Do note that is_readable/is_writable evaluates permissions in an attempt to produce the result.

This WILL fail in certain situations, while the file is actually accessible to the user, but manual evaluation fails to connect the dots.

The only trusted way to detect if a file is readable is to actually open it for reading. The only trusted way to detect if a file is writable is to actually open it for writing. And catch the error in case of failure.