

---

[AlexStocks]

@CodiumAI-Agent /improve

[CodiumAI-Agent]

PR Code Suggestions ✨

Category	Suggestion	Score
Possible bug	Add validation for non-negative playback points Consider adding a check for self_playback_point being non-negative in set_self_playplay_point method to ensure that the playback point is valid. This is crucial as negative values could lead to undefined behavior or errors in log term retrieval. src/braft/fsm_caller.cpp [552-556] void FSMCaller::set_self_playback_point(int64_t self_playback_point) { + if (self_playback_point < 0) { + return; // Optionally handle error or log this situation + } _last_applied_index.store(self_playback_point, butil::memory_order_relaxed); _last_applied_term = _log_manager->get_term(self_playback_point); } Suggestion importance[1-10]: 9 Why: Adding a check for non-negative values is crucial for preventing undefined behavior and potential errors. This suggestion addresses a possible bug and improves the robustness of the code.	9
Enhancement	Add error logging for invalid playback points Implement error handling or logging when the self_playback_point is out of the valid range in set_self_playback_point. This will help in debugging and ensure that the system behaves predictably under erroneous conditions. src/braft/node.cpp [970-976] void NodeImpl::set_self_playback_point(int64_t self_playback_point) { if (self_playback_point <= _fsm_caller->last_applied_index() || self_playback_point > _log_manager->last_log_index()) { + LOG(ERROR) << "Invalid self_playback_point: " << self_playback_point; return; } _fsm_caller->set_self_playback_point(self_playback_point); } Suggestion importance[1-10]: 8 Why: Adding error logging for invalid playback points enhances debugging and ensures predictable system behavior. This is a valuable enhancement for maintainability and reliability.	8
Best practice	Strengthen memory order for critical index updates Use a stronger memory order for storing _last_applied_index in set_self_playback_point to ensure visibility of changes across threads, considering the critical nature of log indexing. src/braft/fsm_caller.cpp [553] _last_applied_index.store(self_playback_point, - butil::memory_order_relaxed); + butil::memory_order_release); Suggestion importance[1-10]: 7 Why: Strengthening the memory order to memory_order_release is a best practice for ensuring visibility of changes across threads, especially for critical updates like log indexing. This improves the reliability of the code.	7
Maintainability	Refactor conditional logic for clarity Refactor the conditional logic in do_snapshot_load to improve readability and maintainability. Extracting conditions into a well-named boolean variable can clarify the intent. src/braft/fsm_caller.cpp [437-440] -if (last_applied_index() < meta.last_included_index()) { +bool shouldUpdateIndex = last_applied_index() < meta.last_included_index(); +if (shouldUpdateIndex) { _last_applied_index.store(meta.last_included_index(), butil::memory_order_release); _last_applied_term = meta.last_included_term(); } Suggestion importance[1-10]: 6 Why: Refactoring the conditional logic improves code readability and maintainability. While this is a good practice, it is not as critical as the other suggestions.	6