rust: qemu-api: add bindings to Error

Provide an implementation of std::error::Error that bridges the Rust
anyhow::Error and std::panic::Location types with QEMU's Error*.

It also has several utility methods, analogous to error_propagate(),
that convert a Result into a return value + Error** pair.  One important
difference is that these propagation methods *panic* if *errp is NULL,
unlike error_propagate() which eats subsequent errors[1].  The reason
for this is that in C you have an error_set*() call at the site where
the error is created, and calls to error_propagate() are relatively rare.

In Rust instead, even though these functions do "propagate" a
qemu_api::Error into a C Error**, there is no error_setg() anywhere that
could check for non-NULL errp and call abort().  error_propagate()'s
behavior of ignoring subsequent errors is generally considered weird,
and there would be a bigger risk of triggering it from Rust code.

[1] This is actually a violation of the preconditions of error_propagate(),
    so it should not happen.  But you never know...

Reviewed-by: Zhao Liu <zhao1.liu@intel.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

rust/qemu-api/Cargo.toml

@@ -15,7 +15,9 @@ rust-version.workspace = true
 
 [dependencies]
 qemu_api_macros = { path = "../qemu-api-macros" }
+anyhow = "~1.0"
 libc = "0.2.162"
+foreign = "~0.3.1"
 
 [features]
 default = ["debug_cell"]

rust/qemu-api/meson.build

@@ -19,6 +19,7 @@ _qemu_api_rs = static_library(
       'src/cell.rs',
       'src/chardev.rs',
       'src/errno.rs',
+      'src/error.rs',
       'src/irq.rs',
       'src/memory.rs',
       'src/module.rs',

rust/qemu-api/src/error.rs

@@ -0,0 +1,312 @@
+// SPDX-License-Identifier: GPL-2.0-or-later
+
+//! Error propagation for QEMU Rust code
+//!
+//! This module contains [`Error`], the bridge between Rust errors and
+//! [`Result`](std::result::Result)s and QEMU's C [`Error`](bindings::Error)
+//! struct.
+//!
+//! For FFI code, [`Error`] provides functions to simplify conversion between
+//! the Rust ([`Result<>`](std::result::Result)) and C (`Error**`) conventions:
+//!
+//! * [`ok_or_propagate`](crate::Error::ok_or_propagate),
+//!   [`bool_or_propagate`](crate::Error::bool_or_propagate),
+//!   [`ptr_or_propagate`](crate::Error::ptr_or_propagate) can be used to build
+//!   a C return value while also propagating an error condition
+//!
+//! * [`err_or_else`](crate::Error::err_or_else) and
+//!   [`err_or_unit`](crate::Error::err_or_unit) can be used to build a `Result`
+//!
+//! This module is most commonly used at the boundary between C and Rust code;
+//! other code will usually access it through the
+//! [`qemu_api::Result`](crate::Result) type alias, and will use the
+//! [`std::error::Error`] interface to let C errors participate in Rust's error
+//! handling functionality.
+//!
+//! Rust code can also create use this module to create an error object that
+//! will be passed up to C code, though in most cases this will be done
+//! transparently through the `?` operator.  Errors can be constructed from a
+//! simple error string, from an [`anyhow::Error`] to pass any other Rust error
+//! type up to C code, or from a combination of the two.
+//!
+//! The third case, corresponding to [`Error::with_error`], is the only one that
+//! requires mentioning [`qemu_api::Error`](crate::Error) explicitly.  Similar
+//! to how QEMU's C code handles errno values, the string and the
+//! `anyhow::Error` object will be concatenated with `:` as the separator.
+
+use std::{
+    borrow::Cow,
+    ffi::{c_char, c_int, c_void, CStr},
+    fmt::{self, Display},
+    panic, ptr,
+};
+
+use foreign::{prelude::*, OwnedPointer};
+
+use crate::bindings;
+
+pub type Result<T> = std::result::Result<T, Error>;
+
+#[derive(Debug)]
+pub struct Error {
+    msg: Option<Cow<'static, str>>,
+    /// Appends the print string of the error to the msg if not None
+    cause: Option<anyhow::Error>,
+    file: &'static str,
+    line: u32,
+}
+
+impl std::error::Error for Error {
+    fn source(&self) -> Option<&(dyn std::error::Error + 'static)> {
+        self.cause.as_ref().map(AsRef::as_ref)
+    }
+
+    #[allow(deprecated)]
+    fn description(&self) -> &str {
+        self.msg
+            .as_deref()
+            .or_else(|| self.cause.as_deref().map(std::error::Error::description))
+            .expect("no message nor cause?")
+    }
+}
+
+impl Display for Error {
+    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
+        let mut prefix = "";
+        if let Some(ref msg) = self.msg {
+            write!(f, "{msg}")?;
+            prefix = ": ";
+        }
+        if let Some(ref cause) = self.cause {
+            write!(f, "{prefix}{cause}")?;
+        } else if prefix.is_empty() {
+            panic!("no message nor cause?");
+        }
+        Ok(())
+    }
+}
+
+impl From<String> for Error {
+    #[track_caller]
+    fn from(msg: String) -> Self {
+        let location = panic::Location::caller();
+        Error {
+            msg: Some(Cow::Owned(msg)),
+            cause: None,
+            file: location.file(),
+            line: location.line(),
+        }
+    }
+}
+
+impl From<&'static str> for Error {
+    #[track_caller]
+    fn from(msg: &'static str) -> Self {
+        let location = panic::Location::caller();
+        Error {
+            msg: Some(Cow::Borrowed(msg)),
+            cause: None,
+            file: location.file(),
+            line: location.line(),
+        }
+    }
+}
+
+impl From<anyhow::Error> for Error {
+    #[track_caller]
+    fn from(error: anyhow::Error) -> Self {
+        let location = panic::Location::caller();
+        Error {
+            msg: None,
+            cause: Some(error),
+            file: location.file(),
+            line: location.line(),
+        }
+    }
+}
+
+impl Error {
+    /// Create a new error, prepending `msg` to the
+    /// description of `cause`
+    #[track_caller]
+    pub fn with_error(msg: impl Into<Cow<'static, str>>, cause: impl Into<anyhow::Error>) -> Self {
+        let location = panic::Location::caller();
+        Error {
+            msg: Some(msg.into()),
+            cause: Some(cause.into()),
+            file: location.file(),
+            line: location.line(),
+        }
+    }
+
+    /// Consume a result, returning `false` if it is an error and
+    /// `true` if it is successful.  The error is propagated into
+    /// `errp` like the C API `error_propagate` would do.
+    ///
+    /// # Safety
+    ///
+    /// `errp` must be a valid argument to `error_propagate`;
+    /// typically it is received from C code and need not be
+    /// checked further at the Rust↔C boundary.
+    pub unsafe fn bool_or_propagate(result: Result<()>, errp: *mut *mut bindings::Error) -> bool {
+        // SAFETY: caller guarantees errp is valid
+        unsafe { Self::ok_or_propagate(result, errp) }.is_some()
+    }
+
+    /// Consume a result, returning a `NULL` pointer if it is an error and
+    /// a C representation of the contents if it is successful.  This is
+    /// similar to the C API `error_propagate`, but it panics if `*errp`
+    /// is not `NULL`.
+    ///
+    /// # Safety
+    ///
+    /// `errp` must be a valid argument to `error_propagate`;
+    /// typically it is received from C code and need not be
+    /// checked further at the Rust↔C boundary.
+    ///
+    /// See [`propagate`](Error::propagate) for more information.
+    #[must_use]
+    pub unsafe fn ptr_or_propagate<T: CloneToForeign>(
+        result: Result<T>,
+        errp: *mut *mut bindings::Error,
+    ) -> *mut T::Foreign {
+        // SAFETY: caller guarantees errp is valid
+        unsafe { Self::ok_or_propagate(result, errp) }.clone_to_foreign_ptr()
+    }
+
+    /// Consume a result in the same way as `self.ok()`, but also propagate
+    /// a possible error into `errp`.  This is similar to the C API
+    /// `error_propagate`, but it panics if `*errp` is not `NULL`.
+    ///
+    /// # Safety
+    ///
+    /// `errp` must be a valid argument to `error_propagate`;
+    /// typically it is received from C code and need not be
+    /// checked further at the Rust↔C boundary.
+    ///
+    /// See [`propagate`](Error::propagate) for more information.
+    pub unsafe fn ok_or_propagate<T>(
+        result: Result<T>,
+        errp: *mut *mut bindings::Error,
+    ) -> Option<T> {
+        result.map_err(|err| unsafe { err.propagate(errp) }).ok()
+    }
+
+    /// Equivalent of the C function `error_propagate`.  Fill `*errp`
+    /// with the information container in `self` if `errp` is not NULL;
+    /// then consume it.
+    ///
+    /// This is similar to the C API `error_propagate`, but it panics if
+    /// `*errp` is not `NULL`.
+    ///
+    /// # Safety
+    ///
+    /// `errp` must be a valid argument to `error_propagate`; it can be
+    /// `NULL` or it can point to any of:
+    /// * `error_abort`
+    /// * `error_fatal`
+    /// * a local variable of (C) type `Error *`
+    ///
+    /// Typically `errp` is received from C code and need not be
+    /// checked further at the Rust↔C boundary.
+    pub unsafe fn propagate(self, errp: *mut *mut bindings::Error) {
+        if errp.is_null() {
+            return;
+        }
+
+        // SAFETY: caller guarantees that errp and *errp are valid
+        unsafe {
+            assert_eq!(*errp, ptr::null_mut());
+            bindings::error_propagate(errp, self.clone_to_foreign_ptr());
+        }
+    }
+
+    /// Convert a C `Error*` into a Rust `Result`, using
+    /// `Ok(())` if `c_error` is NULL.  Free the `Error*`.
+    ///
+    /// # Safety
+    ///
+    /// `c_error` must be `NULL` or valid; typically it was initialized
+    /// with `ptr::null_mut()` and passed by reference to a C function.
+    pub unsafe fn err_or_unit(c_error: *mut bindings::Error) -> Result<()> {
+        // SAFETY: caller guarantees c_error is valid
+        unsafe { Self::err_or_else(c_error, || ()) }
+    }
+
+    /// Convert a C `Error*` into a Rust `Result`, calling `f()` to
+    /// obtain an `Ok` value if `c_error` is NULL.  Free the `Error*`.
+    ///
+    /// # Safety
+    ///
+    /// `c_error` must be `NULL` or point to a valid C [`struct
+    /// Error`](bindings::Error); typically it was initialized with
+    /// `ptr::null_mut()` and passed by reference to a C function.
+    pub unsafe fn err_or_else<T, F: FnOnce() -> T>(
+        c_error: *mut bindings::Error,
+        f: F,
+    ) -> Result<T> {
+        // SAFETY: caller guarantees c_error is valid
+        let err = unsafe { Option::<Self>::from_foreign(c_error) };
+        match err {
+            None => Ok(f()),
+            Some(err) => Err(err),
+        }
+    }
+}
+
+impl FreeForeign for Error {
+    type Foreign = bindings::Error;
+
+    unsafe fn free_foreign(p: *mut bindings::Error) {
+        // SAFETY: caller guarantees p is valid
+        unsafe {
+            bindings::error_free(p);
+        }
+    }
+}
+
+impl CloneToForeign for Error {
+    fn clone_to_foreign(&self) -> OwnedPointer<Self> {
+        // SAFETY: all arguments are controlled by this function
+        unsafe {
+            let err: *mut c_void = libc::malloc(std::mem::size_of::<bindings::Error>());
+            let err: &mut bindings::Error = &mut *err.cast();
+            *err = bindings::Error {
+                msg: format!("{self}").clone_to_foreign_ptr(),
+                err_class: bindings::ERROR_CLASS_GENERIC_ERROR,
+                src_len: self.file.len() as c_int,
+                src: self.file.as_ptr().cast::<c_char>(),
+                line: self.line as c_int,
+                func: ptr::null_mut(),
+                hint: ptr::null_mut(),
+            };
+            OwnedPointer::new(err)
+        }
+    }
+}
+
+impl FromForeign for Error {
+    unsafe fn cloned_from_foreign(c_error: *const bindings::Error) -> Self {
+        // SAFETY: caller guarantees c_error is valid
+        unsafe {
+            let error = &*c_error;
+            let file = if error.src_len < 0 {
+                // NUL-terminated
+                CStr::from_ptr(error.src).to_str()
+            } else {
+                // Can become str::from_utf8 with Rust 1.87.0
+                std::str::from_utf8(std::slice::from_raw_parts(
+                    &*error.src.cast::<u8>(),
+                    error.src_len as usize,
+                ))
+            };
+
+            Error {
+                msg: FromForeign::cloned_from_foreign(error.msg),
+                cause: None,
+                file: file.unwrap(),
+                line: error.line as u32,
+            }
+        }
+    }
+}

rust/qemu-api/src/lib.rs

@@ -19,6 +19,7 @@ pub mod callbacks;
 pub mod cell;
 pub mod chardev;
 pub mod errno;
+pub mod error;
 pub mod irq;
 pub mod memory;
 pub mod module;
@@ -34,6 +35,8 @@ use std::{
     ffi::c_void,
 };
 
+pub use error::{Error, Result};
+
 #[cfg(HAVE_GLIB_WITH_ALIGNED_ALLOC)]
 extern "C" {
     fn g_aligned_alloc0(