backtrace.rs.html -- source
Description: Source to the Rust file `src/libstd/backtrace.rs`.
Keywords: rust, rustlang, rust-lang

      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
    321
    322
    323
    324
    325
    326
    327
    328
    329
    330
    331
    332
    333
    334
    335
    336
    337
    338
    339
    340
    341
    342
    343
    344
    345
    346
    347
    348
    349
    350
    351
    352
    353
    354
    355
    356
    357
    358
    359
    360
    361
    362
    363
    364
    365
    366
    367
    368
    369
    370
    371
    372
    373
    374
    375
    376
    377
    378
    379
    380
    381
    382
    383
    384
    385
    386
    387
    388
    389
    390
    391
    392
    393
    394
    395
    396
    397
    398
    399
    400
    401
    402
    403
    404
    405
    406
    407
    408
    409
    410

    //! Support for capturing a stack backtrace of an OS thread
    //!
    //! This module contains the support necessary to capture a stack backtrace of a
    //! running OS thread from the OS thread itself. The `Backtrace` type supports
    //! capturing a stack trace via the `Backtrace::capture` and
    //! `Backtrace::force_capture` functions.
    //!
    //! A backtrace is typically quite handy to attach to errors (e.g. types
    //! implementing `std::error::Error`) to get a causal chain of where an error
    //! was generated.
    //!
    //! > **Note**: this module is unstable and is designed in [RFC 2504], and you
    //! > can learn more about its status in the [tracking issue].
    //!
    //! [RFC 2504]: https://github.com/rust-lang/rfcs/blob/master/text/2504-fix-error.md
    //! [tracking issue]: https://github.com/rust-lang/rust/issues/53487
    //!
    //! ## Accuracy
    //!
    //! Backtraces are attempted to be as accurate as possible, but no guarantees
    //! are provided about the exact accuracy of a backtrace. Instruction pointers,
    //! symbol names, filenames, line numbers, etc, may all be incorrect when
    //! reported. Accuracy is attempted on a best-effort basis, however, and bugs
    //! are always welcome to indicate areas of improvement!
    //!
    //! For most platforms a backtrace with a filename/line number requires that
    //! programs be compiled with debug information. Without debug information
    //! filenames/line numbers will not be reported.
    //!
    //! ## Platform support
    //!
    //! Not all platforms that libstd compiles for support capturing backtraces.
    //! Some platforms simply do nothing when capturing a backtrace. To check
    //! whether the platform supports capturing backtraces you can consult the
    //! `BacktraceStatus` enum as a result of `Backtrace::status`.
    //!
    //! Like above with accuracy platform support is done on a best effort basis.
    //! Sometimes libraries may not be available at runtime or something may go
    //! wrong which would cause a backtrace to not be captured. Please feel free to
    //! report issues with platforms where a backtrace cannot be captured though!
    //!
    //! ## Environment Variables
    //!
    //! The `Backtrace::capture` function may not actually capture a backtrace by
    //! default. Its behavior is governed by two environment variables:
    //!
    //! * `RUST_LIB_BACKTRACE` - if this is set to `0` then `Backtrace::capture`
    //!   will never capture a backtrace. Any other value this is set to will enable
    //!   `Backtrace::capture`.
    //!
    //! * `RUST_BACKTRACE` - if `RUST_LIB_BACKTRACE` is not set, then this variable
    //!   is consulted with the same rules of `RUST_LIB_BACKTRACE`.
    //!
    //! * If neither of the above env vars are set, then `Backtrace::capture` will
    //!   be disabled.
    //!
    //! Capturing a backtrace can be a quite expensive runtime operation, so the
    //! environment variables allow either forcibly disabling this runtime
    //! performance hit or allow selectively enabling it in some programs.
    //!
    //! Note that the `Backtrace::force_capture` function can be used to ignore
    //! these environment variables. Also note that the state of environment
    //! variables is cached once the first backtrace is created, so altering
    //! `RUST_LIB_BACKTRACE` or `RUST_BACKTRACE` at runtime may not actually change
    //! how backtraces are captured.

    #![unstable(feature = "backtrace", issue = "53487")]

    // NB: A note on resolution of a backtrace:
    //
    // Backtraces primarily happen in two steps, one is where we actually capture
    // the stack backtrace, giving us a list of instruction pointers corresponding
    // to stack frames. Next we take these instruction pointers and, one-by-one,
    // turn them into a human readable name (like `main`).
    //
    // The first phase can be somewhat expensive (walking the stack), especially
    // on MSVC where debug information is consulted to return inline frames each as
    // their own frame. The second phase, however, is almost always extremely
    // expensive (on the order of milliseconds sometimes) when it's consulting debug
    // information.
    //
    // We attempt to amortize this cost as much as possible by delaying resolution
    // of an address to a human readable name for as long as possible. When
    // `Backtrace::create` is called to capture a backtrace it doesn't actually
    // perform any symbol resolution, but rather we lazily resolve symbols only just
    // before they're needed for printing. This way we can make capturing a
    // backtrace and throwing it away much cheaper, but actually printing a
    // backtrace is still basically the same cost.
    //
    // This strategy comes at the cost of some synchronization required inside of a
    // `Backtrace`, but that's a relatively small price to pay relative to capturing
    // a backtrace or actually symbolizing it.

    use crate::env;
    use crate::fmt;
    use crate::sync::atomic::{AtomicUsize, Ordering::SeqCst};
    use crate::sync::Mutex;
    use crate::sys_common::backtrace::{lock, output_filename};
    use crate::vec::Vec;
    use backtrace::BytesOrWideString;
    use backtrace_rs as backtrace;

    /// A captured OS thread stack backtrace.
    ///
    /// This type represents a stack backtrace for an OS thread captured at a
    /// previous point in time. In some instances the `Backtrace` type may
    /// internally be empty due to configuration. For more information see
    /// `Backtrace::capture`.
    pub struct Backtrace {
        inner: Inner,
    }

    /// The current status of a backtrace, indicating whether it was captured or
    /// whether it is empty for some other reason.
    #[non_exhaustive]
    #[derive(Debug, PartialEq, Eq)]
    pub enum BacktraceStatus {
        /// Capturing a backtrace is not supported, likely because it's not
        /// implemented for the current platform.
        Unsupported,
        /// Capturing a backtrace has been disabled through either the
        /// `RUST_LIB_BACKTRACE` or `RUST_BACKTRACE` environment variables.
        Disabled,
        /// A backtrace has been captured and the `Backtrace` should print
        /// reasonable information when rendered.
        Captured,
    }

    enum Inner {
        Unsupported,
        Disabled,
        Captured(Mutex<Capture>),
    }

    struct Capture {
        actual_start: usize,
        resolved: bool,
        frames: Vec<BacktraceFrame>,
    }

    fn _assert_send_sync() {
        fn _assert<T: Send + Sync>() {}
        _assert::<Backtrace>();
    }

    struct BacktraceFrame {
        frame: backtrace::Frame,
        symbols: Vec<BacktraceSymbol>,
    }

    struct BacktraceSymbol {
        name: Option<Vec<u8>>,
        filename: Option<BytesOrWide>,
        lineno: Option<u32>,
    }

    enum BytesOrWide {
        Bytes(Vec<u8>),
        Wide(Vec<u16>),
    }

    impl fmt::Debug for Backtrace {
        fn fmt(&self, fmt: &mut fmt::Formatter<'_>) -> fmt::Result {
            let mut capture = match &self.inner {
                Inner::Unsupported => return fmt.write_str("unsupported backtrace"),
                Inner::Disabled => return fmt.write_str("disabled backtrace"),
                Inner::Captured(c) => c.lock().unwrap(),
            };
            capture.resolve();

            let frames = &capture.frames[capture.actual_start..];

            write!(fmt, "Backtrace ")?;

            let mut dbg = fmt.debug_list();

            for frame in frames {
                if frame.frame.ip().is_null() {
                    continue;
                }

                dbg.entries(&frame.symbols);
            }

            dbg.finish()
        }
    }

    impl fmt::Debug for BacktraceSymbol {
        fn fmt(&self, fmt: &mut fmt::Formatter<'_>) -> fmt::Result {
            write!(fmt, "{{ ")?;

            if let Some(fn_name) = self.name.as_ref().map(|b| backtrace::SymbolName::new(b)) {
                write!(fmt, "fn: \"{:#}\"", fn_name)?;
            } else {
                write!(fmt, "fn: \"<unknown>\"")?;
            }

            if let Some(fname) = self.filename.as_ref() {
                write!(fmt, ", file: {:?}", fname)?;
            }

            if let Some(line) = self.lineno.as_ref() {
                write!(fmt, ", line: {:?}", line)?;
            }

            write!(fmt, " }}")
        }
    }

    impl fmt::Debug for BytesOrWide {
        fn fmt(&self, fmt: &mut fmt::Formatter<'_>) -> fmt::Result {
            output_filename(
                fmt,
                match self {
                    BytesOrWide::Bytes(w) => BytesOrWideString::Bytes(w),
                    BytesOrWide::Wide(w) => BytesOrWideString::Wide(w),
                },
                backtrace::PrintFmt::Short,
                crate::env::current_dir().as_ref().ok(),
            )
        }
    }

    impl Backtrace {
        /// Returns whether backtrace captures are enabled through environment
        /// variables.
        fn enabled() -> bool {
            // Cache the result of reading the environment variables to make
            // backtrace captures speedy, because otherwise reading environment
            // variables every time can be somewhat slow.
            static ENABLED: AtomicUsize = AtomicUsize::new(0);
            match ENABLED.load(SeqCst) {
                0 => {}
                1 => return false,
                _ => return true,
            }
            let enabled = match env::var("RUST_LIB_BACKTRACE") {
                Ok(s) => s != "0",
                Err(_) => match env::var("RUST_BACKTRACE") {
                    Ok(s) => s != "0",
                    Err(_) => false,
                },
            };
            ENABLED.store(enabled as usize + 1, SeqCst);
            return enabled;
        }

        /// Capture a stack backtrace of the current thread.
        ///
        /// This function will capture a stack backtrace of the current OS thread of
        /// execution, returning a `Backtrace` type which can be later used to print
        /// the entire stack trace or render it to a string.
        ///
        /// This function will be a noop if the `RUST_BACKTRACE` or
        /// `RUST_LIB_BACKTRACE` backtrace variables are both not set. If either
        /// environment variable is set and enabled then this function will actually
        /// capture a backtrace. Capturing a backtrace can be both memory intensive
        /// and slow, so these environment variables allow liberally using
        /// `Backtrace::capture` and only incurring a slowdown when the environment
        /// variables are set.
        ///
        /// To forcibly capture a backtrace regardless of environment variables, use
        /// the `Backtrace::force_capture` function.
        #[inline(never)] // want to make sure there's a frame here to remove
        pub fn capture() -> Backtrace {
            if !Backtrace::enabled() {
                return Backtrace { inner: Inner::Disabled };
            }
            Backtrace::create(Backtrace::capture as usize)
        }

        /// Forcibly captures a full backtrace, regardless of environment variable
        /// configuration.
        ///
        /// This function behaves the same as `capture` except that it ignores the
        /// values of the `RUST_BACKTRACE` and `RUST_LIB_BACKTRACE` environment
        /// variables, always capturing a backtrace.
        ///
        /// Note that capturing a backtrace can be an expensive operation on some
        /// platforms, so this should be used with caution in performance-sensitive
        /// parts of code.
        #[inline(never)] // want to make sure there's a frame here to remove
        pub fn force_capture() -> Backtrace {
            Backtrace::create(Backtrace::force_capture as usize)
        }

        // Capture a backtrace which start just before the function addressed by
        // `ip`
        fn create(ip: usize) -> Backtrace {
            let _lock = lock();
            let mut frames = Vec::new();
            let mut actual_start = None;
            unsafe {
                backtrace::trace_unsynchronized(|frame| {
                    frames.push(BacktraceFrame { frame: frame.clone(), symbols: Vec::new() });
                    if frame.symbol_address() as usize == ip && actual_start.is_none() {
                        actual_start = Some(frames.len());
                    }
                    true
                });
            }

            // If no frames came out assume that this is an unsupported platform
            // since `backtrace` doesn't provide a way of learning this right now,
            // and this should be a good enough approximation.
            let inner = if frames.is_empty() {
                Inner::Unsupported
            } else {
                Inner::Captured(Mutex::new(Capture {
                    actual_start: actual_start.unwrap_or(0),
                    frames,
                    resolved: false,
                }))
            };

            Backtrace { inner }
        }

        /// Returns the status of this backtrace, indicating whether this backtrace
        /// request was unsupported, disabled, or a stack trace was actually
        /// captured.
        pub fn status(&self) -> BacktraceStatus {
            match self.inner {
                Inner::Unsupported => BacktraceStatus::Unsupported,
                Inner::Disabled => BacktraceStatus::Disabled,
                Inner::Captured(_) => BacktraceStatus::Captured,
            }
        }
    }

    impl fmt::Display for Backtrace {
        fn fmt(&self, fmt: &mut fmt::Formatter<'_>) -> fmt::Result {
            let mut capture = match &self.inner {
                Inner::Unsupported => return fmt.write_str("unsupported backtrace"),
                Inner::Disabled => return fmt.write_str("disabled backtrace"),
                Inner::Captured(c) => c.lock().unwrap(),
            };
            capture.resolve();

            let full = fmt.alternate();
            let (frames, style) = if full {
                (&capture.frames[..], backtrace::PrintFmt::Full)
            } else {
                (&capture.frames[capture.actual_start..], backtrace::PrintFmt::Short)
            };

            // When printing paths we try to strip the cwd if it exists, otherwise
            // we just print the path as-is. Note that we also only do this for the
            // short format, because if it's full we presumably want to print
            // everything.
            let cwd = crate::env::current_dir();
            let mut print_path = move |fmt: &mut fmt::Formatter<'_>, path: BytesOrWideString<'_>| {
                output_filename(fmt, path, style, cwd.as_ref().ok())
            };

            let mut f = backtrace::BacktraceFmt::new(fmt, style, &mut print_path);
            f.add_context()?;
            for frame in frames {
                let mut f = f.frame();
                if frame.symbols.is_empty() {
                    f.print_raw(frame.frame.ip(), None, None, None)?;
                } else {
                    for symbol in frame.symbols.iter() {
                        f.print_raw(
                            frame.frame.ip(),
                            symbol.name.as_ref().map(|b| backtrace::SymbolName::new(b)),
                            symbol.filename.as_ref().map(|b| match b {
                                BytesOrWide::Bytes(w) => BytesOrWideString::Bytes(w),
                                BytesOrWide::Wide(w) => BytesOrWideString::Wide(w),
                            }),
                            symbol.lineno,
                        )?;
                    }
                }
            }
            f.finish()?;
            Ok(())
        }
    }

    impl Capture {
        fn resolve(&mut self) {
            // If we're already resolved, nothing to do!
            if self.resolved {
                return;
            }
            self.resolved = true;

            // Use the global backtrace lock to synchronize this as it's a
            // requirement of the `backtrace` crate, and then actually resolve
            // everything.
            let _lock = lock();
            for frame in self.frames.iter_mut() {
                let symbols = &mut frame.symbols;
                unsafe {
                    backtrace::resolve_frame_unsynchronized(&frame.frame, |symbol| {
                        symbols.push(BacktraceSymbol {
                            name: symbol.name().map(|m| m.as_bytes().to_vec()),
                            filename: symbol.filename_raw().map(|b| match b {
                                BytesOrWideString::Bytes(b) => BytesOrWide::Bytes(b.to_owned()),
                                BytesOrWideString::Wide(b) => BytesOrWide::Wide(b.to_owned()),
                            }),
                            lineno: symbol.lineno(),
                        });
                    });
                }
            }
        }
    }