Fix to potential pre-commit order inversion

include/libnuraft/raft_server.hxx

@@ -668,6 +668,8 @@ protected:
     void handle_reconnect_resp(resp_msg& resp);
     void handle_custom_notification_resp(resp_msg& resp);
 
+    bool try_update_precommit_index(ulong desired, const size_t MAX_ATTEMPTS = 10);
+
     void handle_ext_resp(ptr<resp_msg>& resp, ptr<rpc_exception>& err);
     void handle_ext_resp_err(rpc_exception& err);
     void handle_join_leave_rpc_err(msg_type t_msg, ptr<peer> p);

src/handle_append_entries.cxx

@@ -720,25 +720,10 @@ ptr<resp_msg> raft_server::handle_append_entries(req_msg& req)
     //   be greater than the last log index this server already has. We should
     //   always compare the target index with current precommit index, and take
     //   it only when it is greater than the previous one.
-    const size_t MAX_ATTEMPTS = 10;
-    size_t num_attempts = 0;
-    ulong prev_precommit_index = precommit_index_;
-    while ( prev_precommit_index < target_precommit_index &&
-            num_attempts < MAX_ATTEMPTS ) {
-        if ( precommit_index_.compare_exchange_strong( prev_precommit_index,
-                                                       target_precommit_index ) ) {
-            break;
-        }
-        // Otherwise: retry until `precommit_index_` is equal to or greater than
-        //            `target_precommit_index`.
-        num_attempts++;
-    }
-    if (num_attempts >= MAX_ATTEMPTS) {
+    bool pc_updated = try_update_precommit_index(target_precommit_index);
+    if (!pc_updated) {
         // If updating `precommit_index_` failed, we SHOULD NOT update
         // commit index as well.
-        p_er("updating precommit_index_ failed after %zu attempts, "
-             "last seen precommit_index_ %zu, target %zu",
-             num_attempts, prev_precommit_index, target_precommit_index);
     } else {
         commit( std::min( req.get_commit_idx(), target_precommit_index ) );
     }
@@ -773,6 +758,29 @@ ptr<resp_msg> raft_server::handle_append_entries(req_msg& req)
     return resp;
 }
 
+bool raft_server::try_update_precommit_index(ulong desired, const size_t MAX_ATTEMPTS) {
+    // If `MAX_ATTEMPTS == 0`, try forever.
+    size_t num_attempts = 0;
+    ulong prev_precommit_index = precommit_index_;
+    while ( prev_precommit_index < desired &&
+            num_attempts < MAX_ATTEMPTS ) {
+        if ( precommit_index_.compare_exchange_strong( prev_precommit_index,
+                                                       desired ) ) {
+            return true;
+        }
+        // Otherwise: retry until `precommit_index_` is equal to or greater than
+        //            `desired`.
+        num_attempts++;
+    }
+    if (precommit_index_ >= desired) {
+        return true;
+    }
+    p_er("updating precommit_index_ failed after %zu/%zu attempts, "
+         "last seen precommit_index_ %zu, target %zu",
+         num_attempts, MAX_ATTEMPTS, prev_precommit_index, desired);
+    return false;
+}
+
 void raft_server::handle_append_entries_resp(resp_msg& resp) {
     peer_itor it = peers_.find(resp.get_src());
     if (it == peers_.end()) {

src/handle_client_request.cxx

@@ -105,7 +105,7 @@ ptr<resp_msg> raft_server::handle_cli_req(req_msg& req) {
     if (num_entries) {
         log_store_->end_of_append_batch(last_idx - num_entries, num_entries);
     }
-    precommit_index_ = last_idx;
+    try_update_precommit_index(last_idx);
     resp_idx = log_store_->next_slot();
 
     // Finished appending logs and pre_commit of itself.

src/raft_server.cxx

@@ -1429,9 +1429,7 @@ ulong raft_server::store_log_entry(ptr<log_entry>& entry, ulong index) {
 
         if ( role_ == srv_role::leader ) {
             // Need to progress precommit index for config.
-            if (precommit_index_ < log_index) {
-                precommit_index_ = log_index;
-            }
+            try_update_precommit_index(log_index);
         }
     }