I looked at `OS.File` earlier and I couldn't find a way to read from a file descriptor (or file handle for Windows) directly. There is a way to access the underlying file descriptor, but not to read from any file descriptor, afaik.

These are backed by POSIX read/write functions on the *nix machines, but on Windows they have crazy hacks to expose an interface that is exactly like POSIX read/write, but backed by the file handle type.