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io: implement [Read|Write]Volatile for &File and co #321

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1e36e8a
io: implement [Read|Write]Volatile for &File and co
roypat Mar 26, 2025
ac9bf67
chore: update coverage
roypat Apr 16, 2025
ce0c619
Merge branch 'main' into read-write-by-ref
roypat May 22, 2025
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68 changes: 59 additions & 9 deletions src/io.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -12,7 +12,7 @@ use std::io::{Cursor, ErrorKind};
use std::io::Stdout;

#[cfg(feature = "rawfd")]
use std::os::fd::AsRawFd;
use std::os::fd::{AsRawFd, RawFd};

macro_rules! retry_eintr {
($io_call: expr) => {
Expand Down Expand Up @@ -127,7 +127,7 @@ pub trait WriteVolatile {
// We explicitly implement our traits for [`std::fs::File`] and [`std::os::unix::net::UnixStream`]
// instead of providing blanket implementation for [`AsRawFd`] due to trait coherence limitations: A
// blanket implementation would prevent us from providing implementations for `&mut [u8]` below, as
// "an upstream crate could implement AsRawFd for &mut [u8]`.
// "an upstream crate could implement AsRawFd for &mut [u8]".

macro_rules! impl_read_write_volatile_for_raw_fd {
($raw_fd_ty:ty) => {
Expand All @@ -137,7 +137,27 @@ macro_rules! impl_read_write_volatile_for_raw_fd {
&mut self,
buf: &mut VolatileSlice<B>,
) -> Result<usize, VolatileMemoryError> {
read_volatile_raw_fd(self, buf)
read_volatile_raw_fd(self.as_raw_fd(), buf)
}
}

#[cfg(feature = "rawfd")]
impl ReadVolatile for &$raw_fd_ty {
fn read_volatile<B: BitmapSlice>(
&mut self,
buf: &mut VolatileSlice<B>,
) -> Result<usize, VolatileMemoryError> {
read_volatile_raw_fd(self.as_raw_fd(), buf)
}
}

#[cfg(feature = "rawfd")]
impl ReadVolatile for &mut $raw_fd_ty {
fn read_volatile<B: BitmapSlice>(
&mut self,
buf: &mut VolatileSlice<B>,
) -> Result<usize, VolatileMemoryError> {
read_volatile_raw_fd(self.as_raw_fd(), buf)
}
}

Expand All @@ -147,7 +167,27 @@ macro_rules! impl_read_write_volatile_for_raw_fd {
&mut self,
buf: &VolatileSlice<B>,
) -> Result<usize, VolatileMemoryError> {
write_volatile_raw_fd(self, buf)
write_volatile_raw_fd(self.as_raw_fd(), buf)
}
}

#[cfg(feature = "rawfd")]
impl WriteVolatile for &$raw_fd_ty {
fn write_volatile<B: BitmapSlice>(
&mut self,
buf: &VolatileSlice<B>,
) -> Result<usize, VolatileMemoryError> {
write_volatile_raw_fd(self.as_raw_fd(), buf)
}
}

#[cfg(feature = "rawfd")]
impl WriteVolatile for &mut $raw_fd_ty {
fn write_volatile<B: BitmapSlice>(
&mut self,
buf: &VolatileSlice<B>,
) -> Result<usize, VolatileMemoryError> {
write_volatile_raw_fd(self.as_raw_fd(), buf)
}
}
};
Expand All @@ -159,7 +199,17 @@ impl WriteVolatile for Stdout {
&mut self,
buf: &VolatileSlice<B>,
) -> Result<usize, VolatileMemoryError> {
write_volatile_raw_fd(self, buf)
write_volatile_raw_fd(self.as_raw_fd(), buf)
}
}

#[cfg(feature = "rawfd")]
impl WriteVolatile for &Stdout {
fn write_volatile<B: BitmapSlice>(
&mut self,
buf: &VolatileSlice<B>,
) -> Result<usize, VolatileMemoryError> {
write_volatile_raw_fd(self.as_raw_fd(), buf)
}
}

Expand All @@ -174,8 +224,8 @@ impl_read_write_volatile_for_raw_fd!(std::os::fd::BorrowedFd<'_>);
///
/// Returns the numbers of bytes read.
#[cfg(feature = "rawfd")]
fn read_volatile_raw_fd<Fd: AsRawFd>(
raw_fd: &mut Fd,
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@bonzini bonzini Mar 26, 2025 •
edited
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I think &mut is more appropriate since you change the seek position into the file, or consume data from a pipe/socket.

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Right, and I think that's why originally I didn't do this. But today I ended up checking the standard library, and they have impl Read for &File, e.g. advancing the seek position/consuming data through an immutable reference is possible for the stdlib version of these traits. I suppose this is considered okay because read-ing from a File doesn't change any Rust state (e.g. the File object itself remains unmodified), only kernel internal state.

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So, I've changed the signature of this function to just take a RawFd instead of any kind of reference, but that doesn't really address your feedback, as it just moves the as_raw_fd() call up to the caller.

But since the standard library has impl Read for &File, do you still have any concerns about providing the same kind of impls here?

fn read_volatile_raw_fd(
raw_fd: RawFd,
buf: &mut VolatileSlice<impl BitmapSlice>,
) -> Result<usize, VolatileMemoryError> {
let fd = raw_fd.as_raw_fd();
Expand Down Expand Up @@ -205,8 +255,8 @@ fn read_volatile_raw_fd<Fd: AsRawFd>(
///
/// Returns the numbers of bytes written.
#[cfg(feature = "rawfd")]
fn write_volatile_raw_fd<Fd: AsRawFd>(
raw_fd: &mut Fd,
fn write_volatile_raw_fd(
raw_fd: RawFd,
buf: &VolatileSlice<impl BitmapSlice>,
) -> Result<usize, VolatileMemoryError> {
let fd = raw_fd.as_raw_fd();
Expand Down