A conditional delete of a key with no current version used to succeed. `DeleteObject` with `If-Match`, and a `DeleteObjects` key with an `ETag`, reported success for a missing key. When the current version was a delete marker, the condition was checked against the ETag the marker kept from the version it hides, so it could pass and create a second marker. S3 fails both with `NoSuchKey` and creates no marker. The posix backend now checks the conditions against the current version in `checkDeletePreconditions`, before its missing-key shortcuts. The azure backend returns `NoSuchKey` for a missing blob instead of skipping the conditions. `ObjectDeletePreconditions.IsSet` reports whether a delete has any condition.
A condition only applies to the current version. `DeleteObject` with a `versionId` and `If-Match` now returns `NotImplemented` with `<Header>If-Match</Header>` and the S3 additional message, ahead of authorization. Before, it checked the condition against that version and deleted the version. In `DeleteObjects`, an empty `<ETag>` fails its key with `InvalidArgument` and a `VersionId` together with an `ETag` fails it with `NotImplemented`, both ahead of the key's authorization.
The `PreconditionFailed` and `NotImplemented` messages drop their trailing period, which S3 doesn't use on any operation.
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