Atomic object undo precondition

RustFS supports a destination-side version precondition for the two S3 operations used to undo changes in a versioned bucket:

x-rustfs-expected-current-version-id: <version-id>

This is a RustFS extension, not a standard S3 header.

Restore a historical object version

Send a same-object CopyObject request whose copy source includes the historical versionId, and set the extension header to the version that was current when the undo was planned. RustFS holds the destination namespace write lock while it compares the current version and creates the restored version.

The request fails without creating a version when the source is not a historical version, the source and destination differ, or the destination current version no longer matches.

Remove a delete marker

Send a version-specific DeleteObject request for the delete marker and set the extension header to the same version ID. RustFS removes it only when it is still the current version and is a delete marker. The namespace write lock covers the check and deletion.

Responses and retries

• A stale, missing, non-current, or non-delete-marker version returns HTTP 412 PreconditionFailed and does not mutate object history.
• An empty or malformed version header returns HTTP 400 InvalidArgument.
• A mismatched operation shape returns HTTP 400 InvalidRequest.
• A successful retry using an old expected version returns HTTP 412 because the first successful request changed the current version.

Object Lock retention, legal hold, authorization, replication, and ordinary S3 behavior remain unchanged. The extension only adds a precondition; it does not bypass existing validation.