#vulnerable code 
                                void block(final long startTimeMs, @SuppressWarnings("unused") final int errorCode) {
        // TODO: Currently we don't care about error_code which indicates why the
        // very RPC fails. To make it better there should be different timeout for
        // each individual error (e.g. we don't need check every
        // heartbeat_timeout_ms whether a dead follower has come back), but it's just
        // fine now.
        final long dueTime = startTimeMs + this.options.getDynamicHeartBeatTimeoutMs();
        try {
            LOG.debug("Blocking {} for {} ms", this.options.getPeerId(), this.options.getDynamicHeartBeatTimeoutMs());
            this.blockTimer = this.timerManager.schedule(() -> onBlockTimeout(this.id), dueTime - Utils.nowMs(),
                TimeUnit.MILLISECONDS);
            this.statInfo.runningState = RunningState.BLOCKING;
            this.id.unlock();
        } catch (final Exception e) {
            LOG.error("Fail to add timer", e);
            // id unlock in sendEmptyEntries.
            sendEmptyEntries(false);
        }
    } 
                            #location 13 
                            #vulnerability type THREAD_SAFETY_VIOLATION
#fixed code
                                long getNextSendIndex() {
        // Fast path
        if (this.inflights.isEmpty()) {
            return this.nextIndex;
        }
        // Too many in-flight requests.
        if (this.inflights.size() > this.raftOptions.getMaxReplicatorInflightMsgs()) {
            return -1L;
        }
        // Last request should be a AppendEntries request and has some entries.
        if (this.rpcInFly != null && this.rpcInFly.isSendingLogEntries()) {
            return this.rpcInFly.startIndex + this.rpcInFly.count;
        }
        return -1L;
    }