Allow to borrow a file descriptor without taking ownership

[cmeissl]

When operating on external provided file descriptors it would be useful if it was possible to create a Memfd that does not
take ownership of the file descriptor.

For example in a graphics pipeline the file descriptor is transported in a separate structure that has the ownership of the
file descriptor. Within the pipeline a filter wants to operate on the file descriptor using Memfd.

impl Filter for GraphicsPipeline {
  fn filter(&self, memory: &Memory) -> Result<(), Error> {
    let mfd = Memfd::try_borrow_from(memory.fd())?;
   ...
  }
}

[lucab]

Thanks for the suggestion. There are indeed a number of improvements that could be done around FD-handling. I'm personally waiting for the Rust io-safety work to stabilize first. Then the feature that you are looking for can be directly built with BorrowedFd. See https://rust-lang.github.io/rfcs/3128-io-safety.html#ownedfd-and-borrowedfdfd.

[sdroege]

For context, this discussion appeared at https://gitlab.freedesktop.org/gstreamer/gstreamer-rs/-/merge_requests/969#note_1294899

[lucab]

/cc @sunfishcode in case they may be interested in this FD-using gstreamer example.