Auto merge of #64154 - alexcrichton:std-backtrace, r=sfackler

std: Add a `backtrace` module

This commit adds a `backtrace` module to the standard library, as
designed in [RFC 2504]. The `Backtrace` type is intentionally very
conservative, effectively only allowing capturing it and printing it.

Additionally this commit also adds a `backtrace` method to the `Error`
trait which defaults to returning `None`, as specified in [RFC 2504].
More information about the design here can be found in [RFC 2504] and in
the [tracking issue].

Implementation-wise this is all based on the `backtrace` crate and very
closely mirrors the `backtrace::Backtrace` type on crates.io. Otherwise
it's pretty standard in how it handles everything internally.

[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

cc #53487
This commit is contained in:
bors 2019-09-11 14:46:08 +00:00
commit fe6d05a8b3
5 changed files with 508 additions and 54 deletions

352
src/libstd/backtrace.rs Normal file
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@ -0,0 +1,352 @@
//! 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::{output_filename, lock};
use crate::vec::Vec;
use backtrace::BytesOrWideString;
/// 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)]
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 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.len() == 0 {
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 {
fmt::Debug::fmt(self, fmt)
}
}
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 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(),
});
});
}
}
}
}

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@ -17,6 +17,7 @@ use core::array;
use crate::alloc::{AllocErr, LayoutErr, CannotReallocInPlace};
use crate::any::TypeId;
use crate::backtrace::Backtrace;
use crate::borrow::Cow;
use crate::cell;
use crate::char;
@ -204,6 +205,20 @@ pub trait Error: Debug + Display {
fn type_id(&self, _: private::Internal) -> TypeId where Self: 'static {
TypeId::of::<Self>()
}
/// Returns a stack backtrace, if available, of where this error ocurred.
///
/// This function allows inspecting the location, in code, of where an error
/// happened. The returned `Backtrace` contains information about the stack
/// trace of the OS thread of execution of where the error originated from.
///
/// Note that not all errors contain a `Backtrace`. Also note that a
/// `Backtrace` may actually be empty. For more information consult the
/// `Backtrace` type itself.
#[unstable(feature = "backtrace", issue = "53487")]
fn backtrace(&self) -> Option<&Backtrace> {
None
}
}
mod private {

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@ -452,6 +452,7 @@ pub mod f64;
#[macro_use]
pub mod thread;
pub mod ascii;
pub mod backtrace;
pub mod collections;
pub mod env;
pub mod error;

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@ -4,8 +4,9 @@
use crate::env;
use crate::fmt;
use crate::io;
use crate::borrow::Cow;
use crate::io::prelude::*;
use crate::path::{self, Path};
use crate::path::{self, Path, PathBuf};
use crate::sync::atomic::{self, Ordering};
use crate::sys::mutex::Mutex;
@ -14,10 +15,26 @@ use backtrace::{BacktraceFmt, BytesOrWideString, PrintFmt};
/// Max number of frames to print.
const MAX_NB_FRAMES: usize = 100;
/// Prints the current backtrace.
pub fn print(w: &mut dyn Write, format: PrintFmt) -> io::Result<()> {
pub fn lock() -> impl Drop {
struct Guard;
static LOCK: Mutex = Mutex::new();
impl Drop for Guard {
fn drop(&mut self) {
unsafe {
LOCK.unlock();
}
}
}
unsafe {
LOCK.lock();
return Guard;
}
}
/// Prints the current backtrace.
pub fn print(w: &mut dyn Write, format: PrintFmt) -> io::Result<()> {
// There are issues currently linking libbacktrace into tests, and in
// general during libstd's own unit tests we're not testing this path. In
// test mode immediately return here to optimize away any references to the
@ -29,71 +46,67 @@ pub fn print(w: &mut dyn Write, format: PrintFmt) -> io::Result<()> {
// Use a lock to prevent mixed output in multithreading context.
// Some platforms also requires it, like `SymFromAddr` on Windows.
unsafe {
LOCK.lock();
let res = _print(w, format);
LOCK.unlock();
res
let _lock = lock();
_print(w, format)
}
}
fn _print(w: &mut dyn Write, format: PrintFmt) -> io::Result<()> {
unsafe fn _print(w: &mut dyn Write, format: PrintFmt) -> io::Result<()> {
struct DisplayBacktrace {
format: PrintFmt,
}
impl fmt::Display for DisplayBacktrace {
fn fmt(&self, fmt: &mut fmt::Formatter<'_>) -> fmt::Result {
_print_fmt(fmt, self.format)
unsafe {
_print_fmt(fmt, self.format)
}
}
}
write!(w, "{}", DisplayBacktrace { format })
}
fn _print_fmt(fmt: &mut fmt::Formatter<'_>, print_fmt: PrintFmt) -> fmt::Result {
unsafe fn _print_fmt(fmt: &mut fmt::Formatter<'_>, print_fmt: PrintFmt) -> fmt::Result {
let cwd = env::current_dir().ok();
let mut print_path = move |fmt: &mut fmt::Formatter<'_>, bows: BytesOrWideString<'_>| {
output_filename(fmt, bows, print_fmt)
output_filename(fmt, bows, print_fmt, cwd.as_ref())
};
let mut bt_fmt = BacktraceFmt::new(fmt, print_fmt, &mut print_path);
bt_fmt.add_context()?;
let mut skipped = false;
unsafe {
let mut idx = 0;
let mut res = Ok(());
backtrace::trace_unsynchronized(|frame| {
if print_fmt == PrintFmt::Short && idx > MAX_NB_FRAMES {
skipped = true;
return false;
}
let mut idx = 0;
let mut res = Ok(());
backtrace::trace_unsynchronized(|frame| {
if print_fmt == PrintFmt::Short && idx > MAX_NB_FRAMES {
return false;
}
let mut hit = false;
let mut stop = false;
backtrace::resolve_frame_unsynchronized(frame, |symbol| {
hit = true;
if print_fmt == PrintFmt::Short {
if let Some(sym) = symbol.name().and_then(|s| s.as_str()) {
if sym.contains("__rust_begin_short_backtrace") {
skipped = true;
stop = true;
return;
}
let mut hit = false;
let mut stop = false;
backtrace::resolve_frame_unsynchronized(frame, |symbol| {
hit = true;
if print_fmt == PrintFmt::Short {
if let Some(sym) = symbol.name().and_then(|s| s.as_str()) {
if sym.contains("__rust_begin_short_backtrace") {
stop = true;
return;
}
}
res = bt_fmt.frame().symbol(frame, symbol);
});
if stop {
return false;
}
if !hit {
res = bt_fmt.frame().print_raw(frame.ip(), None, None, None);
}
idx += 1;
res.is_ok()
res = bt_fmt.frame().symbol(frame, symbol);
});
res?;
}
if stop {
return false;
}
if !hit {
res = bt_fmt.frame().print_raw(frame.ip(), None, None, None);
}
idx += 1;
res.is_ok()
});
res?;
bt_fmt.finish()?;
if skipped {
if print_fmt == PrintFmt::Short {
writeln!(
fmt,
"note: Some details are omitted, \
@ -153,36 +166,34 @@ pub fn log_enabled() -> Option<PrintFmt> {
/// Prints the filename of the backtrace frame.
///
/// See also `output`.
fn output_filename(
pub fn output_filename(
fmt: &mut fmt::Formatter<'_>,
bows: BytesOrWideString<'_>,
print_fmt: PrintFmt,
cwd: Option<&PathBuf>,
) -> fmt::Result {
#[cfg(windows)]
let path_buf;
let file = match bows {
let file: Cow<'_, Path> = match bows {
#[cfg(unix)]
BytesOrWideString::Bytes(bytes) => {
use crate::os::unix::prelude::*;
Path::new(crate::ffi::OsStr::from_bytes(bytes))
Path::new(crate::ffi::OsStr::from_bytes(bytes)).into()
}
#[cfg(not(unix))]
BytesOrWideString::Bytes(bytes) => {
Path::new(crate::str::from_utf8(bytes).unwrap_or("<unknown>"))
Path::new(crate::str::from_utf8(bytes).unwrap_or("<unknown>")).into()
}
#[cfg(windows)]
BytesOrWideString::Wide(wide) => {
use crate::os::windows::prelude::*;
path_buf = crate::ffi::OsString::from_wide(wide);
Path::new(&path_buf)
Cow::Owned(crate::ffi::OsString::from_wide(wide).into())
}
#[cfg(not(windows))]
BytesOrWideString::Wide(_wide) => {
Path::new("<unknown>")
Path::new("<unknown>").into()
}
};
if print_fmt == PrintFmt::Short && file.is_absolute() {
if let Ok(cwd) = env::current_dir() {
if let Some(cwd) = cwd {
if let Ok(stripped) = file.strip_prefix(&cwd) {
if let Some(s) = stripped.to_str() {
return write!(fmt, ".{}{}", path::MAIN_SEPARATOR, s);

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@ -0,0 +1,75 @@
// run-pass
// ignore-android FIXME #17520
// ignore-cloudabi spawning processes is not supported
// ignore-emscripten spawning processes is not supported
// ignore-openbsd no support for libbacktrace without filename
// ignore-sgx no processes
// ignore-msvc see #62897 and `backtrace-debuginfo.rs` test
// compile-flags:-g
#![feature(backtrace)]
use std::env;
use std::process::Command;
use std::str;
fn main() {
let args: Vec<String> = env::args().collect();
if args.len() >= 2 && args[1] == "force" {
println!("{}", std::backtrace::Backtrace::force_capture());
} else if args.len() >= 2 {
println!("{}", std::backtrace::Backtrace::capture());
} else {
runtest(&args[0]);
println!("test ok");
}
}
fn runtest(me: &str) {
env::remove_var("RUST_BACKTRACE");
env::remove_var("RUST_LIB_BACKTRACE");
let p = Command::new(me).arg("a").env("RUST_BACKTRACE", "1").output().unwrap();
assert!(p.status.success());
assert!(String::from_utf8_lossy(&p.stdout).contains("stack backtrace:\n"));
assert!(String::from_utf8_lossy(&p.stdout).contains("backtrace::main"));
let p = Command::new(me).arg("a").env("RUST_BACKTRACE", "0").output().unwrap();
assert!(p.status.success());
assert!(String::from_utf8_lossy(&p.stdout).contains("disabled backtrace\n"));
let p = Command::new(me).arg("a").output().unwrap();
assert!(p.status.success());
assert!(String::from_utf8_lossy(&p.stdout).contains("disabled backtrace\n"));
let p = Command::new(me)
.arg("a")
.env("RUST_LIB_BACKTRACE", "1")
.env("RUST_BACKTRACE", "1")
.output()
.unwrap();
assert!(p.status.success());
assert!(String::from_utf8_lossy(&p.stdout).contains("stack backtrace:\n"));
let p = Command::new(me)
.arg("a")
.env("RUST_LIB_BACKTRACE", "0")
.env("RUST_BACKTRACE", "1")
.output()
.unwrap();
assert!(p.status.success());
assert!(String::from_utf8_lossy(&p.stdout).contains("disabled backtrace\n"));
let p = Command::new(me)
.arg("force")
.env("RUST_LIB_BACKTRACE", "0")
.env("RUST_BACKTRACE", "0")
.output()
.unwrap();
assert!(p.status.success());
assert!(String::from_utf8_lossy(&p.stdout).contains("stack backtrace:\n"));
let p = Command::new(me).arg("force").output().unwrap();
assert!(p.status.success());
assert!(String::from_utf8_lossy(&p.stdout).contains("stack backtrace:\n"));
}