lldpRemTable not exposed until snmp container is restarted.

[pierrecdn]

Description

lldpRemTable not exposed until snmp container is restarted.

Environment
SONiC Software Version: SONiC.criteo.0-ecb63b1 - HwSku: Arista-7060CX-32S-C32 - Distribution: Debian 9.6 - Kernel: 4.9.0-7-amd64

Steps to reproduce the issue:

1. compile using sonic-net/sonic-buildimage@465ebba (201807 as well).
2. try to snmpwalk and search for OIDs in 1.0.8802, should fail.
3. restart SNMP container: docker restart snmp
4. redo snmpwalk, should get results for OIDs in 1.0.8802

Describe the results you expected:

Getting lldpRemTable contents right after boot.

Explanation

I think there's an issue in poll_lldp_entry_updates in some way when the entry are not available from the start.

But I noticed that this container start is delayed due to https://github.com/Azure/sonic-buildimage/blob/d57bef55dbb584352c05e7d0bdb2cea215a537a9/files/build_templates/snmp.timer#L5, so it should definitely be ready.
Got these logs BEFORE the first restart which probably explain the behavior:

Nov 22 14:09:41.683104 sp01 WARNING snmp#snmp-subagent [sonic_ax_impl] WARNING: Invalid interface name in *' in APP_DB, skipping
Nov 22 14:09:48.879592 sp01 WARNING snmp#snmp-subagent [sonic_ax_impl] WARNING: Invalid interface name in TRANSCEIVER_INFO|*                      in STATE_DB, skipping

sonic-py-swsssdk seems involved here, will try to dig a bit more later.

[pierrecdn]

This is apparently due to switch interfaces being all down during startup.
Then daemon restart through docker restart snmp is mandatory to get exports working.

Maybe #64 would be a simple way to solve this?