Comment 2 for bug 1451558

Reviewed: https://review.openstack.org/179955
Committed: https://git.openstack.org/cgit/openstack/neutron/commit/?id=3682e3391f188845d0c7f382f0ccd4b38db3904e
Submitter: Jenkins
Branch: master

commit 3682e3391f188845d0c7f382f0ccd4b38db3904e
Author: Cedric Brandily <email address hidden>
Date: Mon May 4 23:36:19 2015 +0200

    Ensure non-overlapping cidrs in subnetpools without galera

    _get_allocated_cidrs[1] locks only allocated subnets in a subnetpool
    (with mysql/postgresql at least). It ensures we don't allocate a cidr
    overlapping with existent cidrs but nothing disallows a concurrent
    subnet allocation to create a subnet in the same subnetpool.

    This change replaces the lock on subnetpool subnets by a lock on the
    subnetpool itself. It disallows to allocate concurrently 2 subnets in
    the same subnetpool and ensure non-overlapping cidrs in the same
    subnetpool.

    Moreover this change solves a trouble with postgresql which disallows
    to lock an empty select with an outer join: it happens on first subnet
    allocation in a subnetpool when no specific cidr is provided. Moving
    the lock ensures the lock is done on a non-empty select.

    But this change does not ensure non-overlapping cidrs in subnetpools
    with galera because galera doesn't support SELECT FOR UPDATE locks. A
    follow-up change will (try to?) remove locks from subnet allocation[1]
    in order to ensure non-overlapping cidrs in subnetpools also with galera.

    [1] in neutron.ipam.subnet_alloc.SubnetAllocator

    Closes-Bug: #1451558
    Partial-Bug: #1451576
    Change-Id: I73854f9863f44621ae0d89c5dc4893ccc16d07e4