[VNET] "zebra" crashes after VNET removal

[volodymyrsamotiy]

Description

"zebra" process crash happens after removing VNET.
It is known FRR issue in 7.2.1 version which is currently in "201911" branch.
Issue is already fixed in FRR 7.2.1-s3 version which is in "master" branch and this version will be cherry-picked to "201911" branch as well.

Steps to reproduce the issue:

• Create 1 VNET

root@sonic:/home/admin# cat vnet.conf.json
{
    "VXLAN_TUNNEL": {
        "tunnel_v4": {
            "src_ip": "10.1.0.32"
        }
    },
    "VNET": {
        "Vnet1": {
            "vxlan_tunnel": "tunnel_v4",
            "vni": "10001",
            "peer_list": ""
        }
    }
}
root@sonic:/home/admin# config load vnet.conf.json

• Remove VNET

redis-cli -n 4 del "VNET|Vnet1"

• Observe "zebra" process crashed

root@sonic:/home/admin# pidof zebra

Describe the results you received:

"zebra" crashed after removing a VNET.

root@sonic:/home/admin# grep -B 7 "exited: zebra" /var/log/syslog
May 18 10:15:55.390252 sonic NOTICE swss#vrfmgrd: :- doTask: Removed vrf netdev Vnet1
...
May 18 10:15:56.754103 sonic INFO bgp#supervisord: fpmsyncd Connection lost, reconnecting...
May 18 10:15:56.754103 sonic INFO bgp#supervisord: fpmsyncd Waiting for fpm-client connection...
May 18 10:16:03.478007 sonic INFO bgp#supervisord 2020-05-18 10:15:56,753 INFO exited: zebra (terminated by SIGABRT (core dumped); not expected)

Describe the results you expected:

VNET removal should be working without any failures

Additional information you deem important (e.g. issue happens only occasionally):

Output of show version:

SONiC Software Version: SONiC.201911.94-e2e3dde3
Distribution: Debian 9.12
Kernel: 4.9.0-11-2-amd64
Build commit: e2e3dde3
Build date: Fri May 15 04:36:14 UTC 2020
Built by: johnar@jenkins-worker-8

Platform: x86_64-mlnx_msn3700c-r0
HwSKU: ACS-MSN3700C
ASIC: mellanox
Serial Number: MT1852X03894
Uptime: 11:32:53 up  2:20,  1 user,  load average: 1.03, 0.48, 0.46

[lguohan]

fixed in 2778363

[volodymyrsamotiy]

Issue is still observed on latest 201911 image:
https://sonic-jenkins.westus2.cloudapp.azure.com/job/mellanox/job/buildimage-mlnx-201911/100/

[volodymyrsamotiy]

Fixed by #3763