gateway: restore original filename in ReadFile error message

All messages returned by os.Open are guaranteed to return a PathError.
However, as these error messages are printed, they include the temporary
directory for the mounted reference which is not useful to the caller.

On an error, we can restore the filename in the PathError to the
requested filename, as also seen in os.DirFS.

Signed-off-by: Justin Chadwell <[email protected]>

cache/util/fsutil.go

@@ -57,16 +57,22 @@ func ReadFile(ctx context.Context, mount snapshot.Mountable, req ReadRequest) ([
  return errors.WithStack(err)
  }
 
+ f, err := os.Open(fp)
+ if err != nil {
+ // The filename here is internal to the mount, so we can restore
+ // the request base path for error reporting.
+ // See os.DirFS.Open for details.
+ err.(*os.PathError).Path = req.Filename
+ return errors.WithStack(err)
+ }
+
  if req.Range == nil {
- dt, err = os.ReadFile(fp)
+ dt, err = io.ReadAll(f)
+ f.Close()
  if err != nil {
  return errors.WithStack(err)
  }
  } else {
- f, err := os.Open(fp)
- if err != nil {
- return errors.WithStack(err)
- }
  dt, err = io.ReadAll(io.NewSectionReader(f, int64(req.Range.Offset), int64(req.Range.Length)))
  f.Close()
  if err != nil {