tracing_error/error.rs
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use crate::SpanTrace;
use std::error::Error;
use std::fmt::{self, Debug, Display};
struct Erased;
/// A wrapper type for `Error`s that bundles a `SpanTrace` with an inner `Error`
/// type.
///
/// This type is a good match for the error-kind pattern where you have an error
/// type with an inner enum of error variants and you would like to capture a
/// span trace that can be extracted during printing without formatting the span
/// trace as part of your display impl.
///
/// An example of implementing an error type for a library using `TracedError`
/// might look like this
///
/// ```rust,compile_fail
/// #[derive(Debug, thiserror::Error)]
/// enum Kind {
/// // ...
/// }
///
/// #[derive(Debug)]
/// pub struct Error {
/// source: TracedError<Kind>,
/// backtrace: Backtrace,
/// }
///
/// impl std::error::Error for Error {
/// fn source(&self) -> Option<&(dyn std::error::Error + 'static)> {
/// self.source.source()
/// }
///
/// fn backtrace(&self) -> Option<&Backtrace> {
/// Some(&self.backtrace)
/// }
/// }
///
/// impl fmt::Display for Error {
/// fn fmt(&self, fmt: &mut fmt::Formatter<'_>) -> fmt::Result {
/// fmt::Display::fmt(&self.source, fmt)
/// }
/// }
///
/// impl<E> From<E> for Error
/// where
/// Kind: From<E>,
/// {
/// fn from(source: E) -> Self {
/// Self {
/// source: Kind::from(source).into(),
/// backtrace: Backtrace::capture(),
/// }
/// }
/// }
/// ```
#[cfg_attr(docsrs, doc(cfg(feature = "traced-error")))]
pub struct TracedError<E> {
inner: ErrorImpl<E>,
}
impl<E> From<E> for TracedError<E>
where
E: Error + Send + Sync + 'static,
{
fn from(error: E) -> Self {
// # SAFETY
//
// This function + the repr(C) on the ErrorImpl make the type erasure throughout the rest
// of this struct's methods safe. This saves a function pointer that is parameterized on the Error type
// being stored inside the ErrorImpl. This lets the object_ref function safely cast a type
// erased `ErrorImpl` back to its original type, which is needed in order to forward our
// error/display/debug impls to the internal error type from the type erased error type.
//
// The repr(C) is necessary to ensure that the struct is layed out in the order we
// specified it so that we can safely access the vtable and spantrace fields thru a type
// erased pointer to the original object.
let vtable = &ErrorVTable {
object_ref: object_ref::<E>,
};
Self {
inner: ErrorImpl {
vtable,
span_trace: SpanTrace::capture(),
error,
},
}
}
}
#[repr(C)]
struct ErrorImpl<E> {
vtable: &'static ErrorVTable,
span_trace: SpanTrace,
// NOTE: Don't use directly. Use only through vtable. Erased type may have
// different alignment.
error: E,
}
impl ErrorImpl<Erased> {
pub(crate) fn error(&self) -> &(dyn Error + Send + Sync + 'static) {
// # SAFETY
//
// this function is used to cast a type-erased pointer to a pointer to error's
// original type. the `ErrorImpl::error` method, which calls this function, requires that
// the type this function casts to be the original erased type of the error; failure to
// uphold this is UB. since the `From` impl is parameterized over the original error type,
// the function pointer we construct here will also retain the original type. therefore,
// when this is consumed by the `error` method, it will be safe to call.
unsafe { &*(self.vtable.object_ref)(self) }
}
}
struct ErrorVTable {
object_ref: unsafe fn(&ErrorImpl<Erased>) -> &(dyn Error + Send + Sync + 'static),
}
// # SAFETY
//
// This function must be parameterized on the type E of the original error that is being stored
// inside of the `ErrorImpl`. When it is parameterized by the correct type, it safely
// casts the erased `ErrorImpl` pointer type back to the original pointer type.
unsafe fn object_ref<E>(e: &ErrorImpl<Erased>) -> &(dyn Error + Send + Sync + 'static)
where
E: Error + Send + Sync + 'static,
{
// Attach E's native Error vtable onto a pointer to e.error.
&(*(e as *const ErrorImpl<Erased> as *const ErrorImpl<E>)).error
}
impl<E> Error for TracedError<E>
where
E: std::error::Error + 'static,
{
// # SAFETY
//
// This function is safe so long as all functions on `ErrorImpl<Erased>` uphold the invariant
// that the wrapped error is only ever accessed by the `error` method. This method uses the
// function in the vtable to safely convert the pointer type back to the original type, and
// then returns the reference to the erased error.
//
// This function is necessary for the `downcast_ref` in `ExtractSpanTrace` to work, because it
// needs a concrete type to downcast to and we cannot downcast to ErrorImpls parameterized on
// errors defined in other crates. By erasing the type here we can always cast back to the
// Erased version of the ErrorImpl pointer, and still access the internal error type safely
// through the vtable.
fn source<'a>(&'a self) -> Option<&'a (dyn Error + 'static)> {
let erased = unsafe { &*(&self.inner as *const ErrorImpl<E> as *const ErrorImpl<Erased>) };
Some(erased)
}
}
impl<E> Debug for TracedError<E>
where
E: std::error::Error,
{
fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
Debug::fmt(&self.inner.error, f)
}
}
impl<E> Display for TracedError<E>
where
E: std::error::Error,
{
fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
Display::fmt(&self.inner.error, f)
}
}
impl Error for ErrorImpl<Erased> {
fn source(&self) -> Option<&(dyn Error + 'static)> {
self.error().source()
}
}
impl Debug for ErrorImpl<Erased> {
fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
f.pad("span backtrace:\n")?;
Debug::fmt(&self.span_trace, f)
}
}
impl Display for ErrorImpl<Erased> {
fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
f.pad("span backtrace:\n")?;
Display::fmt(&self.span_trace, f)
}
}
/// Extension trait for instrumenting errors with `SpanTrace`s
#[cfg_attr(docsrs, doc(cfg(feature = "traced-error")))]
pub trait InstrumentError {
/// The type of the wrapped error after instrumentation
type Instrumented;
/// Instrument an Error by bundling it with a SpanTrace
///
/// # Examples
///
/// ```rust
/// use tracing_error::{TracedError, InstrumentError};
///
/// fn wrap_error<E>(e: E) -> TracedError<E>
/// where
/// E: std::error::Error + Send + Sync + 'static
/// {
/// e.in_current_span()
/// }
/// ```
fn in_current_span(self) -> Self::Instrumented;
}
/// Extension trait for instrumenting errors in `Result`s with `SpanTrace`s
#[cfg_attr(docsrs, doc(cfg(feature = "traced-error")))]
pub trait InstrumentResult<T> {
/// The type of the wrapped error after instrumentation
type Instrumented;
/// Instrument an Error by bundling it with a SpanTrace
///
/// # Examples
///
/// ```rust
/// # use std::{io, fs};
/// use tracing_error::{TracedError, InstrumentResult};
///
/// # fn fallible_fn() -> io::Result<()> { fs::read_dir("......").map(drop) };
///
/// fn do_thing() -> Result<(), TracedError<io::Error>> {
/// fallible_fn().in_current_span()
/// }
/// ```
fn in_current_span(self) -> Result<T, Self::Instrumented>;
}
impl<T, E> InstrumentResult<T> for Result<T, E>
where
E: InstrumentError,
{
type Instrumented = <E as InstrumentError>::Instrumented;
fn in_current_span(self) -> Result<T, Self::Instrumented> {
self.map_err(E::in_current_span)
}
}
/// A trait for extracting SpanTraces created by `in_current_span()` from `dyn
/// Error` trait objects
#[cfg_attr(docsrs, doc(cfg(feature = "traced-error")))]
pub trait ExtractSpanTrace {
/// Attempts to downcast to a `TracedError` and return a reference to its
/// SpanTrace
///
/// # Examples
///
/// ```rust
/// use tracing_error::ExtractSpanTrace;
/// use std::error::Error;
///
/// fn print_span_trace(e: &(dyn Error + 'static)) {
/// let span_trace = e.span_trace();
/// if let Some(span_trace) = span_trace {
/// println!("{}", span_trace);
/// }
/// }
/// ```
fn span_trace(&self) -> Option<&SpanTrace>;
}
impl<E> InstrumentError for E
where
TracedError<E>: From<E>,
{
type Instrumented = TracedError<E>;
fn in_current_span(self) -> Self::Instrumented {
TracedError::from(self)
}
}
impl ExtractSpanTrace for dyn Error + 'static {
fn span_trace(&self) -> Option<&SpanTrace> {
self.downcast_ref::<ErrorImpl<Erased>>()
.map(|inner| &inner.span_trace)
}
}