diff --git a/library/std/src/backtrace.rs b/library/std/src/backtrace.rs index 7e8e0a621e3..95e18ef2a65 100644 --- a/library/std/src/backtrace.rs +++ b/library/std/src/backtrace.rs @@ -95,11 +95,12 @@ // 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::SeqCst}; -use crate::sync::Mutex; +use crate::sync::Once; use crate::sys_common::backtrace::{lock, output_filename}; use crate::vec::Vec; @@ -132,7 +133,7 @@ pub enum BacktraceStatus { enum Inner { Unsupported, Disabled, - Captured(Mutex), + Captured(LazilyResolvedCapture), } struct Capture { @@ -171,12 +172,11 @@ enum BytesOrWide { impl fmt::Debug for Backtrace { fn fmt(&self, fmt: &mut fmt::Formatter<'_>) -> fmt::Result { - let mut capture = match &self.inner { + let capture = match &self.inner { Inner::Unsupported => return fmt.write_str(""), Inner::Disabled => return fmt.write_str(""), - Inner::Captured(c) => c.lock().unwrap(), + Inner::Captured(c) => c.force(), }; - capture.resolve(); let frames = &capture.frames[capture.actual_start..]; @@ -331,7 +331,7 @@ fn create(ip: usize) -> Backtrace { let inner = if frames.is_empty() { Inner::Unsupported } else { - Inner::Captured(Mutex::new(Capture { + Inner::Captured(LazilyResolvedCapture::new(Capture { actual_start: actual_start.unwrap_or(0), frames, resolved: false, @@ -355,12 +355,11 @@ pub fn status(&self) -> BacktraceStatus { impl fmt::Display for Backtrace { fn fmt(&self, fmt: &mut fmt::Formatter<'_>) -> fmt::Result { - let mut capture = match &self.inner { + 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.lock().unwrap(), + Inner::Captured(c) => c.force(), }; - capture.resolve(); let full = fmt.alternate(); let (frames, style) = if full { @@ -404,6 +403,33 @@ fn fmt(&self, fmt: &mut fmt::Formatter<'_>) -> fmt::Result { } } +struct LazilyResolvedCapture { + sync: Once, + capture: UnsafeCell, +} + +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! diff --git a/library/std/src/backtrace/tests.rs b/library/std/src/backtrace/tests.rs index f5f74d1eb9a..31cf0f70218 100644 --- a/library/std/src/backtrace/tests.rs +++ b/library/std/src/backtrace/tests.rs @@ -3,7 +3,7 @@ #[test] fn test_debug() { let backtrace = Backtrace { - inner: Inner::Captured(Mutex::new(Capture { + inner: Inner::Captured(LazilyResolvedCapture::new(Capture { actual_start: 1, resolved: true, frames: vec![ @@ -54,4 +54,7 @@ fn test_debug() { \n]"; assert_eq!(format!("{:#?}", backtrace), expected); + + // Format the backtrace a second time, just to make sure lazily resolved state is stable + assert_eq!(format!("{:#?}", backtrace), expected); }