src/libstd/panicking.rs

   1 // Copyright 2014 The Rust Project Developers. See the COPYRIGHT
   2 // file at the top-level directory of this distribution and at
   3 // http://rust-lang.org/COPYRIGHT.
   4 //
   5 // Licensed under the Apache License, Version 2.0 <LICENSE-APACHE or
   6 // http://www.apache.org/licenses/LICENSE-2.0> or the MIT license
   7 // <LICENSE-MIT or http://opensource.org/licenses/MIT>, at your
   8 // option. This file may not be copied, modified, or distributed
   9 // except according to those terms.
  10
  11 //! Implementation of various bits and pieces of the `panic!` macro and
  12 //! associated runtime pieces.
  13 //!
  14 //! Specifically, this module contains the implementation of:
  15 //!
  16 //! * Panic hooks
  17 //! * Executing a panic up to doing the actual implementation
  18 //! * Shims around "try"
  19
  20 use io::prelude::*;
  21
  22 use any::Any;
  23 use cell::RefCell;
  24 use core::panic::{PanicInfo, Location};
  25 use fmt;
  26 use intrinsics;
  27 use mem;
  28 use ptr;
  29 use raw;
  30 use sys::stdio::Stderr;
  31 use sys_common::rwlock::RWLock;
  32 use sys_common::thread_info;
  33 use sys_common::util;
  34 use thread;
  35
  36 thread_local! {
  37     pub static LOCAL_STDERR: RefCell<Option<Box<Write + Send>>> = {
  38         RefCell::new(None)
  39     }
  40 }
  41
  42 // Binary interface to the panic runtime that the standard library depends on.
  43 //
  44 // The standard library is tagged with `#![needs_panic_runtime]` (introduced in
  45 // RFC 1513) to indicate that it requires some other crate tagged with
  46 // `#![panic_runtime]` to exist somewhere. Each panic runtime is intended to
  47 // implement these symbols (with the same signatures) so we can get matched up
  48 // to them.
  49 //
  50 // One day this may look a little less ad-hoc with the compiler helping out to
  51 // hook up these functions, but it is not this day!
  52 #[allow(improper_ctypes)]
  53 extern {
  54     fn __rust_maybe_catch_panic(f: fn(*mut u8),
  55                                 data: *mut u8,
  56                                 data_ptr: *mut usize,
  57                                 vtable_ptr: *mut usize) -> u32;
  58     #[cfg_attr(stage0, unwind)]
  59     #[cfg_attr(not(stage0), unwind(allowed))]
  60     fn __rust_start_panic(data: usize, vtable: usize) -> u32;
  61 }
  62
  63 #[derive(Copy, Clone)]
  64 enum Hook {
  65     Default,
  66     Custom(*mut (Fn(&PanicInfo) + 'static + Sync + Send)),
  67 }
  68
  69 static HOOK_LOCK: RWLock = RWLock::new();
  70 static mut HOOK: Hook = Hook::Default;
  71
  72 /// Registers a custom panic hook, replacing any that was previously registered.
  73 ///
  74 /// The panic hook is invoked when a thread panics, but before the panic runtime
  75 /// is invoked. As such, the hook will run with both the aborting and unwinding
  76 /// runtimes. The default hook prints a message to standard error and generates
  77 /// a backtrace if requested, but this behavior can be customized with the
  78 /// `set_hook` and `take_hook` functions.
  79 ///
  80 /// The hook is provided with a `PanicInfo` struct which contains information
  81 /// about the origin of the panic, including the payload passed to `panic!` and
  82 /// the source code location from which the panic originated.
  83 ///
  84 /// The panic hook is a global resource.
  85 ///
  86 /// # Panics
  87 ///
  88 /// Panics if called from a panicking thread.
  89 ///
  90 /// # Examples
  91 ///
  92 /// The following will print "Custom panic hook":
  93 ///
  94 /// ```should_panic
  95 /// use std::panic;
  96 ///
  97 /// panic::set_hook(Box::new(|_| {
  98 ///     println!("Custom panic hook");
  99 /// }));
 100 ///
 101 /// panic!("Normal panic");
 102 /// ```
 103 #[stable(feature = "panic_hooks", since = "1.10.0")]
 104 pub fn set_hook(hook: Box<Fn(&PanicInfo) + 'static + Sync + Send>) {
 105     if thread::panicking() {
 106         panic!("cannot modify the panic hook from a panicking thread");
 107     }
 108
 109     unsafe {
 110         HOOK_LOCK.write();
 111         let old_hook = HOOK;
 112         HOOK = Hook::Custom(Box::into_raw(hook));
 113         HOOK_LOCK.write_unlock();
 114
 115         if let Hook::Custom(ptr) = old_hook {
 116             Box::from_raw(ptr);
 117         }
 118     }
 119 }
 120
 121 /// Unregisters the current panic hook, returning it.
 122 ///
 123 /// If no custom hook is registered, the default hook will be returned.
 124 ///
 125 /// # Panics
 126 ///
 127 /// Panics if called from a panicking thread.
 128 ///
 129 /// # Examples
 130 ///
 131 /// The following will print "Normal panic":
 132 ///
 133 /// ```should_panic
 134 /// use std::panic;
 135 ///
 136 /// panic::set_hook(Box::new(|_| {
 137 ///     println!("Custom panic hook");
 138 /// }));
 139 ///
 140 /// let _ = panic::take_hook();
 141 ///
 142 /// panic!("Normal panic");
 143 /// ```
 144 #[stable(feature = "panic_hooks", since = "1.10.0")]
 145 pub fn take_hook() -> Box<Fn(&PanicInfo) + 'static + Sync + Send> {
 146     if thread::panicking() {
 147         panic!("cannot modify the panic hook from a panicking thread");
 148     }
 149
 150     unsafe {
 151         HOOK_LOCK.write();
 152         let hook = HOOK;
 153         HOOK = Hook::Default;
 154         HOOK_LOCK.write_unlock();
 155
 156         match hook {
 157             Hook::Default => Box::new(default_hook),
 158             Hook::Custom(ptr) => Box::from_raw(ptr),
 159         }
 160     }
 161 }
 162
 163 fn default_hook(info: &PanicInfo) {
 164     #[cfg(feature = "backtrace")]
 165     use sys_common::backtrace;
 166
 167     // If this is a double panic, make sure that we print a backtrace
 168     // for this panic. Otherwise only print it if logging is enabled.
 169     #[cfg(feature = "backtrace")]
 170     let log_backtrace = {
 171         let panics = update_panic_count(0);
 172
 173         if panics >= 2 {
 174             Some(backtrace::PrintFormat::Full)
 175         } else {
 176             backtrace::log_enabled()
 177         }
 178     };
 179
 180     let location = info.location().unwrap();  // The current implementation always returns Some
 181     let file = location.file();
 182     let line = location.line();
 183     let col = location.column();
 184
 185     let msg = match info.payload().downcast_ref::<&'static str>() {
 186         Some(s) => *s,
 187         None => match info.payload().downcast_ref::<String>() {
 188             Some(s) => &s[..],
 189             None => "Box<Any>",
 190         }
 191     };
 192     let mut err = Stderr::new().ok();
 193     let thread = thread_info::current_thread();
 194     let name = thread.as_ref().and_then(|t| t.name()).unwrap_or("<unnamed>");
 195
 196     let write = |err: &mut ::io::Write| {
 197         let _ = writeln!(err, "thread '{}' panicked at '{}', {}:{}:{}",
 198                          name, msg, file, line, col);
 199
 200         #[cfg(feature = "backtrace")]
 201         {
 202             use sync::atomic::{AtomicBool, Ordering};
 203
 204             static FIRST_PANIC: AtomicBool = AtomicBool::new(true);
 205
 206             if let Some(format) = log_backtrace {
 207                 let _ = backtrace::print(err, format);
 208             } else if FIRST_PANIC.compare_and_swap(true, false, Ordering::SeqCst) {
 209                 let _ = writeln!(err, "note: Run with `RUST_BACKTRACE=1` for a backtrace.");
 210             }
 211         }
 212     };
 213
 214     let prev = LOCAL_STDERR.with(|s| s.borrow_mut().take());
 215     match (prev, err.as_mut()) {
 216         (Some(mut stderr), _) => {
 217             write(&mut *stderr);
 218             let mut s = Some(stderr);
 219             LOCAL_STDERR.with(|slot| {
 220                 *slot.borrow_mut() = s.take();
 221             });
 222         }
 223         (None, Some(ref mut err)) => { write(err) }
 224         _ => {}
 225     }
 226 }
 227
 228
 229 #[cfg(not(test))]
 230 #[doc(hidden)]
 231 #[unstable(feature = "update_panic_count", issue = "0")]
 232 pub fn update_panic_count(amt: isize) -> usize {
 233     use cell::Cell;
 234     thread_local! { static PANIC_COUNT: Cell<usize> = Cell::new(0) }
 235
 236     PANIC_COUNT.with(|c| {
 237         let next = (c.get() as isize + amt) as usize;
 238         c.set(next);
 239         return next
 240     })
 241 }
 242
 243 #[cfg(test)]
 244 pub use realstd::rt::update_panic_count;
 245
 246 /// Invoke a closure, capturing the cause of an unwinding panic if one occurs.
 247 pub unsafe fn try<R, F: FnOnce() -> R>(f: F) -> Result<R, Box<Any + Send>> {
 248     #[allow(unions_with_drop_fields)]
 249     union Data<F, R> {
 250         f: F,
 251         r: R,
 252     }
 253
 254     // We do some sketchy operations with ownership here for the sake of
 255     // performance. We can only  pass pointers down to
 256     // `__rust_maybe_catch_panic` (can't pass objects by value), so we do all
 257     // the ownership tracking here manually using a union.
 258     //
 259     // We go through a transition where:
 260     //
 261     // * First, we set the data to be the closure that we're going to call.
 262     // * When we make the function call, the `do_call` function below, we take
 263     //   ownership of the function pointer. At this point the `Data` union is
 264     //   entirely uninitialized.
 265     // * If the closure successfully returns, we write the return value into the
 266     //   data's return slot. Note that `ptr::write` is used as it's overwriting
 267     //   uninitialized data.
 268     // * Finally, when we come back out of the `__rust_maybe_catch_panic` we're
 269     //   in one of two states:
 270     //
 271     //      1. The closure didn't panic, in which case the return value was
 272     //         filled in. We move it out of `data` and return it.
 273     //      2. The closure panicked, in which case the return value wasn't
 274     //         filled in. In this case the entire `data` union is invalid, so
 275     //         there is no need to drop anything.
 276     //
 277     // Once we stack all that together we should have the "most efficient'
 278     // method of calling a catch panic whilst juggling ownership.
 279     let mut any_data = 0;
 280     let mut any_vtable = 0;
 281     let mut data = Data {
 282         f,
 283     };
 284
 285     let r = __rust_maybe_catch_panic(do_call::<F, R>,
 286                                      &mut data as *mut _ as *mut u8,
 287                                      &mut any_data,
 288                                      &mut any_vtable);
 289
 290     return if r == 0 {
 291         debug_assert!(update_panic_count(0) == 0);
 292         Ok(data.r)
 293     } else {
 294         update_panic_count(-1);
 295         debug_assert!(update_panic_count(0) == 0);
 296         Err(mem::transmute(raw::TraitObject {
 297             data: any_data as *mut _,
 298             vtable: any_vtable as *mut _,
 299         }))
 300     };
 301
 302     fn do_call<F: FnOnce() -> R, R>(data: *mut u8) {
 303         unsafe {
 304             let data = data as *mut Data<F, R>;
 305             let f = ptr::read(&mut (*data).f);
 306             ptr::write(&mut (*data).r, f());
 307         }
 308     }
 309 }
 310
 311 /// Determines whether the current thread is unwinding because of panic.
 312 pub fn panicking() -> bool {
 313     update_panic_count(0) != 0
 314 }
 315
 316 /// Entry point of panic from the libcore crate.
 317 #[cfg(not(test))]
 318 #[lang = "panic_fmt"]
 319 #[cfg_attr(stage0, unwind)]
 320 #[cfg_attr(not(stage0), unwind(allowed))]
 321 pub extern fn rust_begin_panic(msg: fmt::Arguments,
 322                                file: &'static str,
 323                                line: u32,
 324                                col: u32) -> ! {
 325     begin_panic_fmt(&msg, &(file, line, col))
 326 }
 327
 328 /// The entry point for panicking with a formatted message.
 329 ///
 330 /// This is designed to reduce the amount of code required at the call
 331 /// site as much as possible (so that `panic!()` has as low an impact
 332 /// on (e.g.) the inlining of other functions as possible), by moving
 333 /// the actual formatting into this shared place.
 334 #[unstable(feature = "libstd_sys_internals",
 335            reason = "used by the panic! macro",
 336            issue = "0")]
 337 #[inline(never)] #[cold]
 338 pub fn begin_panic_fmt(msg: &fmt::Arguments,
 339                        file_line_col: &(&'static str, u32, u32)) -> ! {
 340     use fmt::Write;
 341
 342     // We do two allocations here, unfortunately. But (a) they're
 343     // required with the current scheme, and (b) we don't handle
 344     // panic + OOM properly anyway (see comment in begin_panic
 345     // below).
 346
 347     let mut s = String::new();
 348     let _ = s.write_fmt(*msg);
 349     rust_panic_with_hook(Box::new(s), Some(msg), file_line_col)
 350 }
 351
 352 /// This is the entry point of panicking for panic!() and assert!().
 353 #[unstable(feature = "libstd_sys_internals",
 354            reason = "used by the panic! macro",
 355            issue = "0")]
 356 #[inline(never)] #[cold] // avoid code bloat at the call sites as much as possible
 357 pub fn begin_panic<M: Any + Send>(msg: M, file_line_col: &(&'static str, u32, u32)) -> ! {
 358     // Note that this should be the only allocation performed in this code path.
 359     // Currently this means that panic!() on OOM will invoke this code path,
 360     // but then again we're not really ready for panic on OOM anyway. If
 361     // we do start doing this, then we should propagate this allocation to
 362     // be performed in the parent of this thread instead of the thread that's
 363     // panicking.
 364
 365     rust_panic_with_hook(Box::new(msg), None, file_line_col)
 366 }
 367
 368 /// Executes the primary logic for a panic, including checking for recursive
 369 /// panics and panic hooks.
 370 ///
 371 /// This is the entry point or panics from libcore, formatted panics, and
 372 /// `Box<Any>` panics. Here we'll verify that we're not panicking recursively,
 373 /// run panic hooks, and then delegate to the actual implementation of panics.
 374 #[inline(never)]
 375 #[cold]
 376 fn rust_panic_with_hook(payload: Box<Any + Send>,
 377                         message: Option<&fmt::Arguments>,
 378                         file_line_col: &(&'static str, u32, u32)) -> ! {
 379     let (file, line, col) = *file_line_col;
 380
 381     let panics = update_panic_count(1);
 382
 383     // If this is the third nested call (e.g. panics == 2, this is 0-indexed),
 384     // the panic hook probably triggered the last panic, otherwise the
 385     // double-panic check would have aborted the process. In this case abort the
 386     // process real quickly as we don't want to try calling it again as it'll
 387     // probably just panic again.
 388     if panics > 2 {
 389         util::dumb_print(format_args!("thread panicked while processing \
 390                                        panic. aborting.\n"));
 391         unsafe { intrinsics::abort() }
 392     }
 393
 394     unsafe {
 395         let info = PanicInfo::internal_constructor(
 396             &*payload,
 397             message,
 398             Location::internal_constructor(file, line, col),
 399         );
 400         HOOK_LOCK.read();
 401         match HOOK {
 402             Hook::Default => default_hook(&info),
 403             Hook::Custom(ptr) => (*ptr)(&info),
 404         }
 405         HOOK_LOCK.read_unlock();
 406     }
 407
 408     if panics > 1 {
 409         // If a thread panics while it's already unwinding then we
 410         // have limited options. Currently our preference is to
 411         // just abort. In the future we may consider resuming
 412         // unwinding or otherwise exiting the thread cleanly.
 413         util::dumb_print(format_args!("thread panicked while panicking. \
 414                                        aborting.\n"));
 415         unsafe { intrinsics::abort() }
 416     }
 417
 418     rust_panic(payload)
 419 }
 420
 421 /// Shim around rust_panic. Called by resume_unwind.
 422 pub fn update_count_then_panic(msg: Box<Any + Send>) -> ! {
 423     update_panic_count(1);
 424     rust_panic(msg)
 425 }
 426
 427 /// A private no-mangle function on which to slap yer breakpoints.
 428 #[no_mangle]
 429 #[allow(private_no_mangle_fns)] // yes we get it, but we like breakpoints
 430 pub fn rust_panic(msg: Box<Any + Send>) -> ! {
 431     let code = unsafe {
 432         let obj = mem::transmute::<_, raw::TraitObject>(msg);
 433         __rust_start_panic(obj.data as usize, obj.vtable as usize)
 434     };
 435     rtabort!("failed to initiate panic, error {}", code)
 436 }