EC GET with unavailable .durable will 404

[clayg]

Until recently if there were less than ndata durables we would refuse to serve a GET for an object.

Unfortunately we discovered it's far too common that io contention under load can cause fragments to not get durables [1] and we decided reduce the requirement to a *single* durable [2].

Why?

If less than ndata durable exists it's totally possible they might be briefly inaccessible on a GET that could otherwise still rebuild the object. I think choosing to not service the GET is in contrast to our one of our primary design goals of high availability.

Probetest attached

1. https://bugs.launchpad.net/swift/+bug/1469094

2. https://review.openstack.org/#/c/215276/43

[Tim Burke (1-tim-z)]

Updated the probetest since https://github.com/openstack/swift/commit/116462d landed.