Since go 1.9, os.Files are backed by a mechanism similar to netpoll, whenever possible. This means that you can set read and write deadlines on them, and that you can safely Close() a file from another goroutine while Read() and / or Write() calls are active.

For working with device nodes (/dev/XXX files, like serial ports /dev/ttySx, but not only) this is very useful.

Sadly it turns out one cannot use this new feature, for a trivial and equally frustrating reason:

Occasionally one needs to access the underlying file-descriptor of a device in order to do a trivial ioctl() or something similar. But calling File.Fd(), sets the underlying filedes to blocking mode and disables (forever) all the cool poller support (deadlines, etc).

One cannot turn the filedes to nonblocking mode himself, because he would also need to set internal File / pfd attributes which he has no access to.

Alternatively, one cannot start with a filedes (i.e. use syscall.Open) and convert it to a file, using NewFile() since Files created with NewFile are not considered pollable.

So please: Provide a way to get a File's underlying filedes without setting it to non-blocking mode, and (most importantly) without loosing all the poller goodies. A new method like File.RawFd() would be perfectly adequate and trivial to implement.

I believe my case (accessing pollable device nodes) is exactly one of those for which the poller support was added to File's. So I should be able to use it.