When mounting or otherwise opening an S3 prefix without a trailing
slash, rclone probes the path with a HEAD request to see whether it is
actually a file. Since v1.72.0 (#8975) any error other than "not
found" from that probe was fatal, so credentials scoped to a prefix -
which return 403 rather than 404 for the prefix key - could no longer
open the prefix at all.
6440052fbd s3: fix single file copying behavior with low permission
A 403 on the probe is ambiguous: it can mean either "this is the file
you named but you may not HEAD it" or "this is a prefix you may list
but not HEAD". When the HEAD is not permitted we now fall back to a
listing to disambiguate: if the path has children it is treated as a
directory, otherwise it is treated as a file.
Fixes#9582
2d6d0da37b