panic when import a large number of files

[IanSmith123]

question mentioned at https://discuss.nebula-graph.com.cn/t/topic/1579/21?u=les1ie

If I try to import a larger number of files, eg: 1000 csv files, with a yaml file more than 10000 lines, nebula_importer will panic error

How to reproduce:

1. ganerate sample csv files

from pathlib import Path
import os

dump_dir = Path('./dump')
if not os.path.exists(dump_dir):
    os.mkdir(dump_dir)


def generate_csv():
    num = 10000
    for i in range(num):
        with open(f'{dump_dir}/vertex_{i}.csv', 'w') as f:
            f.write("123\n")


generate_csv()

2. download error config.yaml
   out.zip

3. import csv

python3 reproduce.py
cp path_to_nebula_impoerter_exec dump/
cd dump
./nebula_importer -c out.yaml

4. screenshot