panicking.rs.html -- source
Description: Source to the Rust file `/usr/obj/ports/rust-1.20.0/rustc-1.20.0-src/src/libstd/panicking.rs`.
Keywords: rust, rustlang, rust-lang

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    // Copyright 2014 The Rust Project Developers. See the COPYRIGHT
    // file at the top-level directory of this distribution and at
    // http://rust-lang.org/COPYRIGHT.
    //
    // Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
    // http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
    // <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
    // option. This file may not be copied, modified, or distributed
    // except according to those terms.

    //! Implementation of various bits and pieces of the `panic!` macro and
    //! associated runtime pieces.
    //!
    //! Specifically, this module contains the implementation of:
    //!
    //! * Panic hooks
    //! * Executing a panic up to doing the actual implementation
    //! * Shims around "try"

    use io::prelude::*;

    use any::Any;
    use cell::RefCell;
    use fmt;
    use intrinsics;
    use mem;
    use ptr;
    use raw;
    use sys::stdio::Stderr;
    use sys_common::rwlock::RWLock;
    use sys_common::thread_info;
    use sys_common::util;
    use thread;

    thread_local! {
        pub static LOCAL_STDERR: RefCell<Option<Box<Write + Send>>> = {
            RefCell::new(None)
        }
    }

    // Binary interface to the panic runtime that the standard library depends on.
    //
    // The standard library is tagged with `#![needs_panic_runtime]` (introduced in
    // RFC 1513) to indicate that it requires some other crate tagged with
    // `#![panic_runtime]` to exist somewhere. Each panic runtime is intended to
    // implement these symbols (with the same signatures) so we can get matched up
    // to them.
    //
    // One day this may look a little less ad-hoc with the compiler helping out to
    // hook up these functions, but it is not this day!
    #[allow(improper_ctypes)]
    extern {
        fn __rust_maybe_catch_panic(f: fn(*mut u8),
                                    data: *mut u8,
                                    data_ptr: *mut usize,
                                    vtable_ptr: *mut usize) -> u32;
        #[unwind]
        fn __rust_start_panic(data: usize, vtable: usize) -> u32;
    }

    #[derive(Copy, Clone)]
    enum Hook {
        Default,
        Custom(*mut (Fn(&PanicInfo) + 'static + Sync + Send)),
    }

    static HOOK_LOCK: RWLock = RWLock::new();
    static mut HOOK: Hook = Hook::Default;

    /// Registers a custom panic hook, replacing any that was previously registered.
    ///
    /// The panic hook is invoked when a thread panics, but before the panic runtime
    /// is invoked. As such, the hook will run with both the aborting and unwinding
    /// runtimes. The default hook prints a message to standard error and generates
    /// a backtrace if requested, but this behavior can be customized with the
    /// `set_hook` and `take_hook` functions.
    ///
    /// The hook is provided with a `PanicInfo` struct which contains information
    /// about the origin of the panic, including the payload passed to `panic!` and
    /// the source code location from which the panic originated.
    ///
    /// The panic hook is a global resource.
    ///
    /// # Panics
    ///
    /// Panics if called from a panicking thread.
    ///
    /// # Examples
    ///
    /// The following will print "Custom panic hook":
    ///
    /// ```should_panic
    /// use std::panic;
    ///
    /// panic::set_hook(Box::new(|_| {
    ///     println!("Custom panic hook");
    /// }));
    ///
    /// panic!("Normal panic");
    /// ```
    #[stable(feature = "panic_hooks", since = "1.10.0")]
    pub fn set_hook(hook: Box<Fn(&PanicInfo) + 'static + Sync + Send>) {
        if thread::panicking() {
            panic!("cannot modify the panic hook from a panicking thread");
        }

        unsafe {
            HOOK_LOCK.write();
            let old_hook = HOOK;
            HOOK = Hook::Custom(Box::into_raw(hook));
            HOOK_LOCK.write_unlock();

            if let Hook::Custom(ptr) = old_hook {
                Box::from_raw(ptr);
            }
        }
    }

    /// Unregisters the current panic hook, returning it.
    ///
    /// If no custom hook is registered, the default hook will be returned.
    ///
    /// # Panics
    ///
    /// Panics if called from a panicking thread.
    ///
    /// # Examples
    ///
    /// The following will print "Normal panic":
    ///
    /// ```should_panic
    /// use std::panic;
    ///
    /// panic::set_hook(Box::new(|_| {
    ///     println!("Custom panic hook");
    /// }));
    ///
    /// let _ = panic::take_hook();
    ///
    /// panic!("Normal panic");
    /// ```
    #[stable(feature = "panic_hooks", since = "1.10.0")]
    pub fn take_hook() -> Box<Fn(&PanicInfo) + 'static + Sync + Send> {
        if thread::panicking() {
            panic!("cannot modify the panic hook from a panicking thread");
        }

        unsafe {
            HOOK_LOCK.write();
            let hook = HOOK;
            HOOK = Hook::Default;
            HOOK_LOCK.write_unlock();

            match hook {
                Hook::Default => Box::new(default_hook),
                Hook::Custom(ptr) => Box::from_raw(ptr),
            }
        }
    }

    /// A struct providing information about a panic.
    ///
    /// `PanicInfo` structure is passed to a panic hook set by the [`set_hook`]
    /// function.
    ///
    /// [`set_hook`]: ../../std/panic/fn.set_hook.html
    ///
    /// # Examples
    ///
    /// ```should_panic
    /// use std::panic;
    ///
    /// panic::set_hook(Box::new(|panic_info| {
    ///     println!("panic occured: {:?}", panic_info.payload().downcast_ref::<&str>().unwrap());
    /// }));
    ///
    /// panic!("Normal panic");
    /// ```
    #[stable(feature = "panic_hooks", since = "1.10.0")]
    #[derive(Debug)]
    pub struct PanicInfo<'a> {
        payload: &'a (Any + Send),
        location: Location<'a>,
    }

    impl<'a> PanicInfo<'a> {
        /// Returns the payload associated with the panic.
        ///
        /// This will commonly, but not always, be a `&'static str` or [`String`].
        ///
        /// [`String`]: ../../std/string/struct.String.html
        ///
        /// # Examples
        ///
        /// ```should_panic
        /// use std::panic;
        ///
        /// panic::set_hook(Box::new(|panic_info| {
        ///     println!("panic occured: {:?}", panic_info.payload().downcast_ref::<&str>().unwrap());
        /// }));
        ///
        /// panic!("Normal panic");
        /// ```
        #[stable(feature = "panic_hooks", since = "1.10.0")]
        pub fn payload(&self) -> &(Any + Send) {
            self.payload
        }

        /// Returns information about the location from which the panic originated,
        /// if available.
        ///
        /// This method will currently always return [`Some`], but this may change
        /// in future versions.
        ///
        /// [`Some`]: ../../std/option/enum.Option.html#variant.Some
        ///
        /// # Examples
        ///
        /// ```should_panic
        /// use std::panic;
        ///
        /// panic::set_hook(Box::new(|panic_info| {
        ///     if let Some(location) = panic_info.location() {
        ///         println!("panic occured in file '{}' at line {}", location.file(), location.line());
        ///     } else {
        ///         println!("panic occured but can't get location information...");
        ///     }
        /// }));
        ///
        /// panic!("Normal panic");
        /// ```
        #[stable(feature = "panic_hooks", since = "1.10.0")]
        pub fn location(&self) -> Option<&Location> {
            Some(&self.location)
        }
    }

    /// A struct containing information about the location of a panic.
    ///
    /// This structure is created by the [`location`] method of [`PanicInfo`].
    ///
    /// [`location`]: ../../std/panic/struct.PanicInfo.html#method.location
    /// [`PanicInfo`]: ../../std/panic/struct.PanicInfo.html
    ///
    /// # Examples
    ///
    /// ```should_panic
    /// use std::panic;
    ///
    /// panic::set_hook(Box::new(|panic_info| {
    ///     if let Some(location) = panic_info.location() {
    ///         println!("panic occured in file '{}' at line {}", location.file(), location.line());
    ///     } else {
    ///         println!("panic occured but can't get location information...");
    ///     }
    /// }));
    ///
    /// panic!("Normal panic");
    /// ```
    #[derive(Debug)]
    #[stable(feature = "panic_hooks", since = "1.10.0")]
    pub struct Location<'a> {
        file: &'a str,
        line: u32,
        col: u32,
    }

    impl<'a> Location<'a> {
        /// Returns the name of the source file from which the panic originated.
        ///
        /// # Examples
        ///
        /// ```should_panic
        /// use std::panic;
        ///
        /// panic::set_hook(Box::new(|panic_info| {
        ///     if let Some(location) = panic_info.location() {
        ///         println!("panic occured in file '{}'", location.file());
        ///     } else {
        ///         println!("panic occured but can't get location information...");
        ///     }
        /// }));
        ///
        /// panic!("Normal panic");
        /// ```
        #[stable(feature = "panic_hooks", since = "1.10.0")]
        pub fn file(&self) -> &str {
            self.file
        }

        /// Returns the line number from which the panic originated.
        ///
        /// # Examples
        ///
        /// ```should_panic
        /// use std::panic;
        ///
        /// panic::set_hook(Box::new(|panic_info| {
        ///     if let Some(location) = panic_info.location() {
        ///         println!("panic occured at line {}", location.line());
        ///     } else {
        ///         println!("panic occured but can't get location information...");
        ///     }
        /// }));
        ///
        /// panic!("Normal panic");
        /// ```
        #[stable(feature = "panic_hooks", since = "1.10.0")]
        pub fn line(&self) -> u32 {
            self.line
        }

        /// Returns the column from which the panic originated.
        ///
        /// # Examples
        ///
        /// ```should_panic
        /// #![feature(panic_col)]
        /// use std::panic;
        ///
        /// panic::set_hook(Box::new(|panic_info| {
        ///     if let Some(location) = panic_info.location() {
        ///         println!("panic occured at column {}", location.column());
        ///     } else {
        ///         println!("panic occured but can't get location information...");
        ///     }
        /// }));
        ///
        /// panic!("Normal panic");
        /// ```
        #[unstable(feature = "panic_col", reason = "recently added", issue = "42939")]
        pub fn column(&self) -> u32 {
            self.col
        }
    }

    fn default_hook(info: &PanicInfo) {
        #[cfg(feature = "backtrace")]
        use sys_common::backtrace;

        // If this is a double panic, make sure that we print a backtrace
        // for this panic. Otherwise only print it if logging is enabled.
        #[cfg(feature = "backtrace")]
        let log_backtrace = {
            let panics = update_panic_count(0);

            if panics >= 2 {
                Some(backtrace::PrintFormat::Full)
            } else {
                backtrace::log_enabled()
            }
        };

        let file = info.location.file;
        let line = info.location.line;
        let col = info.location.col;

        let msg = match info.payload.downcast_ref::<&'static str>() {
            Some(s) => *s,
            None => match info.payload.downcast_ref::<String>() {
                Some(s) => &s[..],
                None => "Box<Any>",
            }
        };
        let mut err = Stderr::new().ok();
        let thread = thread_info::current_thread();
        let name = thread.as_ref().and_then(|t| t.name()).unwrap_or("<unnamed>");

        let write = |err: &mut ::io::Write| {
            let _ = writeln!(err, "thread '{}' panicked at '{}', {}:{}:{}",
                             name, msg, file, line, col);

            #[cfg(feature = "backtrace")]
            {
                use sync::atomic::{AtomicBool, Ordering};

                static FIRST_PANIC: AtomicBool = AtomicBool::new(true);

                if let Some(format) = log_backtrace {
                    let _ = backtrace::print(err, format);
                } else if FIRST_PANIC.compare_and_swap(true, false, Ordering::SeqCst) {
                    let _ = writeln!(err, "note: Run with `RUST_BACKTRACE=1` for a backtrace.");
                }
            }
        };

        let prev = LOCAL_STDERR.with(|s| s.borrow_mut().take());
        match (prev, err.as_mut()) {
            (Some(mut stderr), _) => {
                write(&mut *stderr);
                let mut s = Some(stderr);
                LOCAL_STDERR.with(|slot| {
                    *slot.borrow_mut() = s.take();
                });
            }
            (None, Some(ref mut err)) => { write(err) }
            _ => {}
        }
    }

    #[cfg(not(test))]
    #[doc(hidden)]
    #[unstable(feature = "update_panic_count", issue = "0")]
    pub fn update_panic_count(amt: isize) -> usize {
        use cell::Cell;
        thread_local! { static PANIC_COUNT: Cell<usize> = Cell::new(0) }

        PANIC_COUNT.with(|c| {
            let next = (c.get() as isize + amt) as usize;
            c.set(next);
            return next
        })
    }

    #[cfg(test)]
    pub use realstd::rt::update_panic_count;

    /// Invoke a closure, capturing the cause of an unwinding panic if one occurs.
    pub unsafe fn try<R, F: FnOnce() -> R>(f: F) -> Result<R, Box<Any + Send>> {
        #[allow(unions_with_drop_fields)]
        union Data<F, R> {
            f: F,
            r: R,
        }

        // We do some sketchy operations with ownership here for the sake of
        // performance. We can only  pass pointers down to
        // `__rust_maybe_catch_panic` (can't pass objects by value), so we do all
        // the ownership tracking here manually using a union.
        //
        // We go through a transition where:
        //
        // * First, we set the data to be the closure that we're going to call.
        // * When we make the function call, the `do_call` function below, we take
        //   ownership of the function pointer. At this point the `Data` union is
        //   entirely uninitialized.
        // * If the closure successfully returns, we write the return value into the
        //   data's return slot. Note that `ptr::write` is used as it's overwriting
        //   uninitialized data.
        // * Finally, when we come back out of the `__rust_maybe_catch_panic` we're
        //   in one of two states:
        //
        //      1. The closure didn't panic, in which case the return value was
        //         filled in. We move it out of `data` and return it.
        //      2. The closure panicked, in which case the return value wasn't
        //         filled in. In this case the entire `data` union is invalid, so
        //         there is no need to drop anything.
        //
        // Once we stack all that together we should have the "most efficient'
        // method of calling a catch panic whilst juggling ownership.
        let mut any_data = 0;
        let mut any_vtable = 0;
        let mut data = Data {
            f: f,
        };

        let r = __rust_maybe_catch_panic(do_call::<F, R>,
                                         &mut data as *mut _ as *mut u8,
                                         &mut any_data,
                                         &mut any_vtable);

        return if r == 0 {
            debug_assert!(update_panic_count(0) == 0);
            Ok(data.r)
        } else {
            update_panic_count(-1);
            debug_assert!(update_panic_count(0) == 0);
            Err(mem::transmute(raw::TraitObject {
                data: any_data as *mut _,
                vtable: any_vtable as *mut _,
            }))
        };

        fn do_call<F: FnOnce() -> R, R>(data: *mut u8) {
            unsafe {
                let data = data as *mut Data<F, R>;
                let f = ptr::read(&mut (*data).f);
                ptr::write(&mut (*data).r, f());
            }
        }
    }

    /// Determines whether the current thread is unwinding because of panic.
    pub fn panicking() -> bool {
        update_panic_count(0) != 0
    }

    /// Entry point of panic from the libcore crate.
    #[cfg(not(test))]
    #[lang = "panic_fmt"]
    #[unwind]
    pub extern fn rust_begin_panic(msg: fmt::Arguments,
                                   file: &'static str,
                                   line: u32,
                                   col: u32) -> ! {
        begin_panic_fmt(&msg, &(file, line, col))
    }

    /// The entry point for panicking with a formatted message.
    ///
    /// This is designed to reduce the amount of code required at the call
    /// site as much as possible (so that `panic!()` has as low an impact
    /// on (e.g.) the inlining of other functions as possible), by moving
    /// the actual formatting into this shared place.
    #[unstable(feature = "libstd_sys_internals",
               reason = "used by the panic! macro",
               issue = "0")]
    #[inline(never)] #[cold]
    pub fn begin_panic_fmt(msg: &fmt::Arguments,
                           file_line_col: &(&'static str, u32, u32)) -> ! {
        use fmt::Write;

        // We do two allocations here, unfortunately. But (a) they're
        // required with the current scheme, and (b) we don't handle
        // panic + OOM properly anyway (see comment in begin_panic
        // below).

        let mut s = String::new();
        let _ = s.write_fmt(*msg);
        begin_panic_new(s, file_line_col)
    }

    // FIXME: In PR #42938, we have added the column as info passed to the panic
    // handling code. For this, we want to break the ABI of begin_panic.
    // This is not possible to do directly, as the stage0 compiler is hardcoded
    // to emit a call to begin_panic in src/libsyntax/ext/build.rs, only
    // with the file and line number being passed, but not the colum number.
    // By changing the compiler source, we can only affect behaviour of higher
    // stages. We need to perform the switch over two stage0 replacements, using
    // a temporary function begin_panic_new while performing the switch:
    // 0. Right now, we tell stage1 onward to emit a call to begin_panic_new.
    // 1. In the first SNAP, stage0 calls begin_panic_new with the new ABI,
    //    begin_panic stops being used. Now we can change begin_panic to
    //    the new ABI, and start emitting calls to begin_panic in higher
    //    stages again, this time with the new ABI.
    // 2. After the second SNAP, stage0 calls begin_panic with the new ABI,
    //    and we can remove the temporary begin_panic_new function.

    /// This is the entry point of panicking for panic!() and assert!().
    #[unstable(feature = "libstd_sys_internals",
               reason = "used by the panic! macro",
               issue = "0")]
    #[inline(never)] #[cold] // avoid code bloat at the call sites as much as possible
    pub fn begin_panic_new<M: Any + Send>(msg: M, file_line_col: &(&'static str, u32, u32)) -> ! {
        // Note that this should be the only allocation performed in this code path.
        // Currently this means that panic!() on OOM will invoke this code path,
        // but then again we're not really ready for panic on OOM anyway. If
        // we do start doing this, then we should propagate this allocation to
        // be performed in the parent of this thread instead of the thread that's
        // panicking.

        rust_panic_with_hook(Box::new(msg), file_line_col)
    }

    /// This is the entry point of panicking for panic!() and assert!().
    #[unstable(feature = "libstd_sys_internals",
               reason = "used by the panic! macro",
               issue = "0")]
    #[inline(never)] #[cold] // avoid code bloat at the call sites as much as possible
    pub fn begin_panic<M: Any + Send>(msg: M, file_line: &(&'static str, u32)) -> ! {
        // Note that this should be the only allocation performed in this code path.
        // Currently this means that panic!() on OOM will invoke this code path,
        // but then again we're not really ready for panic on OOM anyway. If
        // we do start doing this, then we should propagate this allocation to
        // be performed in the parent of this thread instead of the thread that's
        // panicking.

        let (file, line) = *file_line;
        let file_line_col = (file, line, 0);

        rust_panic_with_hook(Box::new(msg), &file_line_col)
    }

    /// Executes the primary logic for a panic, including checking for recursive
    /// panics and panic hooks.
    ///
    /// This is the entry point or panics from libcore, formatted panics, and
    /// `Box<Any>` panics. Here we'll verify that we're not panicking recursively,
    /// run panic hooks, and then delegate to the actual implementation of panics.
    #[inline(never)]
    #[cold]
    fn rust_panic_with_hook(msg: Box<Any + Send>,
                            file_line_col: &(&'static str, u32, u32)) -> ! {
        let (file, line, col) = *file_line_col;

        let panics = update_panic_count(1);

        // If this is the third nested call (e.g. panics == 2, this is 0-indexed),
        // the panic hook probably triggered the last panic, otherwise the
        // double-panic check would have aborted the process. In this case abort the
        // process real quickly as we don't want to try calling it again as it'll
        // probably just panic again.
        if panics > 2 {
            util::dumb_print(format_args!("thread panicked while processing \
                                           panic. aborting.\n"));
            unsafe { intrinsics::abort() }
        }

        unsafe {
            let info = PanicInfo {
                payload: &*msg,
                location: Location {
                    file,
                    line,
                    col,
                },
            };
            HOOK_LOCK.read();
            match HOOK {
                Hook::Default => default_hook(&info),
                Hook::Custom(ptr) => (*ptr)(&info),
            }
            HOOK_LOCK.read_unlock();
        }

        if panics > 1 {
            // If a thread panics while it's already unwinding then we
            // have limited options. Currently our preference is to
            // just abort. In the future we may consider resuming
            // unwinding or otherwise exiting the thread cleanly.
            util::dumb_print(format_args!("thread panicked while panicking. \
                                           aborting.\n"));
            unsafe { intrinsics::abort() }
        }

        rust_panic(msg)
    }

    /// Shim around rust_panic. Called by resume_unwind.
    pub fn update_count_then_panic(msg: Box<Any + Send>) -> ! {
        update_panic_count(1);
        rust_panic(msg)
    }

    /// A private no-mangle function on which to slap yer breakpoints.
    #[no_mangle]
    #[allow(private_no_mangle_fns)] // yes we get it, but we like breakpoints
    pub fn rust_panic(msg: Box<Any + Send>) -> ! {
        let code = unsafe {
            let obj = mem::transmute::<_, raw::TraitObject>(msg);
            __rust_start_panic(obj.data as usize, obj.vtable as usize)
        };
        rtabort!("failed to initiate panic, error {}", code)
    }

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