rewriteObject operation metadata copy semantics differ from actual behaviour

[alexrjones]

In the implementation of the object rewrite operation, metadata provided in the request is merged(-ish) with metadata from the source object to produce the final metadata set.

fake-gcs-server/fakestorage/object.go

Lines 958 to 995 in 7503535

	// Only supplied metadata overwrites the new object's metadata
	if len(metadata.Metadata) == 0 {
	metadata.Metadata = obj.Metadata
	}
	if metadata.ContentType == "" {
	metadata.ContentType = obj.ContentType
	}
	if metadata.ContentEncoding == "" {
	metadata.ContentEncoding = obj.ContentEncoding
	}
	if metadata.ContentDisposition == "" {
	metadata.ContentDisposition = obj.ContentDisposition
	}
	if metadata.ContentLanguage == "" {
	metadata.ContentLanguage = obj.ContentLanguage
	}
	if metadata.CacheControl == "" {
	metadata.CacheControl = obj.CacheControl
	}
	dstBucket := vars["destinationBucket"]
	if _, err := s.backend.GetBucket(dstBucket); err != nil {
	return jsonResponse{status: http.StatusNotFound}
	}
	newObject := StreamingObject{
	ObjectAttrs: ObjectAttrs{
	BucketName: dstBucket,
	Name: vars["destinationObject"],
	ACL: obj.ACL,
	ContentType: metadata.ContentType,
	ContentEncoding: metadata.ContentEncoding,
	ContentDisposition: metadata.ContentDisposition,
	ContentLanguage: metadata.ContentLanguage,
	CacheControl: metadata.CacheControl,
	Metadata: metadata.Metadata,
	},
	Content: obj.Content,
	}

However, this differs from how the operation is described (and anecdotally, how I've seen it behave):

If the request body is empty, editable metadata from the source object is applied to the destination object [...]

The corollary to this statement is that if the request body is not empty, then the editable metadata is not copied - which is the behaviour I have observed with real GCS. So, setting any editable metadata fields should cause the request to be the sole source of the editable metadata fields (as defined here).