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
411
412
413
414
415
416
417
418
419
420
421
422
423
424
425
426
427
428
429
430
431
432
433
434
435
436
437
438
439
440
441
442
443
444
445
446
447
448
449
450
451
452
453
454
455
456
457
458
459
460
461
462
463
464
465
466
467
468
469
470
471
472
473
474
475
476
477
478
479
480
481
482
483
484
485
486
487
488
489
490
491
492
493
494
495
496
497
498
499
500
501
502
503
504
505
506
//! 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.
//!
//! ## 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 might 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 might 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 might not actually change
//! how backtraces are captured.

#![stable(feature = "backtrace", since = "CURRENT_RUSTC_VERSION")]

#[cfg(test)]
mod tests;

// 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::backtrace_rs::{self, BytesOrWideString};
use crate::cell::UnsafeCell;
use crate::env;
use crate::ffi::c_void;
use crate::fmt;
use crate::sync::atomic::{AtomicUsize, Ordering::Relaxed};
use crate::sync::Once;
use crate::sys_common::backtrace::{lock, output_filename};
use crate::vec::Vec;

/// 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`.
#[stable(feature = "backtrace", since = "CURRENT_RUSTC_VERSION")]
#[must_use]
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.
#[stable(feature = "backtrace", since = "CURRENT_RUSTC_VERSION")]
#[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.
    #[stable(feature = "backtrace", since = "CURRENT_RUSTC_VERSION")]
    Unsupported,
    /// Capturing a backtrace has been disabled through either the
    /// `RUST_LIB_BACKTRACE` or `RUST_BACKTRACE` environment variables.
    #[stable(feature = "backtrace", since = "CURRENT_RUSTC_VERSION")]
    Disabled,
    /// A backtrace has been captured and the `Backtrace` should print
    /// reasonable information when rendered.
    #[stable(feature = "backtrace", since = "CURRENT_RUSTC_VERSION")]
    Captured,
}

enum Inner {
    Unsupported,
    Disabled,
    Captured(LazilyResolvedCapture),
}

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

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

/// A single frame of a backtrace.
#[unstable(feature = "backtrace_frames", issue = "79676")]
pub struct BacktraceFrame {
    frame: RawFrame,
    symbols: Vec<BacktraceSymbol>,
}

#[derive(Debug)]
enum RawFrame {
    Actual(backtrace_rs::Frame),
    #[cfg(test)]
    Fake,
}

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

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

#[stable(feature = "backtrace", since = "CURRENT_RUSTC_VERSION")]
impl fmt::Debug for Backtrace {
    fn fmt(&self, fmt: &mut fmt::Formatter<'_>) -> fmt::Result {
        let capture = match &self.inner {
            Inner::Unsupported => return fmt.write_str("<unsupported>"),
            Inner::Disabled => return fmt.write_str("<disabled>"),
            Inner::Captured(c) => c.force(),
        };

        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()
    }
}

#[unstable(feature = "backtrace_frames", issue = "79676")]
impl fmt::Debug for BacktraceFrame {
    fn fmt(&self, fmt: &mut fmt::Formatter<'_>) -> fmt::Result {
        let mut dbg = fmt.debug_list();
        dbg.entries(&self.symbols);
        dbg.finish()
    }
}

impl fmt::Debug for BacktraceSymbol {
    fn fmt(&self, fmt: &mut fmt::Formatter<'_>) -> fmt::Result {
        // FIXME: improve formatting: https://github.com/rust-lang/rust/issues/65280
        // FIXME: Also, include column numbers into the debug format as Display already has them.
        // Until there are stable per-frame accessors, the format shouldn't be changed:
        // https://github.com/rust-lang/rust/issues/65280#issuecomment-638966585
        write!(fmt, "{{ ")?;

        if let Some(fn_name) = self.name.as_ref().map(|b| backtrace_rs::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 {
            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_rs::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(Relaxed) {
            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, Relaxed);
        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.
    #[stable(feature = "backtrace", since = "CURRENT_RUSTC_VERSION")]
    #[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.
    #[stable(feature = "backtrace", since = "CURRENT_RUSTC_VERSION")]
    #[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)
    }

    /// Forcibly captures a disabled backtrace, regardless of environment
    /// variable configuration.
    #[stable(feature = "backtrace", since = "CURRENT_RUSTC_VERSION")]
    #[rustc_const_stable(feature = "backtrace", since = "CURRENT_RUSTC_VERSION")]
    pub const fn disabled() -> Backtrace {
        Backtrace { inner: Inner::Disabled }
    }

    // Capture a backtrace which start just before the function addressed by
    // `ip`
    fn create(ip: usize) -> Backtrace {
        // SAFETY: We don't attempt to lock this reentrantly.
        let _lock = unsafe { lock() };
        let mut frames = Vec::new();
        let mut actual_start = None;
        unsafe {
            backtrace_rs::trace_unsynchronized(|frame| {
                frames.push(BacktraceFrame {
                    frame: RawFrame::Actual(frame.clone()),
                    symbols: Vec::new(),
                });
                if frame.symbol_address().addr() == 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(LazilyResolvedCapture::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.
    #[stable(feature = "backtrace", since = "CURRENT_RUSTC_VERSION")]
    #[must_use]
    pub fn status(&self) -> BacktraceStatus {
        match self.inner {
            Inner::Unsupported => BacktraceStatus::Unsupported,
            Inner::Disabled => BacktraceStatus::Disabled,
            Inner::Captured(_) => BacktraceStatus::Captured,
        }
    }
}

impl<'a> Backtrace {
    /// Returns an iterator over the backtrace frames.
    #[must_use]
    #[unstable(feature = "backtrace_frames", issue = "79676")]
    pub fn frames(&'a self) -> &'a [BacktraceFrame] {
        if let Inner::Captured(c) = &self.inner { &c.force().frames } else { &[] }
    }
}

#[stable(feature = "backtrace", since = "CURRENT_RUSTC_VERSION")]
impl fmt::Display for Backtrace {
    fn fmt(&self, fmt: &mut fmt::Formatter<'_>) -> fmt::Result {
        let capture = match &self.inner {
            Inner::Unsupported => return fmt.write_str("unsupported backtrace"),
            Inner::Disabled => return fmt.write_str("disabled backtrace"),
            Inner::Captured(c) => c.force(),
        };

        let full = fmt.alternate();
        let (frames, style) = if full {
            (&capture.frames[..], backtrace_rs::PrintFmt::Full)
        } else {
            (&capture.frames[capture.actual_start..], backtrace_rs::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_rs::BacktraceFmt::new(fmt, style, &mut print_path);
        f.add_context()?;
        for frame in frames {
            if frame.symbols.is_empty() {
                f.frame().print_raw(frame.frame.ip(), None, None, None)?;
            } else {
                for symbol in frame.symbols.iter() {
                    f.frame().print_raw_with_column(
                        frame.frame.ip(),
                        symbol.name.as_ref().map(|b| backtrace_rs::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,
                        symbol.colno,
                    )?;
                }
            }
        }
        f.finish()?;
        Ok(())
    }
}

struct LazilyResolvedCapture {
    sync: Once,
    capture: UnsafeCell<Capture>,
}

impl LazilyResolvedCapture {
    fn new(capture: Capture) -> Self {
        LazilyResolvedCapture { sync: Once::new(), capture: UnsafeCell::new(capture) }
    }

    fn force(&self) -> &Capture {
        self.sync.call_once(|| {
            // SAFETY: This exclusive reference can't overlap with any others
            // `Once` guarantees callers will block until this closure returns
            // `Once` also guarantees only a single caller will enter this closure
            unsafe { &mut *self.capture.get() }.resolve();
        });

        // SAFETY: This shared reference can't overlap with the exclusive reference above
        unsafe { &*self.capture.get() }
    }
}

// SAFETY: Access to the inner value is synchronized using a thread-safe `Once`
// So long as `Capture` is `Sync`, `LazilyResolvedCapture` is too
unsafe impl Sync for LazilyResolvedCapture where Capture: Sync {}

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.
        // SAFETY: We don't attempt to lock this reentrantly.
        let _lock = unsafe { lock() };
        for frame in self.frames.iter_mut() {
            let symbols = &mut frame.symbols;
            let frame = match &frame.frame {
                RawFrame::Actual(frame) => frame,
                #[cfg(test)]
                RawFrame::Fake => unimplemented!(),
            };
            unsafe {
                backtrace_rs::resolve_frame_unsynchronized(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(),
                        colno: symbol.colno(),
                    });
                });
            }
        }
    }
}

impl RawFrame {
    fn ip(&self) -> *mut c_void {
        match self {
            RawFrame::Actual(frame) => frame.ip(),
            #[cfg(test)]
            RawFrame::Fake => crate::ptr::invalid_mut(1),
        }
    }
}