Network won't be synced when scale a new network node

[Yang Li]

If we have some networks(with dhcp enabled subnets) created, when we want to scale a new network nodes, and the dhcp-agent hasn't been started before in that node. If we start dhcp-agent, sometimes the network resource won't be synced in this nodes, no network related ns and dnsmasq
process created. The root cause is the function _report_state and sync_state are almost executed at the same time, if _report_state executed first, everything is fine, but if sync_state executed first, no network resource will be allocated.

L3-agent will do report_state before resource sync, so there is no problem with router resources.

[Lajos Katona (lajos-katona)]

Am I understand right that this is a scaling operation, when you add a new host where you start dhcp-agent?

[Yang Li (yang-li)]

Yes, this bug only affect on the first start of neutron-dhcp-agent for the new network node, the second start will sync all the network data on the node because the first start has already registered this agent to neutron-server.

[Lajos Katona (lajos-katona)]

I reproduced it with multihost devstack and even if I use dhcp_agents_per_network = 2, if I delay _report_state the namespaces will not be created.

[OpenStack Infra (hudson-openstack)]

Change abandoned by "liuyulong <email address hidden>" on branch: master
Review: https://review.opendev.org/c/openstack/neutron/+/657214
Reason: Restore if needed