avoid appending directory as file path in s3 driver Walk

when a directory is empty, the s3 api lists it with a trailing slash.
this causes the path to be appended twice to the walkInfo slice, causing
purge uploads path transformations to panic when the `_uploads` is
emtpy.

this adds a check for file paths ending on slash, and do not append
those as regular files to the walkInfo slice.

fixes distribution#4358

Signed-off-by: Flavian Missi <fmissi@redhat.com>

registry/storage/driver/s3-aws/s3.go

@@ -1178,6 +1178,16 @@ func (d *driver) doWalk(parentCtx context.Context, objectCount *int64, from stri
 				prevDir = dir
 			}
 
+			// in some cases the _uploads dir might be empty. when this happens, it would
+			// be appended twice to the walkInfos slice, once as [...]/_uploads and
+			// once more erroneously as [...]/_uploads/. the easiest way to avoid this is
+			// to skip appending filePath to walkInfos if it ends in "/". the loop through
+			// dirs will already have handled it in that case, so it's safe to continue this
+			// loop.
+			if strings.HasSuffix(filePath, "/") {
+				continue
+			}
+
 			walkInfos = append(walkInfos, storagedriver.FileInfoInternal{
 				FileInfoFields: storagedriver.FileInfoFields{
 					IsDir:   false,
@@ -1541,6 +1551,7 @@ func (w *writer) Write(p []byte) (int, error) {
 func (w *writer) Size() int64 {
 	return w.size
 }
+
 func (w *writer) Close() error {
 	if w.closed {
 		return fmt.Errorf("already closed")