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Diffstat (limited to 'library/std/src/backtrace.rs')
| -rw-r--r-- | library/std/src/backtrace.rs | 493 |
1 files changed, 493 insertions, 0 deletions
diff --git a/library/std/src/backtrace.rs b/library/std/src/backtrace.rs new file mode 100644 index 00000000000..02e6811bc3f --- /dev/null +++ b/library/std/src/backtrace.rs @@ -0,0 +1,493 @@ +//! 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::ffi::c_void; +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: RawFrame, + symbols: Vec<BacktraceSymbol>, +} + +enum RawFrame { + Actual(backtrace::Frame), + #[cfg(test)] + Fake, +} + +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>"), + Inner::Disabled => return fmt.write_str("<disabled>"), + 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); + 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) + } + + /// Forcibly captures a disabled backtrace, regardless of environment + /// variable configuration. + 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 { + let _lock = lock(); + let mut frames = Vec::new(); + let mut actual_start = None; + unsafe { + backtrace::trace_unsynchronized(|frame| { + frames.push(BacktraceFrame { + frame: RawFrame::Actual(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; + let frame = match &frame.frame { + RawFrame::Actual(frame) => frame, + #[cfg(test)] + RawFrame::Fake => unimplemented!(), + }; + unsafe { + backtrace::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(), + }); + }); + } + } + } +} + +impl RawFrame { + fn ip(&self) -> *mut c_void { + match self { + RawFrame::Actual(frame) => frame.ip(), + #[cfg(test)] + RawFrame::Fake => 1 as *mut c_void, + } + } +} + +#[test] +fn test_debug() { + let backtrace = Backtrace { + inner: Inner::Captured(Mutex::new(Capture { + actual_start: 1, + resolved: true, + frames: vec![ + BacktraceFrame { + frame: RawFrame::Fake, + symbols: vec![BacktraceSymbol { + name: Some(b"std::backtrace::Backtrace::create".to_vec()), + filename: Some(BytesOrWide::Bytes(b"rust/backtrace.rs".to_vec())), + lineno: Some(100), + }], + }, + BacktraceFrame { + frame: RawFrame::Fake, + symbols: vec![BacktraceSymbol { + name: Some(b"__rust_maybe_catch_panic".to_vec()), + filename: None, + lineno: None, + }], + }, + BacktraceFrame { + frame: RawFrame::Fake, + symbols: vec![ + BacktraceSymbol { + name: Some(b"std::rt::lang_start_internal".to_vec()), + filename: Some(BytesOrWide::Bytes(b"rust/rt.rs".to_vec())), + lineno: Some(300), + }, + BacktraceSymbol { + name: Some(b"std::rt::lang_start".to_vec()), + filename: Some(BytesOrWide::Bytes(b"rust/rt.rs".to_vec())), + lineno: Some(400), + }, + ], + }, + ], + })), + }; + + #[rustfmt::skip] + let expected = "Backtrace [\ + \n { fn: \"__rust_maybe_catch_panic\" },\ + \n { fn: \"std::rt::lang_start_internal\", file: \"rust/rt.rs\", line: 300 },\ + \n { fn: \"std::rt::lang_start\", file: \"rust/rt.rs\", line: 400 },\ + \n]"; + + assert_eq!(format!("{:#?}", backtrace), expected); +} |