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Implement IntoRawFd for SharedFd and File #228

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Implement IntoRawFd for SharedFd and File #228

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11 changes: 11 additions & 0 deletions src/fs/file.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -855,6 +855,17 @@ impl File {
}
}

impl IntoRawFd for File {
// Consumes this object, returning the raw underlying file descriptor.
// Since all in-flight operations hold a reference to &self, the type-checker
// will ensure that there are no in-flight operations when this method is
// called.
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I don't understand this comment that says the in-flight operations hold a reference to &self. The in-flight operations I think you mean actually hold a cloned SharedFd, like in

tokio-uring/src/io/write.rs

Line 10 in afab7f2

fd: SharedFd,

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@thomasbarrett thomasbarrett Feb 6, 2023 •
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Ah yeah... when I was trying to wrap my head around SharedFd, I came to the following conclusion (please correct me If I am wrong): The SharedFd close method will hang forever if there is any in-flight IO. However, the actual File::close API does not appear to share this issue because the Rust borrow-checker already actively prevents in-flight asynchronous methods (or even just references to File) when transferring ownership of a tokio_uring::File (either by File::close or by File::into_raw_fd).

Example 1: OK

fn main() -> std::io::Result<()> {
    tokio_uring::start(async {
        let file = File::open("test.txt").await?;
        let fd = file.into_raw_fd();
        let file = unsafe { File::from_raw_fd(fd) };
        file.close().await?;

        Ok(())
    })
}

Example 2: Compilation Error

An async block that isn't awaited before calling into_raw_fd or close will result in an error.

fn main() -> std::io::Result<()> {
    tokio_uring::start(async {
        let file = File::open("test.txt").await?;

        tokio_uring::spawn(async {
            let buf: Vec<u8> = Vec::with_capacity(16);
            let (buf, res) = file.read_at(buf, 0).await;
        });

        let fd = file.into_raw_fd();
        let file = unsafe { File::from_raw_fd(fd) };
        file.close().await?;

        Ok(())
    })
}

Example 3: Compilation Error

cannot move out of an Rc

fn main() -> std::io::Result<()> {
    tokio_uring::start(async {
        let file = std::rc::Rc::new(File::open("test.txt").await?);
        let file2 = file.clone();
        tokio_uring::spawn(async move {
            let buf: Vec<u8> = Vec::with_capacity(16);
            let (buf, res) = file2.read_at(buf, 0).await;
        });

        let fd = file.into_raw_fd();
        let file = unsafe { File::from_raw_fd(fd) };
        file.close().await?;

        Ok(())
    })
}

Example 4: Ok

This compiles since we use try_unwrap (which ensures that there are no other references to File).

fn main() -> std::io::Result<()> {
    tokio_uring::start(async {
        let file = std::rc::Rc::new(File::open("test.txt").await?);
        let file2 = file.clone();
        tokio_uring::spawn(async move {
            let buf: Vec<u8> = Vec::with_capacity(16);
            let (buf, res) = file2.read_at(buf, 0).await;
        });

        match std::rc::Rc::try_unwrap(file) {
            Ok(file) => {
                let fd = file.into_raw_fd();
                let file = unsafe { File::from_raw_fd(fd) };
                file.close().await;
            },
            Err(err) => {
                panic!("unexpected io in-flight")
            }
        }

        Ok(())
    })
}

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@thomasbarrett thomasbarrett Feb 6, 2023 •
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Example 5: In-flight IO possible only with (incorrect usage of) unsafe code.

fn main() -> std::io::Result<()> {
    tokio_uring::start(async {
        let file = File::open("test.txt").await?;
        let fd = file.into_raw_fd();

        tokio_uring::spawn(async move {
            let file = unsafe { File::from_raw_fd(fd) };
            let buf: Vec<u8> = Vec::with_capacity(16);
            let (buf, res) = file.read_at(buf, 0).await;
        });

        let file = unsafe { File::from_raw_fd(fd) };
        file.close().await;

        Ok(())
    })
}

fn into_raw_fd(self) -> RawFd {
// This should never panic.
self.fd.into_raw_fd()
}
}

impl FromRawFd for File {
unsafe fn from_raw_fd(fd: RawFd) -> Self {
File::from_shared_fd(SharedFd::new(fd))
Expand Down
25 changes: 24 additions & 1 deletion src/io/shared_fd.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -2,7 +2,7 @@ use crate::io::Close;
use std::future::poll_fn;

use std::cell::RefCell;
use std::os::unix::io::{FromRawFd, RawFd};
use std::os::unix::io::{FromRawFd, IntoRawFd, RawFd};
use std::rc::Rc;
use std::task::Waker;

Expand Down Expand Up @@ -39,6 +39,26 @@ enum State {

/// The FD is fully closed
Closed,

// The FD is leaked. A SharedFd in this state will not be closed on drop.
// A SharedFd will only enter this state as a result of the `into_raw_fd`
// method.
Leaked,
}

impl IntoRawFd for SharedFd {
// Consumes this object, returning the raw underlying file descriptor.
// This method will panic if there are any in-flight operations.
fn into_raw_fd(mut self) -> RawFd {
// Change the SharedFd state to `Leaked` so that the file-descriptor is
// not closed on drop.
if let Some(inner) = Rc::get_mut(&mut self.inner) {
let state = RefCell::get_mut(&mut inner.state);
*state = State::Leaked;
return self.raw_fd();
}
panic!("unexpected operation in-flight")
}
}

impl SharedFd {
Expand Down Expand Up @@ -139,6 +159,9 @@ impl Inner {
Poll::Ready(())
}
State::Closed => Poll::Ready(()),
// By definition, a SharedFd in a Leaked state is not closed, so we should
// never encounter this case while waiting for a SharedFd to close.
State::Leaked => unreachable!(),
}
})
.await;
Expand Down