This change introduces the concept of a "versioning mode" for
versioned_writes. The following modes are supported:
* stack
When deleting, check whether any previous versions exist in the
versions container. If none is found, the object is deleted. If the
most-recent version in the versions container is not a delete
marker, it is copied into the versioned container (overwriting the
current version if one exists) and then deleted from the versions
container. This preserves the previous behavior.
If the most-recent version in the versions container is a delete
marker and a current version exists in the versioned container, the
current version is deleted. If the most-recent version in the
versions container is a delete marker and no current version exists
in the versioned container, we copy the next-most-recent version
from the versions container into the versioned container (assuming
it exists and is not a delete marker) and delete both the
most-recent version (i.e., the delete marker) and the just-copied
next-most-recent version from the versions container.
With this mode, DELETEs to versioned containers "undo" operations
on containers. Previously this was limited to undoing PUTs, but now
it will also undo DELETEs performed while in "history" mode.
* history
When deleting, check whether a current version exists in the
versioned container. If one is found, it is copied to the versions
container. Then an empty "delete marker" object is also put into the
versions container; this records when the object was deleted.
Finally, the original current version is deleted from the versioned
container. As a result, subsequent GETs or HEADs will return a 404,
and container listings for the versioned container do not include
the object.
With this mode, DELETEs to versioned containers behave like DELETEs
to other containers, but with a history of what has happened.
Clients may specify (via a new X-Versions-Mode header) which mode a
container should use. By default, the existing "stack" mode is used.
Upgrade consideration:
======================
Clients should not use the "history" mode until all proxies in the
cluster have been upgraded. Attempting to use the "history" mode during
a rolling upgrade may result in some requests being served by proxies
running old code (which necessarily uses the "stack" mode), leading to
data loss.
Change-Id: I555dc17fefd0aa9ade681aa156da24e018ebe74b
c7283be4fe