Steps to reproduce:

1. Upgraded zabbix agent from 6.0 to 6.20 bate
2. I used the command "for i in {1..100};do sleep 60;ss -p state close-wait | grep "zabbix" | wc -l >> /tmp/a.txt;echo "    ";date >> /tmp/a.txt;done" to generate a text file.

Result:

1、Active checking becomes unknown

2、The "Cannot create data pipe: [24] Too many open files" error message appears

3、There are too many tcp connections that are in close-wait.

4、The number only decreased after I restarted agent service.But if I restart the server service it doesn't work,the agent is still connected to the old server 
See screenshot...
Expected:
The tcp connection is disconnected after the zabbix agent finishes sending the data.