Before this change, operations.CopyDirMetadata would fail with: `internal error:
expecting directory string from combine root '' to have SetMetadata method:
optional feature not implemented` if the dst was the root directory of a combine
upstream. This is because combine was returning a *fs.Dir, which does not
satisfy the fs.SetMetadataer interface.
While it is true that combine cannot set metadata on the root of an upstream
(see also #7652), this should not be considered an error that causes sync to do
high-level retries, abort without doing deletes, etc.
This change addresses the issue by creating a new type of DirWrapper that is
allowed to fail silently, for exceptional cases such as this where certain
special directories have more limited abilities than what the Fs usually
supports.
It is possible that other similar wrapping backends (Union?) may need this same
fix.
Set this automatically for any backend which implements UnWrap and
manually for combine and union which can't implement UnWrap but do
overlay other backends.
Before this patch backends could be shutdown when they fell out of the
cache when they were in use with combine. This was particularly
noticeable with the dropbox backend which gave this error when
uploading files after the backend was Shutdown.
Failed to copy: upload failed: batcher is shutting down
This patch gets the combine remote to pin them until it is finished.
See: https://forum.rclone.org/t/rclone-combine-upload-failed-batcher-is-shutting-down/32168
