summaryrefslogtreecommitdiffstats
path: root/vendor/tracing-error/src/backtrace.rs
blob: 0a2c31d292304426cec3731bf22188044bc965be (plain)
1
2
3
4
5
6
7
8
9
10
11
12
13
14
15
16
17
18
19
20
21
22
23
24
25
26
27
28
29
30
31
32
33
34
35
36
37
38
39
40
41
42
43
44
45
46
47
48
49
50
51
52
53
54
55
56
57
58
59
60
61
62
63
64
65
66
67
68
69
70
71
72
73
74
75
76
77
78
79
80
81
82
83
84
85
86
87
88
89
90
91
92
93
94
95
96
97
98
99
100
101
102
103
104
105
106
107
108
109
110
111
112
113
114
115
116
117
118
119
120
121
122
123
124
125
126
127
128
129
130
131
132
133
134
135
136
137
138
139
140
141
142
143
144
145
146
147
148
149
150
151
152
153
154
155
156
157
158
159
160
161
162
163
164
165
166
167
168
169
170
171
172
173
174
175
176
177
178
179
180
181
182
183
184
185
186
187
188
189
190
191
192
193
194
195
196
197
198
199
200
201
202
203
204
205
206
207
208
209
210
211
212
213
214
215
216
217
218
219
220
221
222
223
224
225
226
227
228
229
230
231
232
233
234
235
236
237
238
239
240
241
242
243
244
245
246
247
248
249
250
251
252
253
254
255
256
257
258
259
260
261
262
263
264
265
266
267
268
269
270
271
272
273
274
275
276
277
278
279
280
281
282
283
284
285
286
287
288
289
290
291
292
293
294
295
296
297
298
299
300
301
302
303
304
305
306
307
308
309
310
311
312
313
314
315
316
317
318
319
320
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.
///
/// [`tracing`]: https://docs.rs/tracing
/// [`Backtrace`]: https://doc.rust-lang.org/std/backtrace/struct.Backtrace.html
/// [span]: https://docs.rs/tracing/latest/tracing/span/index.html
/// [parents]: https://docs.rs/tracing/latest/tracing/span/index.html#span-relationships
/// [fields]: https://docs.rs/tracing/latest/tracing/field/index.html
/// [futures]: https://doc.rust-lang.org/std/future/trait.Future.html
/// [`tracing-futures`]: https://docs.rs/tracing-futures/
/// [`with_spans`]: #method.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]: https://docs.rs/tracing/latest/tracing/field/index.html
    /// [`Metadata`]: https://docs.rs/tracing/latest/tracing/struct.Metadata.html
    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<'a> fmt::Debug for DebugSpan<'a> {
            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())
        });
    }
}