Re: [PATCH 2/2] rust: add module to convert between -errno and io::Error

[Kevin Wolf <kwolf@redhat.com>]

Am 13.02.2025 um 15:32 hat Paolo Bonzini geschrieben:
> It is a common convention in QEMU to return a positive value in case of
> success, and a negated errno value in case of error.  Unfortunately,
> using errno portably in Rust is a bit complicated; on Unix the errno
> values are supported natively by io::Error, but on Windows they are not;
> so, use the libc crate.
> 
> This is a set of utility functions that are used by both chardev and
> block layer bindings.
> 
> Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

In my series, this allows a simplification of sorts:

     let mapping = match qemu_co_run_future(s.map(&req)) {
         Ok(mapping) => mapping,
-        Err(e) => {
-            return -e
-                .raw_os_error()
-                .unwrap_or(bindings::EIO as std::os::raw::c_int)
-        }
+        Err(e) => return -i32::from(Errno::from(e).0),
     };

Not really nice yet, though.

> +pub fn into_neg_errno<T: MergeErrno, E: Into<Errno>>(value: Result<T, E>) -> T::Out {
> +    match value {
> +        Ok(x) => x.map_ok(),
> +        Err(err) => -T::Out::from(err.into().0),
> +    }
> +}

How about making this a method of a trait that is implemented for
Result<T, E> similar to what you already have, but also directly for
io::Error (or actually E: Into<Errno>)?

Then the above could become:

     let mapping = match qemu_co_run_future(s.map(&req)) {
         Ok(mapping) => mapping,
         Err(e) => return e.into_neg_errno(),
     };

Kevin