Comment 19 for bug 1722126

Reviewed: https://review.opendev.org/c/openstack/neutron/+/762874
Committed: https://opendev.org/openstack/neutron/commit/6bc1c00d66ced82dd142739790222e8408684696
Submitter: "Zuul (22348)"
Branch: master

commit 6bc1c00d66ced82dd142739790222e8408684696
Author: Gaudenz Steinlin <email address hidden>
Date: Mon Nov 16 17:41:18 2020 +0100

    Copy existing IPv6 leases to generated lease file

    Because the DHCP agent does not know the IAID (identity association
    identifier) of assigned IPv6 addresses it's not possible to generate the
    lease file including IPv6 leases. Because of this IPv6 addresses are
    excluded when generating the lease file in case of DHCP agent restarts.
    This causes DHCPv6 clients to fail to RENEW their lease and to go
    through a full address discovery cycle with possible short connectivity
    disruption.

    This commit copies the existing IPv6 leaes from an already existing
    lease file if present. While this does not allow for DHCP agent
    failover, this is still better than just skipping the IPv6 addresses.

    A lease file without the IPv6 addresses is still generated if an agent
    is migrated to a different host.

    This commit complements the fix implemented in
    Ib1b2f284ab81f1c4af7b08b5257b45a3f6e79c3e which just skips the IPv6
    leases as otherwise the lease file would be invalid and all leases would
    be lost. It does not change the behavior for still valid IPv4 leases.

    With this issue fixed an additional fix is required to not loose DHCPv6
    leases when the agent restarts dnsmasq. Currently the DHCP agent
    regenerates all configuration files on restart. This means that DHCPv6
    leases are lost as they can't be regenerated. This changes the agent to
    only delete the config files if the agent's ports are also removed.

    Closes-Bug: #1722126
    Related-Change: Ib1b2f284ab81f1c4af7b08b5257b45a3f6e79c3e

    Change-Id: I40761b30563749251b9d74731bbe7a80a124da89