Before this change a streamed multipart upload wrote its parts
straight to the object's final path on the underlying remote. That
meant an in-progress upload overwrote any object already stored under
that name, and aborting or failing the upload destroyed it. The
opposite of the S3 guarantee that an object only changes on a
successful CompleteMultipartUpload.
Remotes that upload atomically already (PartialUploads is false, e.g.
object stores) are safe to stream straight to the destination, so they
still do. Remotes where a partial upload is visible (PartialUploads is
true, e.g. local) now stream the parts to a temporary object instead
and move it, server-side, into its final place only when the upload
completes. A failed or aborted upload then just removes the temporary
object and leaves any pre-existing object untouched.
The temporary-object path needs the remote to support a server-side
move or copy in addition to PutStream uploads fall back to being
buffered in memory as before. The temporary objects are named with a
leading ".rclone_multipart_upload_" and hidden from listings.
Nick Craig-Wood
060b10c8cc