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...
Signed-off-by: Paolo Bonzini <pbonz...@redhat.com>
---
docs/devel/rust.rst | 5 +
rust/Cargo.lock | 17 ++
rust/Cargo.toml | 1 +
rust/qemu-api/Cargo.toml | 2 +
rust/qemu-api/meson.build | 1 +
rust/qemu-api/src/error.rs | 312 +++++++++++++++++++++++++++++++++++++
rust/qemu-api/src/lib.rs | 3 +
7 files changed, 341 insertions(+)
create mode 100644 rust/qemu-api/src/error.rs
diff --git a/docs/devel/rust.rst b/docs/devel/rust.rst
index 34d9c7945b7..98803d5e8dd 100644
--- a/docs/devel/rust.rst
+++ b/docs/devel/rust.rst
@@ -96,6 +96,11 @@ are missing:
architecture (VMState). Right now, VMState lacks type safety because
it is hard to place the ``VMStateField`` definitions in traits.
+* NUL-terminated file names with ``#[track_caller]`` are scheduled for
+ inclusion as `#![feature(location_file_nul)]`, but it will be a while
+ before QEMU can use them. For now, there is special code in
+ ``util/error.c`` to support non-NUL-terminated file names.
+
* associated const equality would be nice to have for some users of
``callbacks::FnCall``, but is still experimental. ``ASSERT_IS_SOME``
replaces it.
diff --git a/rust/Cargo.lock b/rust/Cargo.lock
index bccfe855a70..b785c718f31 100644
--- a/rust/Cargo.lock
+++ b/rust/Cargo.lock
@@ -2,6 +2,12 @@
# It is not intended for manual editing.
version = 3
+[[package]]
+name = "anyhow"
+version = "1.0.98"
+source = "registry+https://github.com/rust-lang/crates.io-index";
+checksum = "e16d2d3311acee920a9eb8d33b8cbc1787ce4a264e85f964c2404b969bdcd487"
+
[[package]]
name = "arbitrary-int"
version = "1.2.7"
@@ -44,6 +50,15 @@ version = "1.12.0"
source = "registry+https://github.com/rust-lang/crates.io-index";
checksum = "3dca9240753cf90908d7e4aac30f630662b02aebaa1b58a3cadabdb23385b58b"
+[[package]]
+name = "foreign"
+version = "0.3.1"
+source = "registry+https://github.com/rust-lang/crates.io-index";
+checksum = "17ca1b5be8c9d320daf386f1809c7acc0cb09accbae795c2001953fa50585846"
+dependencies = [
+ "libc",
+]
+
[[package]]
name = "hpet"
version = "0.1.0"
@@ -114,6 +129,8 @@ dependencies = [
name = "qemu_api"
version = "0.1.0"
dependencies = [
+ "anyhow",
+ "foreign",
"libc",
"qemu_api_macros",
]
diff --git a/rust/Cargo.toml b/rust/Cargo.toml
index fd4c2fbf84b..0868e1b4268 100644
--- a/rust/Cargo.toml
+++ b/rust/Cargo.toml
@@ -67,6 +67,7 @@ missing_safety_doc = "deny"
mut_mut = "deny"
needless_bitwise_bool = "deny"
needless_pass_by_ref_mut = "deny"
+needless_update = "deny"
no_effect_underscore_binding = "deny"
option_option = "deny"
or_fun_call = "deny"
diff --git a/rust/qemu-api/Cargo.toml b/rust/qemu-api/Cargo.toml
index c96cf50e7a1..db7000dee44 100644
--- a/rust/qemu-api/Cargo.toml
+++ b/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"]
diff --git a/rust/qemu-api/meson.build b/rust/qemu-api/meson.build
index 2f0f3b2aae1..cac8595a148 100644
--- a/rust/qemu-api/meson.build
+++ b/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',
diff --git a/rust/qemu-api/src/error.rs b/rust/qemu-api/src/error.rs
new file mode 100644
index 00000000000..80157f6ea1b
--- /dev/null
+++ b/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,
+ }
+ }
+ }
+}
diff --git a/rust/qemu-api/src/lib.rs b/rust/qemu-api/src/lib.rs
index 234a94e3c29..93902fc94bc 100644
--- a/rust/qemu-api/src/lib.rs
+++ b/rust/qemu-api/src/lib.rs
@@ -19,6 +19,7 @@
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 @@
ffi::c_void,
};
+pub use error::{Error, Result};
+
#[cfg(HAVE_GLIB_WITH_ALIGNED_ALLOC)]
extern "C" {
fn g_aligned_alloc0(
--
2.49.0
