object-server returns 404 for POST to existing object

[Alistair Coles]

When a DiskFile is opened, its on-disk files are first listed and then
xattr metadata is read. If a listed file no longer exists when its
metadata is read then a DiskFileNotExist exception is raised which
would previously cause the object server to return a 404.

This behavior is appropriate when the missing file is a data file (it
could have been removed by a concurrent request) but not when it is a
meta file (it could have been replaced by another meta file, but the
object still exists). For example, during a period of many concurrent
POST requests to an object in a production system, a significant
number of the POSTs returned 404 despite the object never being
deleted.

[Alistair Coles (alistair-coles)]

https://review.opendev.org/c/openstack/swift/+/909674