Using memmap2 with RawFd

[mripard]

Even though files are the most obvious target for an mmap, we might want to call mmap on a file descriptor that isn't backed by a File. Buffers on Linux can be allocated that way for example.

The current API that takes a File doesn't really allow to use the crate in such a case.

Something that would work would be

let buffer_fd = allocate_buffer(whatever);
let file = unsafe { File::from_raw_fd(buffer_fd) };
let mut mmap = unsafe {
    MmapOptions::new().map_mut(&file)?
};

However, FrowRawFd consumes the ownership of the FD, therefore making it fairly hard to integrate into a larger buffer management codebase.

We could switch from File to AsRawFd the API, but I'm not sure whether that change in the API would be welcome?

[RazrFalcon]

Well, we have to support Windows too and AsRawFd is Unix-only. I guess we have to provide some kind of wrapper that would accept File, RawFd and RawHandle. Basically:

pub unsafe fn map<T: Into<OurWrapper>>(&self, src: T) -> Result<Mmap> {
    // ...
}

Patches are welcome.