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Add AsFd implementations for stdio lock types on WASI. #101768

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merged 3 commits into from
Oct 5, 2022
Merged

Add AsFd implementations for stdio lock types on WASI. #101768

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ed812c7
Add `AsFd` implementations for stdio lock types on WASI.
sunfishcode Sep 13, 2022
625ebf6
Add stability attributes.
sunfishcode Sep 22, 2022
905ebc3
Add SAFETY comments for AsFd implementations on stdin/stdout/stderr
joshtriplett Oct 3, 2022
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27 changes: 27 additions & 0 deletions library/std/src/sys/wasi/stdio.rs
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Original file line number Diff line number Diff line change
Expand Up @@ -30,6 +30,15 @@ impl AsFd for Stdin {
}
}

#[stable(feature = "io_safety", since = "1.63.0")]
impl<'a> AsFd for io::StdinLock<'a> {
#[inline]
fn as_fd(&self) -> BorrowedFd<'_> {
// SAFETY: user code should not close stdin out from under the standard library
unsafe { BorrowedFd::borrow_raw(0) }
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Why exactly is this correct? A safety comment here would be great to have for someone stumbling upon this without having prior knowledge of the API and WASI spec

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I agree, but note that the implementations of AsFd for UNIX don't have such comments either. Nonetheless, added as a suggestion.

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It's a little tricky. WASI itself doesn't make such a guarantees (Unix doesn't either, for that matter). It's efffectively Rust's std that makes this guarantee/requirement, and for its part, its own API doesn't provide a way to close stdin/stdout/stderr without using unsafe. @joshtriplett's added comments look ok for now, though eventually we should add public documentation about this.

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}
}

impl io::Read for Stdin {
fn read(&mut self, data: &mut [u8]) -> io::Result<usize> {
self.read_vectored(&mut [IoSliceMut::new(data)])
Expand Down Expand Up @@ -65,6 +74,15 @@ impl AsFd for Stdout {
}
}

#[stable(feature = "io_safety", since = "1.63.0")]
impl<'a> AsFd for io::StdoutLock<'a> {
#[inline]
fn as_fd(&self) -> BorrowedFd<'_> {
// SAFETY: user code should not close stdout out from under the standard library
unsafe { BorrowedFd::borrow_raw(1) }
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}
}

impl io::Write for Stdout {
fn write(&mut self, data: &[u8]) -> io::Result<usize> {
self.write_vectored(&[IoSlice::new(data)])
Expand Down Expand Up @@ -103,6 +121,15 @@ impl AsFd for Stderr {
}
}

#[stable(feature = "io_safety", since = "1.63.0")]
impl<'a> AsFd for io::StderrLock<'a> {
#[inline]
fn as_fd(&self) -> BorrowedFd<'_> {
// SAFETY: user code should not close stderr out from under the standard library
unsafe { BorrowedFd::borrow_raw(2) }
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}
}

impl io::Write for Stderr {
fn write(&mut self, data: &[u8]) -> io::Result<usize> {
self.write_vectored(&[IoSlice::new(data)])
Expand Down