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    //! 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)
        }

        // 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);
    }