cannot reuse tcp connection for the external storage

[Leavrth]

Bug Report

Please answer these questions before submitting your issue. Thanks!

1. Minimal reproduce step (Required)

1. run br truncate

2. What did you expect to see? (Required)

netstat | grep "TIME_WAIT"
0

3. What did you see instead (Required)

netstat | grep "TIME_WAIT"
11111

4. What is your TiDB version? (Required)

master