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>
---
 docs/devel/rust.rst             |   1 +
 rust/qemu-api/meson.build       |   4 +
 rust/qemu-api/src/assertions.rs |  28 +++
 rust/qemu-api/src/errno.rs      | 343 ++++++++++++++++++++++++++++++++
 rust/qemu-api/src/lib.rs        |   1 +
 rust/qemu-api/src/prelude.rs    |   2 +
 6 files changed, 379 insertions(+)
 create mode 100644 rust/qemu-api/src/errno.rs

diff --git a/docs/devel/rust.rst b/docs/devel/rust.rst
index 90958e5a306..c75dccdbb7c 100644
--- a/docs/devel/rust.rst
+++ b/docs/devel/rust.rst
@@ -179,6 +179,7 @@ module           status
 ``callbacks``    complete
 ``cell``         stable
 ``c_str``        complete
+``errno``        complete
 ``irq``          complete
 ``memory``       stable
 ``module``       complete
diff --git a/rust/qemu-api/meson.build b/rust/qemu-api/meson.build
index 2e9c1078b9b..bcf1cf780f3 100644
--- a/rust/qemu-api/meson.build
+++ b/rust/qemu-api/meson.build
@@ -2,6 +2,8 @@ _qemu_api_cfg = run_command(rustc_args,
   '--config-headers', config_host_h, '--features', files('Cargo.toml'),
   capture: true, check: true).stdout().strip().splitlines()
 
+libc_dep = dependency('libc-0.2-rs')
+
 # _qemu_api_cfg += ['--cfg', 'feature="allocator"']
 if rustc.version().version_compare('>=1.77.0')
   _qemu_api_cfg += ['--cfg', 'has_offset_of']
@@ -22,6 +24,7 @@ _qemu_api_rs = static_library(
       'src/cell.rs',
       'src/chardev.rs',
       'src/c_str.rs',
+      'src/errno.rs',
       'src/irq.rs',
       'src/memory.rs',
       'src/module.rs',
@@ -39,6 +42,7 @@ _qemu_api_rs = static_library(
   override_options: ['rust_std=2021', 'build.rust_std=2021'],
   rust_abi: 'rust',
   rust_args: _qemu_api_cfg,
+  dependencies: libc_dep,
 )
 
 rust.test('rust-qemu-api-tests', _qemu_api_rs,
diff --git a/rust/qemu-api/src/assertions.rs b/rust/qemu-api/src/assertions.rs
index fa1a18de6fe..104dec39774 100644
--- a/rust/qemu-api/src/assertions.rs
+++ b/rust/qemu-api/src/assertions.rs
@@ -92,3 +92,31 @@ fn types_must_be_equal<T, U>(_: T)
         };
     };
 }
+
+/// Assert that an expression matches a pattern.  This can also be
+/// useful to compare enums that do not implement `Eq`.
+///
+/// # Examples
+///
+/// ```
+/// # use qemu_api::assert_match;
+/// // JoinHandle does not implement `Eq`, therefore the result
+/// // does not either.
+/// let result: Result<std::thread::JoinHandle<()>, u32> = Err(42);
+/// assert_match!(result, Err(42));
+/// ```
+#[macro_export]
+macro_rules! assert_match {
+    ($a:expr, $b:pat) => {
+        assert!(
+            match $a {
+                $b => true,
+                _ => false,
+            },
+            "{} = {:?} does not match {}",
+            stringify!($a),
+            $a,
+            stringify!($b)
+        );
+    };
+}
diff --git a/rust/qemu-api/src/errno.rs b/rust/qemu-api/src/errno.rs
new file mode 100644
index 00000000000..bfd972df951
--- /dev/null
+++ b/rust/qemu-api/src/errno.rs
@@ -0,0 +1,343 @@
+// SPDX-License-Identifier: GPL-2.0-or-later
+
+//! Utility functions to convert `errno` to and from
+//! [`io::Error`]/[`io::Result`]
+//!
+//! QEMU C functions often have a "positive success/negative `errno`" calling
+//! convention.  This module provides functions to portably convert an integer
+//! into an [`io::Result`] and back.
+
+use std::{convert::TryFrom, io, io::ErrorKind};
+
+/// An `errno` value that can be converted into an [`io::Error`]
+pub struct Errno(pub u16);
+
+// On Unix, from_raw_os_error takes an errno value and OS errors
+// are printed using strerror.  On Windows however it takes a
+// GetLastError() value; therefore we need to convert errno values
+// into io::Error by hand.  This is the same mapping that the
+// standard library uses to retrieve the kind of OS errors
+// (`std::sys::pal::unix::decode_error_kind`).
+impl From<Errno> for ErrorKind {
+    fn from(value: Errno) -> ErrorKind {
+        let Errno(errno) = value;
+        match i32::from(errno) {
+            libc::EPERM | libc::EACCES => ErrorKind::PermissionDenied,
+            libc::ENOENT => ErrorKind::NotFound,
+            libc::EINTR => ErrorKind::Interrupted,
+            x if x == libc::EAGAIN || x == libc::EWOULDBLOCK => ErrorKind::WouldBlock,
+            libc::ENOMEM => ErrorKind::OutOfMemory,
+            libc::EEXIST => ErrorKind::AlreadyExists,
+            libc::EINVAL => ErrorKind::InvalidInput,
+            libc::EPIPE => ErrorKind::BrokenPipe,
+            libc::EADDRINUSE => ErrorKind::AddrInUse,
+            libc::EADDRNOTAVAIL => ErrorKind::AddrNotAvailable,
+            libc::ECONNABORTED => ErrorKind::ConnectionAborted,
+            libc::ECONNREFUSED => ErrorKind::ConnectionRefused,
+            libc::ECONNRESET => ErrorKind::ConnectionReset,
+            libc::ENOTCONN => ErrorKind::NotConnected,
+            libc::ENOTSUP => ErrorKind::Unsupported,
+            libc::ETIMEDOUT => ErrorKind::TimedOut,
+            _ => ErrorKind::Other,
+        }
+    }
+}
+
+// This is used on Windows for all io::Errors, but also on Unix if the
+// io::Error does not have a raw OS error.  This is the reversed
+// mapping of the above.
+impl From<io::ErrorKind> for Errno {
+    fn from(value: io::ErrorKind) -> Errno {
+        let errno = match value {
+            // can be both EPERM or EACCES :( pick one
+            ErrorKind::PermissionDenied => libc::EPERM,
+            ErrorKind::NotFound => libc::ENOENT,
+            ErrorKind::Interrupted => libc::EINTR,
+            ErrorKind::WouldBlock => libc::EAGAIN,
+            ErrorKind::OutOfMemory => libc::ENOMEM,
+            ErrorKind::AlreadyExists => libc::EEXIST,
+            ErrorKind::InvalidInput => libc::EINVAL,
+            ErrorKind::BrokenPipe => libc::EPIPE,
+            ErrorKind::AddrInUse => libc::EADDRINUSE,
+            ErrorKind::AddrNotAvailable => libc::EADDRNOTAVAIL,
+            ErrorKind::ConnectionAborted => libc::ECONNABORTED,
+            ErrorKind::ConnectionRefused => libc::ECONNREFUSED,
+            ErrorKind::ConnectionReset => libc::ECONNRESET,
+            ErrorKind::NotConnected => libc::ENOTCONN,
+            ErrorKind::Unsupported => libc::ENOTSUP,
+            ErrorKind::TimedOut => libc::ETIMEDOUT,
+            _ => libc::EIO,
+        };
+        Errno(errno as u16)
+    }
+}
+
+impl From<Errno> for io::Error {
+    #[cfg(unix)]
+    fn from(value: Errno) -> io::Error {
+        let Errno(errno) = value;
+        io::Error::from_raw_os_error(errno.into())
+    }
+
+    #[cfg(windows)]
+    fn from(value: Errno) -> io::Error {
+        let error_kind: ErrorKind = value.into();
+        error_kind.into()
+    }
+}
+
+impl From<io::Error> for Errno {
+    fn from(value: io::Error) -> Errno {
+        if cfg!(unix) {
+            if let Some(errno) = value.raw_os_error() {
+                return Errno(u16::try_from(errno).unwrap());
+            }
+        }
+        value.kind().into()
+    }
+}
+
+/// Internal traits; used to enable [`into_io_result`] and [`into_neg_errno`]
+/// for the "right" set of types.
+mod traits {
+    use super::Errno;
+
+    /// A signed type that can be converted into an
+    /// [`io::Result`](std::io::Result)
+    pub trait GetErrno {
+        /// Unsigned variant of `Self`, used as the type for the `Ok` case.
+        type Out;
+
+        /// Return `Ok(self)` if positive, `Err(Errno(-self))` if negative
+        fn into_errno_result(self) -> Result<Self::Out, Errno>;
+    }
+
+    /// A type that can be taken out of an [`io::Result`](std::io::Result) and
+    /// converted into "positive success/negative `errno`" convention.
+    pub trait MergeErrno {
+        /// Signed variant of `Self`, used as the return type of
+        /// [`into_neg_errno`](super::into_neg_errno).
+        type Out: From<u16> + std::ops::Neg<Output = Self::Out>;
+
+        /// Return `self`, asserting that it is in range
+        fn map_ok(self) -> Self::Out;
+    }
+
+    macro_rules! get_errno {
+        ($t:ty, $out:ty) => {
+            impl GetErrno for $t {
+                type Out = $out;
+                fn into_errno_result(self) -> Result<Self::Out, Errno> {
+                    match self {
+                        0.. => Ok(self as $out),
+                        -65535..=-1 => Err(Errno(-self as u16)),
+                        _ => panic!("{self} is not a negative errno"),
+                    }
+                }
+            }
+        };
+    }
+
+    get_errno!(i32, u32);
+    get_errno!(i64, u64);
+    get_errno!(isize, usize);
+
+    macro_rules! merge_errno {
+        ($t:ty, $out:ty) => {
+            impl MergeErrno for $t {
+                type Out = $out;
+                fn map_ok(self) -> Self::Out {
+                    self.try_into().unwrap()
+                }
+            }
+        };
+    }
+
+    merge_errno!(u8, i32);
+    merge_errno!(u16, i32);
+    merge_errno!(u32, i32);
+    merge_errno!(u64, i64);
+
+    impl MergeErrno for () {
+        type Out = i32;
+        fn map_ok(self) -> i32 {
+            0
+        }
+    }
+}
+
+use traits::{GetErrno, MergeErrno};
+
+/// Convert an integer value into a [`io::Result`].
+///
+/// Positive values are turned into an `Ok` result; negative values
+/// are interpreted as negated `errno` and turned into an `Err`.
+///
+/// ```
+/// # use qemu_api::errno::into_io_result;
+/// # use std::io::ErrorKind;
+/// let ok = into_io_result(1i32).unwrap();
+/// assert_eq!(ok, 1u32);
+///
+/// let err = into_io_result(-1i32).unwrap_err(); // -EPERM
+/// assert_eq!(err.kind(), ErrorKind::PermissionDenied);
+/// ```
+///
+/// # Panics
+///
+/// Since the result is an unsigned integer, negative values must
+/// be close to 0; values that are too far away are considered
+/// likely overflows and will panic:
+///
+/// ```should_panic
+/// # use qemu_api::errno::into_io_result;
+/// # #[allow(dead_code)]
+/// let err = into_io_result(-0x1234_5678i32); // panic
+/// ```
+pub fn into_io_result<T: GetErrno>(value: T) -> io::Result<T::Out> {
+    value.into_errno_result().map_err(Into::into)
+}
+
+/// Convert a [`Result`] into an integer value, using negative `errno`
+/// values to report errors.
+///
+/// ```
+/// # use qemu_api::errno::into_neg_errno;
+/// # use std::io::{self, ErrorKind};
+/// let ok: io::Result<()> = Ok(());
+/// assert_eq!(into_neg_errno(ok), 0);
+///
+/// let err: io::Result<()> = Err(ErrorKind::InvalidInput.into());
+/// assert_eq!(into_neg_errno(err), -22); // -EINVAL
+/// ```
+///
+/// Since this module also provides the ability to convert [`io::Error`]
+/// to an `errno` value, [`io::Result`] is the most commonly used type
+/// for the argument of this function:
+///
+/// # Panics
+///
+/// Since the result is a signed integer, integer `Ok` values must remain
+/// positive:
+///
+/// ```should_panic
+/// # use qemu_api::errno::into_neg_errno;
+/// # use std::io;
+/// let err: io::Result<u32> = Ok(0x8899_AABB);
+/// into_neg_errno(err) // panic
+/// # ;
+/// ```
+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),
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use std::io::ErrorKind;
+
+    use super::*;
+    use crate::assert_match;
+
+    #[test]
+    pub fn test_from_u8() {
+        let ok: io::Result<_> = Ok(42u8);
+        assert_eq!(into_neg_errno(ok), 42);
+
+        let err: io::Result<u8> = Err(io::ErrorKind::PermissionDenied.into());
+        assert_eq!(into_neg_errno(err), -1);
+
+        if cfg!(unix) {
+            let os_err: io::Result<u8> = Err(io::Error::from_raw_os_error(10));
+            assert_eq!(into_neg_errno(os_err), -10);
+        }
+    }
+
+    #[test]
+    pub fn test_from_u16() {
+        let ok: io::Result<_> = Ok(1234u16);
+        assert_eq!(into_neg_errno(ok), 1234);
+
+        let err: io::Result<u16> = Err(io::ErrorKind::PermissionDenied.into());
+        assert_eq!(into_neg_errno(err), -1);
+
+        if cfg!(unix) {
+            let os_err: io::Result<u16> = Err(io::Error::from_raw_os_error(10));
+            assert_eq!(into_neg_errno(os_err), -10);
+        }
+    }
+
+    #[test]
+    pub fn test_i32() {
+        assert_match!(into_io_result(1234i32), Ok(1234));
+
+        let err = into_io_result(-1i32).unwrap_err();
+        #[cfg(unix)]
+        assert_match!(err.raw_os_error(), Some(1));
+        assert_match!(err.kind(), ErrorKind::PermissionDenied);
+    }
+
+    #[test]
+    pub fn test_from_u32() {
+        let ok: io::Result<_> = Ok(1234u32);
+        assert_eq!(into_neg_errno(ok), 1234);
+
+        let err: io::Result<u32> = Err(io::ErrorKind::PermissionDenied.into());
+        assert_eq!(into_neg_errno(err), -1);
+
+        if cfg!(unix) {
+            let os_err: io::Result<u32> = Err(io::Error::from_raw_os_error(10));
+            assert_eq!(into_neg_errno(os_err), -10);
+        }
+    }
+
+    #[test]
+    pub fn test_i64() {
+        assert_match!(into_io_result(1234i64), Ok(1234));
+
+        let err = into_io_result(-22i64).unwrap_err();
+        #[cfg(unix)]
+        assert_match!(err.raw_os_error(), Some(22));
+        assert_match!(err.kind(), ErrorKind::InvalidInput);
+    }
+
+    #[test]
+    pub fn test_from_u64() {
+        let ok: io::Result<_> = Ok(1234u64);
+        assert_eq!(into_neg_errno(ok), 1234);
+
+        let err: io::Result<u64> = Err(io::ErrorKind::InvalidInput.into());
+        assert_eq!(into_neg_errno(err), -22);
+
+        if cfg!(unix) {
+            let os_err: io::Result<u64> = Err(io::Error::from_raw_os_error(6));
+            assert_eq!(into_neg_errno(os_err), -6);
+        }
+    }
+
+    #[test]
+    pub fn test_isize() {
+        assert_match!(into_io_result(1234isize), Ok(1234));
+
+        let err = into_io_result(-4isize).unwrap_err();
+        #[cfg(unix)]
+        assert_match!(err.raw_os_error(), Some(4));
+        assert_match!(err.kind(), ErrorKind::Interrupted);
+    }
+
+    #[test]
+    pub fn test_from_unit() {
+        let ok: io::Result<_> = Ok(());
+        assert_eq!(into_neg_errno(ok), 0);
+
+        let err: io::Result<()> = Err(io::ErrorKind::OutOfMemory.into());
+        assert_eq!(into_neg_errno(err), -12);
+
+        if cfg!(unix) {
+            let os_err: io::Result<()> = Err(io::Error::from_raw_os_error(2));
+            assert_eq!(into_neg_errno(os_err), -2);
+        }
+    }
+}
diff --git a/rust/qemu-api/src/lib.rs b/rust/qemu-api/src/lib.rs
index ed1a8f9a2b4..05f38b51d30 100644
--- a/rust/qemu-api/src/lib.rs
+++ b/rust/qemu-api/src/lib.rs
@@ -19,6 +19,7 @@
 pub mod callbacks;
 pub mod cell;
 pub mod chardev;
+pub mod errno;
 pub mod irq;
 pub mod memory;
 pub mod module;
diff --git a/rust/qemu-api/src/prelude.rs b/rust/qemu-api/src/prelude.rs
index fbf0ee23e0b..634acf37a85 100644
--- a/rust/qemu-api/src/prelude.rs
+++ b/rust/qemu-api/src/prelude.rs
@@ -9,6 +9,8 @@
 pub use crate::cell::BqlCell;
 pub use crate::cell::BqlRefCell;
 
+pub use crate::errno;
+
 pub use crate::qdev::DeviceMethods;
 
 pub use crate::qom::InterfaceType;
-- 
2.48.1

Hi Paolo,

> 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.

I'm a bit confused. The doc of error.h just said the negative value for
failure:

• integer-valued functions return non-negative / negative.

Why do we need to using libc's -errno for Windows as well?

Converting `io::Error::last_os_error().raw_os_error().unwrap()` to a
negative value seems compatible with Windows, except it returns Windows
error codes.

> This is a set of utility functions that are used by both chardev and
> block layer bindings.

...

> +// On Unix, from_raw_os_error takes an errno value and OS errors
> +// are printed using strerror.  On Windows however it takes a
> +// GetLastError() value; therefore we need to convert errno values
> +// into io::Error by hand.  This is the same mapping that the
> +// standard library uses to retrieve the kind of OS errors
> +// (`std::sys::pal::unix::decode_error_kind`).
> +impl From<Errno> for ErrorKind {
> +    fn from(value: Errno) -> ErrorKind {

What about `use ErrorKind::*;` to oimt the following "ErrorKind::"
prefix?

> +        let Errno(errno) = value;
> +        match i32::from(errno) {

Maybe `match i32::from(errno.0)` ?

> +            libc::EPERM | libc::EACCES => ErrorKind::PermissionDenied,
> +            libc::ENOENT => ErrorKind::NotFound,
> +            libc::EINTR => ErrorKind::Interrupted,
> +            x if x == libc::EAGAIN || x == libc::EWOULDBLOCK => ErrorKind::WouldBlock,
> +            libc::ENOMEM => ErrorKind::OutOfMemory,
> +            libc::EEXIST => ErrorKind::AlreadyExists,
> +            libc::EINVAL => ErrorKind::InvalidInput,
> +            libc::EPIPE => ErrorKind::BrokenPipe,
> +            libc::EADDRINUSE => ErrorKind::AddrInUse,
> +            libc::EADDRNOTAVAIL => ErrorKind::AddrNotAvailable,
> +            libc::ECONNABORTED => ErrorKind::ConnectionAborted,
> +            libc::ECONNREFUSED => ErrorKind::ConnectionRefused,
> +            libc::ECONNRESET => ErrorKind::ConnectionReset,
> +            libc::ENOTCONN => ErrorKind::NotConnected,
> +            libc::ENOTSUP => ErrorKind::Unsupported,
> +            libc::ETIMEDOUT => ErrorKind::TimedOut,
> +            _ => ErrorKind::Other,

Are these errno cases specifically selected? It seems to have fewer than
`decode_error_kind` lists. Why not support all the cases `decode_error_kind`
mentions? Or do we need to try to cover as many errno cases as possible
from rust/kernel/error.rs?

> +        }
> +    }
> +}
> +
> +// This is used on Windows for all io::Errors, but also on Unix if the
> +// io::Error does not have a raw OS error.  This is the reversed
> +// mapping of the above.

Maybe:

This is the "almost" reversed (except the default case) mapping

?

> +impl From<io::ErrorKind> for Errno {
> +    fn from(value: io::ErrorKind) -> Errno {

`use ErrorKind::*;` could save some words, too.

> +        let errno = match value {
> +            // can be both EPERM or EACCES :( pick one
> +            ErrorKind::PermissionDenied => libc::EPERM,
> +            ErrorKind::NotFound => libc::ENOENT,
> +            ErrorKind::Interrupted => libc::EINTR,
> +            ErrorKind::WouldBlock => libc::EAGAIN,
> +            ErrorKind::OutOfMemory => libc::ENOMEM,
> +            ErrorKind::AlreadyExists => libc::EEXIST,
> +            ErrorKind::InvalidInput => libc::EINVAL,
> +            ErrorKind::BrokenPipe => libc::EPIPE,
> +            ErrorKind::AddrInUse => libc::EADDRINUSE,
> +            ErrorKind::AddrNotAvailable => libc::EADDRNOTAVAIL,
> +            ErrorKind::ConnectionAborted => libc::ECONNABORTED,
> +            ErrorKind::ConnectionRefused => libc::ECONNREFUSED,
> +            ErrorKind::ConnectionReset => libc::ECONNRESET,
> +            ErrorKind::NotConnected => libc::ENOTCONN,
> +            ErrorKind::Unsupported => libc::ENOTSUP,
> +            ErrorKind::TimedOut => libc::ETIMEDOUT,
> +            _ => libc::EIO,
> +        };
> +        Errno(errno as u16)
> +    }
> +}
> +
> +impl From<Errno> for io::Error {
> +    #[cfg(unix)]
> +    fn from(value: Errno) -> io::Error {
> +        let Errno(errno) = value;
> +        io::Error::from_raw_os_error(errno.into())

Maybe `io::Error::from_raw_os_error(value.0.into())`?

> +    }
> +
> +    #[cfg(windows)]
> +    fn from(value: Errno) -> io::Error {
> +        let error_kind: ErrorKind = value.into();
> +        error_kind.into()

Even this works:

     fn from(value: Errno) -> io::Error {
-        let error_kind: ErrorKind = value.into();
-        error_kind.into()
+        value.into()

However, it's less readability, so I still prefer your current codes.
:-)

Thanks,
Zhao