fix(storage): bounds-check the file basename in PurgeUploads Walk callback (#4860)

registry/storage/purgeuploads.go

@@ -71,7 +71,12 @@ func getOutstandingUploads(ctx context.Context, driver storageDriver.StorageDriv
 	err = driver.Walk(ctx, root, func(fileInfo storageDriver.FileInfo) error {
 		filePath := fileInfo.Path()
 		_, file := path.Split(filePath)
-		if file[0] == '_' {
+		// path.Split returns file == "" for paths that end in "/" (a
+		// bare directory such as the bucket root when S3 reports an
+		// empty Key / common prefix). Indexing file[0] then panics
+		// with 'index out of range [0] with length 0' and takes the
+		// whole PurgeUploads goroutine down (#4713).
+		if len(file) > 0 && file[0] == '_' {
 			// Reserved directory
 			inUploadDir = (file == "_uploads")