get_container_info 503s shouldn't try to clear memcache

[Tim Burke]

I had a container-server get overloaded, which caused a thundering herd problem when it fell out of cache. That deserves its own bug, but it had some weird side effects; in particular, my graph of memcache sets went *way up,* which seemed odd.

Tracing through get_container_info and set_info_cache, we can see why, though:

[0] we check cache, but it's a miss
[1] so we go to origin, but we get a 503
[2] we then check cache again, only that's still not populated
    so we make an info dict based on the response and see about caching it. Now
[3] we have a response in hand, and it's unsuccessful
[4] so we kill the key in memcache (even though we *just checked* and had a miss!)

That seems useless, but it's actually *worse* than useless -- if you've got a thousand or so clients all trying to get container info for that DB, some of them might actually have been able to get through -- and if they managed to set info in memcache between our second get and our kill, we just destroyed valuable information that would've helped the next guy.

[0] https://github.com/openstack/swift/blob/2.25.0/swift/proxy/controllers/base.py#L434
[1] https://github.com/openstack/swift/blob/2.25.0/swift/proxy/controllers/base.py#L439-L445
[2] https://github.com/openstack/swift/blob/2.25.0/swift/proxy/controllers/base.py#L461
[3] https://github.com/openstack/swift/blob/2.25.0/swift/proxy/controllers/base.py#L555-L559
[4] https://github.com/openstack/swift/blob/2.25.0/swift/proxy/controllers/base.py#L563-L567

[OpenStack Infra (hudson-openstack)]

Reviewed: https://review.opendev.org/735359
Committed: https://git.openstack.org/cgit/openstack/swift/commit/?id=7be7cc966b5a98a35cbbccfc426a4efb65583c12
Submitter: Zuul
Branch: master

commit 7be7cc966b5a98a35cbbccfc426a4efb65583c12
Author: Tim Burke <email address hidden>
Date: Fri Jun 12 08:35:06 2020 -0700

    proxy: Stop killing memcache entries on 5xx responses

    When you've got a container server that's struggling to respond to HEAD
    requests, it's really not helpful to keep evicting the cache when some
    of the hundreds of concurrent requests trying to repopulate the cache
    return 503.

    Change-Id: I49174a21a854a4e8e564a7bbf997e1841f9dda71
    Closes-Bug: #1883211