Comment 41 for bug 1953165

Reviewed: https://review.opendev.org/c/openstack/neutron/+/880957
Committed: https://opendev.org/openstack/neutron/commit/1c615281f7632f3f1cf4bd37eefe90c50c6dfe25
Submitter: "Zuul (22348)"
Branch: stable/zed

commit 1c615281f7632f3f1cf4bd37eefe90c50c6dfe25
Author: Bence Romsics <email address hidden>
Date: Mon Mar 6 13:04:01 2023 +0100

    Suppress IPv6 metadata DAD failure and delete address

    IPv4 DAD is non-existent in Linux or its failure is silent, so we
    never needed to catch and ignore it. On the other hand IPv6 DAD
    failure is explicit, hence comes this change.

    This of course leaves the metadata service dead on hosts where
    duplicate address detection failed. But if we catch the
    DADFailed exception and delete the address, at least other
    functions of the dhcp-agent should not be affected.

    With this the IPv6 isolated metadata service is not redundant, which
    is the best we can do without a redesign.

    Also document the promised service level of isolated metadata.

    Added additional tests for the metadata driver as well.

    Conflicts:
        neutron/tests/unit/agent/linux/test_dhcp.py
            conflict with 74224e79e031636018b970fac9c2aa72516eb12d
        neutron/tests/unit/agent/metadata/test_driver.py
            conflict with 3d575f8bd066ce2eb46353a49a8c6850ba9e4387

    Change-Id: I6b544c5528cb22e5e8846fc47dfb8b05f70f975c
    Partial-Bug: #1953165
    (cherry picked from commit 2aee961ab6942ab59aeacdc93d918c8c19023041)
    (cherry picked from commit 071255f098e0e73fd5220f83cbbc8ac1c421f3ab)