tracing-error 0.2.1

use crate::layer::WithContext;
use std::fmt;
use tracing::{Metadata, Span};

/// A captured trace of [`tracing`] spans.
///
/// This type can be thought of as a relative of
/// [`std::backtrace::Backtrace`][`Backtrace`].
/// However, rather than capturing the current call stack when it is
/// constructed, a `SpanTrace` instead captures the current [span] and its
/// [parents].
///
/// In many cases, span traces may be as useful as stack backtraces useful in
/// pinpointing where an error occurred and why, if not moreso:
///
/// * A span trace captures only the user-defined, human-readable `tracing`
///   spans, rather than _every_ frame in the call stack, often cutting out a
///   lot of noise.
/// * Span traces include the [fields] recorded by each span in the trace, as
///   well as their names and source code location, so different invocations of
///   a function can be distinguished,
/// * In asynchronous code, backtraces for errors that occur in [futures] often
///   consist not of the stack frames that _spawned_ a future, but the stack
///   frames of the executor that is responsible for running that future. This
///   means that if an `async fn` calls another `async fn` which generates an
///   error, the calling async function will not appear in the stack trace (and
///   often, the callee won't either!). On the other hand, when the
///   [`tracing-futures`] crate is used to instrument async code, the span trace
///   will represent the logical application context a future was running in,
///   rather than the stack trace of the executor that was polling a future when
///   an error occurred.
///
/// Finally, unlike stack [`Backtrace`]s, capturing a `SpanTrace` is fairly
/// lightweight, and the resulting struct is not large. The `SpanTrace` struct
/// is formatted lazily; instead, it simply stores a copy of the current span,
/// and allows visiting the spans in that span's trace tree by calling the
/// [`with_spans` method][`with_spans`].
///
/// # Formatting
///
/// The `SpanTrace` type implements `fmt::Display`, formatting the span trace
/// similarly to how Rust formats panics. For example:
///
/// ```text
///    0: custom_error::do_another_thing
///         with answer=42 will_succeed=false
///           at examples/examples/custom_error.rs:42
///    1: custom_error::do_something
///         with foo="hello world"
///           at examples/examples/custom_error.rs:37
/// ```
///
/// Additionally, if custom formatting is desired, the [`with_spans`] method can
/// be used to visit each span in the trace, formatting them in order.
///
/// [`Backtrace`]: std::backtrace::Backtrace
/// [span]: mod@tracing::span
/// [parents]: mod@tracing::span#span-relationships
/// [fields]: tracing::field
/// [futures]: std::future::Future
/// [`tracing-futures`]: https://docs.rs/tracing-futures/
/// [`with_spans`]: SpanTrace::with_spans()
#[derive(Clone)]
pub struct SpanTrace {
    span: Span,
}

// === impl SpanTrace ===

impl SpanTrace {
    /// Create a new span trace with the given span as the innermost span.
    pub fn new(span: Span) -> Self {
        SpanTrace { span }
    }

    /// Capture the current span trace.
    ///
    /// # Examples
    /// ```rust
    /// use tracing_error::SpanTrace;
    ///
    /// pub struct MyError {
    ///     span_trace: SpanTrace,
    ///     // ...
    /// }
    ///
    /// # fn some_error_condition() -> bool { true }
    ///
    /// #[tracing::instrument]
    /// pub fn my_function(arg: &str) -> Result<(), MyError> {
    ///     if some_error_condition() {
    ///         return Err(MyError {
    ///             span_trace: SpanTrace::capture(),
    ///             // ...
    ///         });
    ///     }
    ///
    ///     // ...
    /// #   Ok(())
    /// }
    /// ```
    pub fn capture() -> Self {
        SpanTrace::new(Span::current())
    }

    /// Apply a function to all captured spans in the trace until it returns
    /// `false`.
    ///
    /// This will call the provided function with a reference to the
    /// [`Metadata`] and a formatted representation of the [fields] of each span
    /// captured in the trace, starting with the span that was current when the
    /// trace was captured. The function may return `true` or `false` to
    /// indicate whether to continue iterating over spans; if it returns
    /// `false`, no additional spans will be visited.
    ///
    /// [fields]: tracing::field
    /// [`Metadata`]: tracing::Metadata
    pub fn with_spans(&self, f: impl FnMut(&'static Metadata<'static>, &str) -> bool) {
        self.span.with_subscriber(|(id, s)| {
            if let Some(getcx) = s.downcast_ref::<WithContext>() {
                getcx.with_context(s, id, f);
            }
        });
    }

    /// Returns the status of this `SpanTrace`.
    ///
    /// The status indicates one of the following:
    /// * the current subscriber does not support capturing `SpanTrace`s
    /// * there was no current span, so a trace was not captured
    /// * a span trace was successfully captured
    pub fn status(&self) -> SpanTraceStatus {
        let inner = if self.span.is_none() {
            SpanTraceStatusInner::Empty
        } else {
            let mut status = None;
            self.span.with_subscriber(|(_, s)| {
                if s.downcast_ref::<WithContext>().is_some() {
                    status = Some(SpanTraceStatusInner::Captured);
                }
            });

            status.unwrap_or(SpanTraceStatusInner::Unsupported)
        };

        SpanTraceStatus(inner)
    }
}

/// The current status of a SpanTrace, indicating whether it was captured or
/// whether it is empty for some other reason.
#[derive(Debug, PartialEq, Eq)]
pub struct SpanTraceStatus(SpanTraceStatusInner);

impl SpanTraceStatus {
    /// Formatting a SpanTrace is not supported, likely because there is no
    /// ErrorLayer or the ErrorLayer is from a different version of
    /// tracing_error
    pub const UNSUPPORTED: SpanTraceStatus = SpanTraceStatus(SpanTraceStatusInner::Unsupported);

    /// The SpanTrace is empty, likely because it was captured outside of any
    /// `span`s
    pub const EMPTY: SpanTraceStatus = SpanTraceStatus(SpanTraceStatusInner::Empty);

    /// A span trace has been captured and the `SpanTrace` should print
    /// reasonable information when rendered.
    pub const CAPTURED: SpanTraceStatus = SpanTraceStatus(SpanTraceStatusInner::Captured);
}

#[derive(Debug, PartialEq, Eq)]
enum SpanTraceStatusInner {
    Unsupported,
    Empty,
    Captured,
}

macro_rules! try_bool {
    ($e:expr, $dest:ident) => {{
        let ret = $e.unwrap_or_else(|e| $dest = Err(e));

        if $dest.is_err() {
            return false;
        }

        ret
    }};
}

impl fmt::Display for SpanTrace {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        let mut err = Ok(());
        let mut span = 0;

        self.with_spans(|metadata, fields| {
            if span > 0 {
                try_bool!(write!(f, "\n",), err);
            }

            try_bool!(
                write!(f, "{:>4}: {}::{}", span, metadata.target(), metadata.name()),
                err
            );

            if !fields.is_empty() {
                try_bool!(write!(f, "\n           with {}", fields), err);
            }

            if let Some((file, line)) = metadata
                .file()
                .and_then(|file| metadata.line().map(|line| (file, line)))
            {
                try_bool!(write!(f, "\n             at {}:{}", file, line), err);
            }

            span += 1;
            true
        });

        err
    }
}

impl fmt::Debug for SpanTrace {
    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
        struct DebugSpan<'a> {
            metadata: &'a Metadata<'a>,
            fields: &'a str,
        }

        impl fmt::Debug for DebugSpan<'_> {
            fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
                write!(
                    f,
                    "{{ target: {:?}, name: {:?}",
                    self.metadata.target(),
                    self.metadata.name()
                )?;

                if !self.fields.is_empty() {
                    write!(f, ", fields: {:?}", self.fields)?;
                }

                if let Some((file, line)) = self
                    .metadata
                    .file()
                    .and_then(|file| self.metadata.line().map(|line| (file, line)))
                {
                    write!(f, ", file: {:?}, line: {:?}", file, line)?;
                }

                write!(f, " }}")?;

                Ok(())
            }
        }

        write!(f, "SpanTrace ")?;
        let mut dbg = f.debug_list();
        self.with_spans(|metadata, fields| {
            dbg.entry(&DebugSpan { metadata, fields });
            true
        });
        dbg.finish()
    }
}

#[cfg(test)]
mod tests {
    use super::*;
    use crate::ErrorLayer;
    use tracing::subscriber::with_default;
    use tracing::{span, Level};
    use tracing_subscriber::{prelude::*, registry::Registry};

    #[test]
    fn capture_supported() {
        let subscriber = Registry::default().with(ErrorLayer::default());

        with_default(subscriber, || {
            let span = span!(Level::ERROR, "test span");
            let _guard = span.enter();

            let span_trace = SpanTrace::capture();

            dbg!(&span_trace);

            assert_eq!(SpanTraceStatus::CAPTURED, span_trace.status())
        });
    }

    #[test]
    fn capture_empty() {
        let subscriber = Registry::default().with(ErrorLayer::default());

        with_default(subscriber, || {
            let span_trace = SpanTrace::capture();

            dbg!(&span_trace);

            assert_eq!(SpanTraceStatus::EMPTY, span_trace.status())
        });
    }

    #[test]
    fn capture_unsupported() {
        let subscriber = Registry::default();

        with_default(subscriber, || {
            let span = span!(Level::ERROR, "test span");
            let _guard = span.enter();

            let span_trace = SpanTrace::capture();

            dbg!(&span_trace);

            assert_eq!(SpanTraceStatus::UNSUPPORTED, span_trace.status())
        });
    }
}