library/std/src/backtrace.rs

   1 //! Support for capturing a stack backtrace of an OS thread
   2 //!
   3 //! This module contains the support necessary to capture a stack backtrace of a
   4 //! running OS thread from the OS thread itself. The `Backtrace` type supports
   5 //! capturing a stack trace via the `Backtrace::capture` and
   6 //! `Backtrace::force_capture` functions.
   7 //!
   8 //! A backtrace is typically quite handy to attach to errors (e.g. types
   9 //! implementing `std::error::Error`) to get a causal chain of where an error
  10 //! was generated.
  11 //!
  12 //! > **Note**: this module is unstable and is designed in [RFC 2504], and you
  13 //! > can learn more about its status in the [tracking issue].
  14 //!
  15 //! [RFC 2504]: https://github.com/rust-lang/rfcs/blob/master/text/2504-fix-error.md
  16 //! [tracking issue]: https://github.com/rust-lang/rust/issues/53487
  17 //!
  18 //! ## Accuracy
  19 //!
  20 //! Backtraces are attempted to be as accurate as possible, but no guarantees
  21 //! are provided about the exact accuracy of a backtrace. Instruction pointers,
  22 //! symbol names, filenames, line numbers, etc, may all be incorrect when
  23 //! reported. Accuracy is attempted on a best-effort basis, however, and bugs
  24 //! are always welcome to indicate areas of improvement!
  25 //!
  26 //! For most platforms a backtrace with a filename/line number requires that
  27 //! programs be compiled with debug information. Without debug information
  28 //! filenames/line numbers will not be reported.
  29 //!
  30 //! ## Platform support
  31 //!
  32 //! Not all platforms that libstd compiles for support capturing backtraces.
  33 //! Some platforms simply do nothing when capturing a backtrace. To check
  34 //! whether the platform supports capturing backtraces you can consult the
  35 //! `BacktraceStatus` enum as a result of `Backtrace::status`.
  36 //!
  37 //! Like above with accuracy platform support is done on a best effort basis.
  38 //! Sometimes libraries may not be available at runtime or something may go
  39 //! wrong which would cause a backtrace to not be captured. Please feel free to
  40 //! report issues with platforms where a backtrace cannot be captured though!
  41 //!
  42 //! ## Environment Variables
  43 //!
  44 //! The `Backtrace::capture` function may not actually capture a backtrace by
  45 //! default. Its behavior is governed by two environment variables:
  46 //!
  47 //! * `RUST_LIB_BACKTRACE` - if this is set to `0` then `Backtrace::capture`
  48 //!   will never capture a backtrace. Any other value this is set to will enable
  49 //!   `Backtrace::capture`.
  50 //!
  51 //! * `RUST_BACKTRACE` - if `RUST_LIB_BACKTRACE` is not set, then this variable
  52 //!   is consulted with the same rules of `RUST_LIB_BACKTRACE`.
  53 //!
  54 //! * If neither of the above env vars are set, then `Backtrace::capture` will
  55 //!   be disabled.
  56 //!
  57 //! Capturing a backtrace can be a quite expensive runtime operation, so the
  58 //! environment variables allow either forcibly disabling this runtime
  59 //! performance hit or allow selectively enabling it in some programs.
  60 //!
  61 //! Note that the `Backtrace::force_capture` function can be used to ignore
  62 //! these environment variables. Also note that the state of environment
  63 //! variables is cached once the first backtrace is created, so altering
  64 //! `RUST_LIB_BACKTRACE` or `RUST_BACKTRACE` at runtime may not actually change
  65 //! how backtraces are captured.
  66
  67 #![unstable(feature = "backtrace", issue = "53487")]
  68
  69 #[cfg(test)]
  70 mod tests;
  71
  72 // NB: A note on resolution of a backtrace:
  73 //
  74 // Backtraces primarily happen in two steps, one is where we actually capture
  75 // the stack backtrace, giving us a list of instruction pointers corresponding
  76 // to stack frames. Next we take these instruction pointers and, one-by-one,
  77 // turn them into a human readable name (like `main`).
  78 //
  79 // The first phase can be somewhat expensive (walking the stack), especially
  80 // on MSVC where debug information is consulted to return inline frames each as
  81 // their own frame. The second phase, however, is almost always extremely
  82 // expensive (on the order of milliseconds sometimes) when it's consulting debug
  83 // information.
  84 //
  85 // We attempt to amortize this cost as much as possible by delaying resolution
  86 // of an address to a human readable name for as long as possible. When
  87 // `Backtrace::create` is called to capture a backtrace it doesn't actually
  88 // perform any symbol resolution, but rather we lazily resolve symbols only just
  89 // before they're needed for printing. This way we can make capturing a
  90 // backtrace and throwing it away much cheaper, but actually printing a
  91 // backtrace is still basically the same cost.
  92 //
  93 // This strategy comes at the cost of some synchronization required inside of a
  94 // `Backtrace`, but that's a relatively small price to pay relative to capturing
  95 // a backtrace or actually symbolizing it.
  96
  97 use crate::backtrace_rs::{self, BytesOrWideString};
  98 use crate::env;
  99 use crate::ffi::c_void;
 100 use crate::fmt;
 101 use crate::sync::atomic::{AtomicUsize, Ordering::SeqCst};
 102 use crate::sync::Mutex;
 103 use crate::sys_common::backtrace::{lock, output_filename};
 104 use crate::vec::Vec;
 105
 106 /// A captured OS thread stack backtrace.
 107 ///
 108 /// This type represents a stack backtrace for an OS thread captured at a
 109 /// previous point in time. In some instances the `Backtrace` type may
 110 /// internally be empty due to configuration. For more information see
 111 /// `Backtrace::capture`.
 112 pub struct Backtrace {
 113     inner: Inner,
 114 }
 115
 116 /// The current status of a backtrace, indicating whether it was captured or
 117 /// whether it is empty for some other reason.
 118 #[non_exhaustive]
 119 #[derive(Debug, PartialEq, Eq)]
 120 pub enum BacktraceStatus {
 121     /// Capturing a backtrace is not supported, likely because it's not
 122     /// implemented for the current platform.
 123     Unsupported,
 124     /// Capturing a backtrace has been disabled through either the
 125     /// `RUST_LIB_BACKTRACE` or `RUST_BACKTRACE` environment variables.
 126     Disabled,
 127     /// A backtrace has been captured and the `Backtrace` should print
 128     /// reasonable information when rendered.
 129     Captured,
 130 }
 131
 132 enum Inner {
 133     Unsupported,
 134     Disabled,
 135     Captured(Mutex<Capture>),
 136 }
 137
 138 struct Capture {
 139     actual_start: usize,
 140     resolved: bool,
 141     frames: Vec<BacktraceFrame>,
 142 }
 143
 144 fn _assert_send_sync() {
 145     fn _assert<T: Send + Sync>() {}
 146     _assert::<Backtrace>();
 147 }
 148
 149 struct BacktraceFrame {
 150     frame: RawFrame,
 151     symbols: Vec<BacktraceSymbol>,
 152 }
 153
 154 enum RawFrame {
 155     Actual(backtrace_rs::Frame),
 156     #[cfg(test)]
 157     Fake,
 158 }
 159
 160 struct BacktraceSymbol {
 161     name: Option<Vec<u8>>,
 162     filename: Option<BytesOrWide>,
 163     lineno: Option<u32>,
 164 }
 165
 166 enum BytesOrWide {
 167     Bytes(Vec<u8>),
 168     Wide(Vec<u16>),
 169 }
 170
 171 impl fmt::Debug for Backtrace {
 172     fn fmt(&self, fmt: &mut fmt::Formatter<'_>) -> fmt::Result {
 173         let mut capture = match &self.inner {
 174             Inner::Unsupported => return fmt.write_str("<unsupported>"),
 175             Inner::Disabled => return fmt.write_str("<disabled>"),
 176             Inner::Captured(c) => c.lock().unwrap(),
 177         };
 178         capture.resolve();
 179
 180         let frames = &capture.frames[capture.actual_start..];
 181
 182         write!(fmt, "Backtrace ")?;
 183
 184         let mut dbg = fmt.debug_list();
 185
 186         for frame in frames {
 187             if frame.frame.ip().is_null() {
 188                 continue;
 189             }
 190
 191             dbg.entries(&frame.symbols);
 192         }
 193
 194         dbg.finish()
 195     }
 196 }
 197
 198 impl fmt::Debug for BacktraceSymbol {
 199     fn fmt(&self, fmt: &mut fmt::Formatter<'_>) -> fmt::Result {
 200         write!(fmt, "{{ ")?;
 201
 202         if let Some(fn_name) = self.name.as_ref().map(|b| backtrace_rs::SymbolName::new(b)) {
 203             write!(fmt, "fn: \"{:#}\"", fn_name)?;
 204         } else {
 205             write!(fmt, "fn: <unknown>")?;
 206         }
 207
 208         if let Some(fname) = self.filename.as_ref() {
 209             write!(fmt, ", file: \"{:?}\"", fname)?;
 210         }
 211
 212         if let Some(line) = self.lineno.as_ref() {
 213             write!(fmt, ", line: {:?}", line)?;
 214         }
 215
 216         write!(fmt, " }}")
 217     }
 218 }
 219
 220 impl fmt::Debug for BytesOrWide {
 221     fn fmt(&self, fmt: &mut fmt::Formatter<'_>) -> fmt::Result {
 222         output_filename(
 223             fmt,
 224             match self {
 225                 BytesOrWide::Bytes(w) => BytesOrWideString::Bytes(w),
 226                 BytesOrWide::Wide(w) => BytesOrWideString::Wide(w),
 227             },
 228             backtrace_rs::PrintFmt::Short,
 229             crate::env::current_dir().as_ref().ok(),
 230         )
 231     }
 232 }
 233
 234 impl Backtrace {
 235     /// Returns whether backtrace captures are enabled through environment
 236     /// variables.
 237     fn enabled() -> bool {
 238         // Cache the result of reading the environment variables to make
 239         // backtrace captures speedy, because otherwise reading environment
 240         // variables every time can be somewhat slow.
 241         static ENABLED: AtomicUsize = AtomicUsize::new(0);
 242         match ENABLED.load(SeqCst) {
 243             0 => {}
 244             1 => return false,
 245             _ => return true,
 246         }
 247         let enabled = match env::var("RUST_LIB_BACKTRACE") {
 248             Ok(s) => s != "0",
 249             Err(_) => match env::var("RUST_BACKTRACE") {
 250                 Ok(s) => s != "0",
 251                 Err(_) => false,
 252             },
 253         };
 254         ENABLED.store(enabled as usize + 1, SeqCst);
 255         enabled
 256     }
 257
 258     /// Capture a stack backtrace of the current thread.
 259     ///
 260     /// This function will capture a stack backtrace of the current OS thread of
 261     /// execution, returning a `Backtrace` type which can be later used to print
 262     /// the entire stack trace or render it to a string.
 263     ///
 264     /// This function will be a noop if the `RUST_BACKTRACE` or
 265     /// `RUST_LIB_BACKTRACE` backtrace variables are both not set. If either
 266     /// environment variable is set and enabled then this function will actually
 267     /// capture a backtrace. Capturing a backtrace can be both memory intensive
 268     /// and slow, so these environment variables allow liberally using
 269     /// `Backtrace::capture` and only incurring a slowdown when the environment
 270     /// variables are set.
 271     ///
 272     /// To forcibly capture a backtrace regardless of environment variables, use
 273     /// the `Backtrace::force_capture` function.
 274     #[inline(never)] // want to make sure there's a frame here to remove
 275     pub fn capture() -> Backtrace {
 276         if !Backtrace::enabled() {
 277             return Backtrace { inner: Inner::Disabled };
 278         }
 279         Backtrace::create(Backtrace::capture as usize)
 280     }
 281
 282     /// Forcibly captures a full backtrace, regardless of environment variable
 283     /// configuration.
 284     ///
 285     /// This function behaves the same as `capture` except that it ignores the
 286     /// values of the `RUST_BACKTRACE` and `RUST_LIB_BACKTRACE` environment
 287     /// variables, always capturing a backtrace.
 288     ///
 289     /// Note that capturing a backtrace can be an expensive operation on some
 290     /// platforms, so this should be used with caution in performance-sensitive
 291     /// parts of code.
 292     #[inline(never)] // want to make sure there's a frame here to remove
 293     pub fn force_capture() -> Backtrace {
 294         Backtrace::create(Backtrace::force_capture as usize)
 295     }
 296
 297     /// Forcibly captures a disabled backtrace, regardless of environment
 298     /// variable configuration.
 299     pub const fn disabled() -> Backtrace {
 300         Backtrace { inner: Inner::Disabled }
 301     }
 302
 303     // Capture a backtrace which start just before the function addressed by
 304     // `ip`
 305     fn create(ip: usize) -> Backtrace {
 306         let _lock = lock();
 307         let mut frames = Vec::new();
 308         let mut actual_start = None;
 309         unsafe {
 310             backtrace_rs::trace_unsynchronized(|frame| {
 311                 frames.push(BacktraceFrame {
 312                     frame: RawFrame::Actual(frame.clone()),
 313                     symbols: Vec::new(),
 314                 });
 315                 if frame.symbol_address() as usize == ip && actual_start.is_none() {
 316                     actual_start = Some(frames.len());
 317                 }
 318                 true
 319             });
 320         }
 321
 322         // If no frames came out assume that this is an unsupported platform
 323         // since `backtrace` doesn't provide a way of learning this right now,
 324         // and this should be a good enough approximation.
 325         let inner = if frames.is_empty() {
 326             Inner::Unsupported
 327         } else {
 328             Inner::Captured(Mutex::new(Capture {
 329                 actual_start: actual_start.unwrap_or(0),
 330                 frames,
 331                 resolved: false,
 332             }))
 333         };
 334
 335         Backtrace { inner }
 336     }
 337
 338     /// Returns the status of this backtrace, indicating whether this backtrace
 339     /// request was unsupported, disabled, or a stack trace was actually
 340     /// captured.
 341     pub fn status(&self) -> BacktraceStatus {
 342         match self.inner {
 343             Inner::Unsupported => BacktraceStatus::Unsupported,
 344             Inner::Disabled => BacktraceStatus::Disabled,
 345             Inner::Captured(_) => BacktraceStatus::Captured,
 346         }
 347     }
 348 }
 349
 350 impl fmt::Display for Backtrace {
 351     fn fmt(&self, fmt: &mut fmt::Formatter<'_>) -> fmt::Result {
 352         let mut capture = match &self.inner {
 353             Inner::Unsupported => return fmt.write_str("unsupported backtrace"),
 354             Inner::Disabled => return fmt.write_str("disabled backtrace"),
 355             Inner::Captured(c) => c.lock().unwrap(),
 356         };
 357         capture.resolve();
 358
 359         let full = fmt.alternate();
 360         let (frames, style) = if full {
 361             (&capture.frames[..], backtrace_rs::PrintFmt::Full)
 362         } else {
 363             (&capture.frames[capture.actual_start..], backtrace_rs::PrintFmt::Short)
 364         };
 365
 366         // When printing paths we try to strip the cwd if it exists, otherwise
 367         // we just print the path as-is. Note that we also only do this for the
 368         // short format, because if it's full we presumably want to print
 369         // everything.
 370         let cwd = crate::env::current_dir();
 371         let mut print_path = move |fmt: &mut fmt::Formatter<'_>, path: BytesOrWideString<'_>| {
 372             output_filename(fmt, path, style, cwd.as_ref().ok())
 373         };
 374
 375         let mut f = backtrace_rs::BacktraceFmt::new(fmt, style, &mut print_path);
 376         f.add_context()?;
 377         for frame in frames {
 378             let mut f = f.frame();
 379             if frame.symbols.is_empty() {
 380                 f.print_raw(frame.frame.ip(), None, None, None)?;
 381             } else {
 382                 for symbol in frame.symbols.iter() {
 383                     f.print_raw(
 384                         frame.frame.ip(),
 385                         symbol.name.as_ref().map(|b| backtrace_rs::SymbolName::new(b)),
 386                         symbol.filename.as_ref().map(|b| match b {
 387                             BytesOrWide::Bytes(w) => BytesOrWideString::Bytes(w),
 388                             BytesOrWide::Wide(w) => BytesOrWideString::Wide(w),
 389                         }),
 390                         symbol.lineno,
 391                     )?;
 392                 }
 393             }
 394         }
 395         f.finish()?;
 396         Ok(())
 397     }
 398 }
 399
 400 impl Capture {
 401     fn resolve(&mut self) {
 402         // If we're already resolved, nothing to do!
 403         if self.resolved {
 404             return;
 405         }
 406         self.resolved = true;
 407
 408         // Use the global backtrace lock to synchronize this as it's a
 409         // requirement of the `backtrace` crate, and then actually resolve
 410         // everything.
 411         let _lock = lock();
 412         for frame in self.frames.iter_mut() {
 413             let symbols = &mut frame.symbols;
 414             let frame = match &frame.frame {
 415                 RawFrame::Actual(frame) => frame,
 416                 #[cfg(test)]
 417                 RawFrame::Fake => unimplemented!(),
 418             };
 419             unsafe {
 420                 backtrace_rs::resolve_frame_unsynchronized(frame, |symbol| {
 421                     symbols.push(BacktraceSymbol {
 422                         name: symbol.name().map(|m| m.as_bytes().to_vec()),
 423                         filename: symbol.filename_raw().map(|b| match b {
 424                             BytesOrWideString::Bytes(b) => BytesOrWide::Bytes(b.to_owned()),
 425                             BytesOrWideString::Wide(b) => BytesOrWide::Wide(b.to_owned()),
 426                         }),
 427                         lineno: symbol.lineno(),
 428                     });
 429                 });
 430             }
 431         }
 432     }
 433 }
 434
 435 impl RawFrame {
 436     fn ip(&self) -> *mut c_void {
 437         match self {
 438             RawFrame::Actual(frame) => frame.ip(),
 439             #[cfg(test)]
 440             RawFrame::Fake => 1 as *mut c_void,
 441         }
 442     }
 443 }