Since we used to allow zero-byte last segments but now we don't, it
can be difficult to deal with some old SLO manifests.
Imagine you're writing some code to sync objects from Swift cluster A
to Swift cluster B. You start off with just a GET from A piped into a
PUT to B, and that works great until you hit a SLO manifest and B
won't accept a 500GB object. So, you write some code to detect SLO
manifests, sync their segments, then take the JSON manifest
(?multipart-manifest=get) and sync *that* over.
Now, life is good... until one day you get an exception notification
that there's this manifest on cluster A that cluster B won't
accept. Turns out that, back when Swift would take zero-byte final
segments on SLOs (before commit 7f636a5), someone uploaded such a SLO
to cluster A. Now, however, zero-byte final segments are invalid, so
that SLO that exists over in cluster A can't just be copied to cluster
B.
A little coding later, your sync tool detects zero-byte final segments
and removes them when copying a manifest. But now your ETags don't
match between clusters, so you have to figure out some way to deal
with that, and so you put it in metadata, but then you realize that
your syncer might encounter a SLO which contains a sub-SLO which has a
zero-byte final segment, and it's right about then that you start
thinking about giving up on programming and getting a job as an
elevator mechanic.
This commit makes life easier for developers of such applications by
allowing SLOs to have zero-byte segments again.
Change-Id: Ia37880bbb435e269ec53b2963eb1b9121696d479
37f23b072e