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[Openstack-security] [Bug 1819957] Fix proposed to oslo.cache (stable/queens)

Fix proposed to branch: stable/queens

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  Caching with stale data when a server disconnects due to network
  partition and reconnects

Status in OpenStack Identity (keystone):
Status in keystonemiddleware:
Status in oslo.cache:
  Fix Released
Status in OpenStack Security Advisory:
  Won't Fix

Bug description:
  The flush_on_reconnect optional flag is not used. This can cause stale
  data to be utilized from a cache server that disconnected due to a
  network partition. This has security concerns as follows:

  1*  Password changes/user changes may be reverted for the cache TTL
  1a* User may get locked out if PCI-DSS is on and the password change happens during the network
  2*  Grant changes may be reverted for the cache TTL
  3*  Resources (all types) may become "undeleted" for the cache TTL
  4*  Tokens (KSM) may become valid again during the cache TTL

  As noted in the python-memcached library:

      @param flush_on_reconnect: optional flag which prevents a
              scenario that can cause stale data to be read: If there's more
              than one memcached server and the connection to one is
              interrupted, keys that mapped to that server will get
              reassigned to another. If the first server comes back, those
              keys will map to it again. If it still has its data, get()s
              can read stale data that was overwritten on another
              server. This flag is off by default for backwards

  The solution is to explicitly pass flush_on_reconnect as an optional
  argument. A concern with this model is that the memcached servers may
  be utilized by other tooling and may lose cache state (in the case the
  oslo.cache connection is the only thing affected by the network

  This similarly needs to be addressed in pymemcache when it is utilized
  in lieu of python-memcached.

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