Fix filepath.Clean for file prefixed URLs

multistep/commonsteps/step_download.go

@@ -221,11 +221,16 @@ func (s *StepDownload) download(ctx context.Context, ui packersdk.Ui, source str
 	}
 	src := u.String()
 	if u.Scheme == "" || strings.ToLower(u.Scheme) == "file" {
-		// If a local filepath, then we need to preprocess to make sure the
-		// path doens't have any multiple successive path separators; if it
+		// If a local filepath, then we need to pre-process to make sure the
+		// path doesn't have any multiple successive path separators; if it
 		// does, go-getter will read this as a specialized go-getter-specific
 		// subdirectory command, which it most likely isn't.
 		src = filepath.Clean(u.String())
+		// Starting with Go 1.21.0 filepath.Clean on Windows treats paths containing file: as invalid
+		// so they are prefixed with a dot (.) followed by the os.PathSeparator
+		if runtime.GOOS == "windows" && (src[0] == '.' && os.IsPathSeparator(src[1])) {
+			src = src[2:]
+		}
 		if _, err := os.Stat(filepath.Clean(u.Path)); err != nil {
 			// Cleaned path isn't present on system so it must be some other
 			// scheme. Don't error right away; see if go-getter can figure it