private boolean transferLeadership(final long logIndex) {
        if (this.hasSucceeded && this.nextIndex > logIndex) {
            // _id is unlock in _send_timeout_now
            sendTimeoutNow(true, false);
            return true;
        }
        // Register log_index so that _on_rpc_return trigger
        // _send_timeout_now if _next_index reaches log_index
        this.timeoutNowIndex = logIndex;
        unlockId();
        return true;
    }
Send log entries to follower, returns true when success, otherwise false and unlock the id.

@param nextSendingIndex next sending index
@return send result.
jraft-core/src/main/java/com/alipay/sofa/jraft/core/Replicator.java