First of all, thanks for this library! The performance gains are very noticeable.

I found that when there is a panic during data fetch inside the handler, then the panic cannot be recovered even if there is a panic recovery middleware present in the middleware chain or if there is a recovery middleware wrapping the stampede cache handler itself.

I believe this is because the singleflight package here creates a goroutine and if a panic occurs in that goroutine, then it can never be recovered in the main goroutine.

The fix should be to recover the panic properly in the callback function within HandlerWithKey and set the error properly.