src/libstd/panicking.rs

   1 //! Implementation of various bits and pieces of the `panic!` macro and
   2 //! associated runtime pieces.
   3 //!
   4 //! Specifically, this module contains the implementation of:
   5 //!
   6 //! * Panic hooks
   7 //! * Executing a panic up to doing the actual implementation
   8 //! * Shims around "try"
   9
  10 use core::panic::BoxMeUp;
  11
  12 use io::prelude::*;
  13
  14 use any::Any;
  15 use cell::RefCell;
  16 use core::panic::{PanicInfo, Location};
  17 use fmt;
  18 use intrinsics;
  19 use mem;
  20 use ptr;
  21 use raw;
  22 use sys::stdio::panic_output;
  23 use sys_common::rwlock::RWLock;
  24 use sys_common::thread_info;
  25 use sys_common::util;
  26 use thread;
  27
  28 thread_local! {
  29     pub static LOCAL_STDERR: RefCell<Option<Box<dyn Write + Send>>> = {
  30         RefCell::new(None)
  31     }
  32 }
  33
  34 // Binary interface to the panic runtime that the standard library depends on.
  35 //
  36 // The standard library is tagged with `#![needs_panic_runtime]` (introduced in
  37 // RFC 1513) to indicate that it requires some other crate tagged with
  38 // `#![panic_runtime]` to exist somewhere. Each panic runtime is intended to
  39 // implement these symbols (with the same signatures) so we can get matched up
  40 // to them.
  41 //
  42 // One day this may look a little less ad-hoc with the compiler helping out to
  43 // hook up these functions, but it is not this day!
  44 #[allow(improper_ctypes)]
  45 extern {
  46     fn __rust_maybe_catch_panic(f: fn(*mut u8),
  47                                 data: *mut u8,
  48                                 data_ptr: *mut usize,
  49                                 vtable_ptr: *mut usize) -> u32;
  50     #[unwind(allowed)]
  51     fn __rust_start_panic(payload: usize) -> u32;
  52 }
  53
  54 #[derive(Copy, Clone)]
  55 enum Hook {
  56     Default,
  57     Custom(*mut (dyn Fn(&PanicInfo) + 'static + Sync + Send)),
  58 }
  59
  60 static HOOK_LOCK: RWLock = RWLock::new();
  61 static mut HOOK: Hook = Hook::Default;
  62
  63 /// Registers a custom panic hook, replacing any that was previously registered.
  64 ///
  65 /// The panic hook is invoked when a thread panics, but before the panic runtime
  66 /// is invoked. As such, the hook will run with both the aborting and unwinding
  67 /// runtimes. The default hook prints a message to standard error and generates
  68 /// a backtrace if requested, but this behavior can be customized with the
  69 /// `set_hook` and [`take_hook`] functions.
  70 ///
  71 /// [`take_hook`]: ./fn.take_hook.html
  72 ///
  73 /// The hook is provided with a `PanicInfo` struct which contains information
  74 /// about the origin of the panic, including the payload passed to `panic!` and
  75 /// the source code location from which the panic originated.
  76 ///
  77 /// The panic hook is a global resource.
  78 ///
  79 /// # Panics
  80 ///
  81 /// Panics if called from a panicking thread.
  82 ///
  83 /// # Examples
  84 ///
  85 /// The following will print "Custom panic hook":
  86 ///
  87 /// ```should_panic
  88 /// use std::panic;
  89 ///
  90 /// panic::set_hook(Box::new(|_| {
  91 ///     println!("Custom panic hook");
  92 /// }));
  93 ///
  94 /// panic!("Normal panic");
  95 /// ```
  96 #[stable(feature = "panic_hooks", since = "1.10.0")]
  97 pub fn set_hook(hook: Box<dyn Fn(&PanicInfo) + 'static + Sync + Send>) {
  98     if thread::panicking() {
  99         panic!("cannot modify the panic hook from a panicking thread");
 100     }
 101
 102     unsafe {
 103         HOOK_LOCK.write();
 104         let old_hook = HOOK;
 105         HOOK = Hook::Custom(Box::into_raw(hook));
 106         HOOK_LOCK.write_unlock();
 107
 108         if let Hook::Custom(ptr) = old_hook {
 109             Box::from_raw(ptr);
 110         }
 111     }
 112 }
 113
 114 /// Unregisters the current panic hook, returning it.
 115 ///
 116 /// *See also the function [`set_hook`].*
 117 ///
 118 /// [`set_hook`]: ./fn.set_hook.html
 119 ///
 120 /// If no custom hook is registered, the default hook will be returned.
 121 ///
 122 /// # Panics
 123 ///
 124 /// Panics if called from a panicking thread.
 125 ///
 126 /// # Examples
 127 ///
 128 /// The following will print "Normal panic":
 129 ///
 130 /// ```should_panic
 131 /// use std::panic;
 132 ///
 133 /// panic::set_hook(Box::new(|_| {
 134 ///     println!("Custom panic hook");
 135 /// }));
 136 ///
 137 /// let _ = panic::take_hook();
 138 ///
 139 /// panic!("Normal panic");
 140 /// ```
 141 #[stable(feature = "panic_hooks", since = "1.10.0")]
 142 pub fn take_hook() -> Box<dyn Fn(&PanicInfo) + 'static + Sync + Send> {
 143     if thread::panicking() {
 144         panic!("cannot modify the panic hook from a panicking thread");
 145     }
 146
 147     unsafe {
 148         HOOK_LOCK.write();
 149         let hook = HOOK;
 150         HOOK = Hook::Default;
 151         HOOK_LOCK.write_unlock();
 152
 153         match hook {
 154             Hook::Default => Box::new(default_hook),
 155             Hook::Custom(ptr) => Box::from_raw(ptr),
 156         }
 157     }
 158 }
 159
 160 fn default_hook(info: &PanicInfo) {
 161     #[cfg(feature = "backtrace")]
 162     use sys_common::backtrace;
 163
 164     // If this is a double panic, make sure that we print a backtrace
 165     // for this panic. Otherwise only print it if logging is enabled.
 166     #[cfg(feature = "backtrace")]
 167     let log_backtrace = {
 168         let panics = update_panic_count(0);
 169
 170         if panics >= 2 {
 171             Some(backtrace::PrintFormat::Full)
 172         } else {
 173             backtrace::log_enabled()
 174         }
 175     };
 176
 177     let location = info.location().unwrap();  // The current implementation always returns Some
 178
 179     let msg = match info.payload().downcast_ref::<&'static str>() {
 180         Some(s) => *s,
 181         None => match info.payload().downcast_ref::<String>() {
 182             Some(s) => &s[..],
 183             None => "Box<Any>",
 184         }
 185     };
 186     let thread = thread_info::current_thread();
 187     let name = thread.as_ref().and_then(|t| t.name()).unwrap_or("<unnamed>");
 188
 189     let write = |err: &mut dyn (::io::Write)| {
 190         let _ = writeln!(err, "thread '{}' panicked at '{}', {}",
 191                          name, msg, location);
 192
 193         #[cfg(feature = "backtrace")]
 194         {
 195             use sync::atomic::{AtomicBool, Ordering};
 196
 197             static FIRST_PANIC: AtomicBool = AtomicBool::new(true);
 198
 199             if let Some(format) = log_backtrace {
 200                 let _ = backtrace::print(err, format);
 201             } else if FIRST_PANIC.compare_and_swap(true, false, Ordering::SeqCst) {
 202                 let _ = writeln!(err, "note: Run with `RUST_BACKTRACE=1` \
 203                                        environment variable to display a backtrace.");
 204             }
 205         }
 206     };
 207
 208     if let Some(mut local) = LOCAL_STDERR.with(|s| s.borrow_mut().take()) {
 209        write(&mut *local);
 210        let mut s = Some(local);
 211        LOCAL_STDERR.with(|slot| {
 212            *slot.borrow_mut() = s.take();
 213        });
 214     } else if let Some(mut out) = panic_output() {
 215         write(&mut out);
 216     }
 217 }
 218
 219
 220 #[cfg(not(test))]
 221 #[doc(hidden)]
 222 #[unstable(feature = "update_panic_count", issue = "0")]
 223 pub fn update_panic_count(amt: isize) -> usize {
 224     use cell::Cell;
 225     thread_local! { static PANIC_COUNT: Cell<usize> = Cell::new(0) }
 226
 227     PANIC_COUNT.with(|c| {
 228         let next = (c.get() as isize + amt) as usize;
 229         c.set(next);
 230         return next
 231     })
 232 }
 233
 234 #[cfg(test)]
 235 pub use realstd::rt::update_panic_count;
 236
 237 /// Invoke a closure, capturing the cause of an unwinding panic if one occurs.
 238 pub unsafe fn try<R, F: FnOnce() -> R>(f: F) -> Result<R, Box<dyn Any + Send>> {
 239     #[allow(unions_with_drop_fields)]
 240     union Data<F, R> {
 241         f: F,
 242         r: R,
 243     }
 244
 245     // We do some sketchy operations with ownership here for the sake of
 246     // performance. We can only  pass pointers down to
 247     // `__rust_maybe_catch_panic` (can't pass objects by value), so we do all
 248     // the ownership tracking here manually using a union.
 249     //
 250     // We go through a transition where:
 251     //
 252     // * First, we set the data to be the closure that we're going to call.
 253     // * When we make the function call, the `do_call` function below, we take
 254     //   ownership of the function pointer. At this point the `Data` union is
 255     //   entirely uninitialized.
 256     // * If the closure successfully returns, we write the return value into the
 257     //   data's return slot. Note that `ptr::write` is used as it's overwriting
 258     //   uninitialized data.
 259     // * Finally, when we come back out of the `__rust_maybe_catch_panic` we're
 260     //   in one of two states:
 261     //
 262     //      1. The closure didn't panic, in which case the return value was
 263     //         filled in. We move it out of `data` and return it.
 264     //      2. The closure panicked, in which case the return value wasn't
 265     //         filled in. In this case the entire `data` union is invalid, so
 266     //         there is no need to drop anything.
 267     //
 268     // Once we stack all that together we should have the "most efficient'
 269     // method of calling a catch panic whilst juggling ownership.
 270     let mut any_data = 0;
 271     let mut any_vtable = 0;
 272     let mut data = Data {
 273         f,
 274     };
 275
 276     let r = __rust_maybe_catch_panic(do_call::<F, R>,
 277                                      &mut data as *mut _ as *mut u8,
 278                                      &mut any_data,
 279                                      &mut any_vtable);
 280
 281     return if r == 0 {
 282         debug_assert!(update_panic_count(0) == 0);
 283         Ok(data.r)
 284     } else {
 285         update_panic_count(-1);
 286         debug_assert!(update_panic_count(0) == 0);
 287         Err(mem::transmute(raw::TraitObject {
 288             data: any_data as *mut _,
 289             vtable: any_vtable as *mut _,
 290         }))
 291     };
 292
 293     fn do_call<F: FnOnce() -> R, R>(data: *mut u8) {
 294         unsafe {
 295             let data = data as *mut Data<F, R>;
 296             let f = ptr::read(&mut (*data).f);
 297             ptr::write(&mut (*data).r, f());
 298         }
 299     }
 300 }
 301
 302 /// Determines whether the current thread is unwinding because of panic.
 303 pub fn panicking() -> bool {
 304     update_panic_count(0) != 0
 305 }
 306
 307 /// Entry point of panic from the libcore crate.
 308 #[cfg(not(test))]
 309 #[panic_handler]
 310 #[unwind(allowed)]
 311 pub fn rust_begin_panic(info: &PanicInfo) -> ! {
 312     continue_panic_fmt(&info)
 313 }
 314
 315 /// The entry point for panicking with a formatted message.
 316 ///
 317 /// This is designed to reduce the amount of code required at the call
 318 /// site as much as possible (so that `panic!()` has as low an impact
 319 /// on (e.g.) the inlining of other functions as possible), by moving
 320 /// the actual formatting into this shared place.
 321 #[unstable(feature = "libstd_sys_internals",
 322            reason = "used by the panic! macro",
 323            issue = "0")]
 324 #[cold]
 325 // If panic_immediate_abort, inline the abort call,
 326 // otherwise avoid inlining because of it is cold path.
 327 #[cfg_attr(not(feature="panic_immediate_abort"),inline(never))]
 328 #[cfg_attr(    feature="panic_immediate_abort" ,inline)]
 329 pub fn begin_panic_fmt(msg: &fmt::Arguments,
 330                        file_line_col: &(&'static str, u32, u32)) -> ! {
 331     if cfg!(feature = "panic_immediate_abort") {
 332         unsafe { intrinsics::abort() }
 333     }
 334
 335     let (file, line, col) = *file_line_col;
 336     let info = PanicInfo::internal_constructor(
 337         Some(msg),
 338         Location::internal_constructor(file, line, col),
 339     );
 340     continue_panic_fmt(&info)
 341 }
 342
 343 fn continue_panic_fmt(info: &PanicInfo) -> ! {
 344     struct PanicPayload<'a> {
 345         inner: &'a fmt::Arguments<'a>,
 346         string: Option<String>,
 347     }
 348
 349     impl<'a> PanicPayload<'a> {
 350         fn new(inner: &'a fmt::Arguments<'a>) -> PanicPayload<'a> {
 351             PanicPayload { inner, string: None }
 352         }
 353
 354         fn fill(&mut self) -> &mut String {
 355             use fmt::Write;
 356
 357             let inner = self.inner;
 358             self.string.get_or_insert_with(|| {
 359                 let mut s = String::new();
 360                 drop(s.write_fmt(*inner));
 361                 s
 362             })
 363         }
 364     }
 365
 366     unsafe impl<'a> BoxMeUp for PanicPayload<'a> {
 367         fn box_me_up(&mut self) -> *mut (dyn Any + Send) {
 368             let contents = mem::replace(self.fill(), String::new());
 369             Box::into_raw(Box::new(contents))
 370         }
 371
 372         fn get(&mut self) -> &(dyn Any + Send) {
 373             self.fill()
 374         }
 375     }
 376
 377     // We do two allocations here, unfortunately. But (a) they're
 378     // required with the current scheme, and (b) we don't handle
 379     // panic + OOM properly anyway (see comment in begin_panic
 380     // below).
 381
 382     let loc = info.location().unwrap(); // The current implementation always returns Some
 383     let msg = info.message().unwrap(); // The current implementation always returns Some
 384     let file_line_col = (loc.file(), loc.line(), loc.column());
 385     rust_panic_with_hook(
 386         &mut PanicPayload::new(msg),
 387         info.message(),
 388         &file_line_col);
 389 }
 390
 391 /// This is the entry point of panicking for panic!() and assert!().
 392 #[unstable(feature = "libstd_sys_internals",
 393            reason = "used by the panic! macro",
 394            issue = "0")]
 395 #[cfg_attr(not(test), lang = "begin_panic")]
 396 // never inline unless panic_immediate_abort to avoid code
 397 // bloat at the call sites as much as possible
 398 #[cfg_attr(not(feature="panic_immediate_abort"),inline(never))]
 399 #[cold]
 400 pub fn begin_panic<M: Any + Send>(msg: M, file_line_col: &(&'static str, u32, u32)) -> ! {
 401     if cfg!(feature = "panic_immediate_abort") {
 402         unsafe { intrinsics::abort() }
 403     }
 404
 405     // Note that this should be the only allocation performed in this code path.
 406     // Currently this means that panic!() on OOM will invoke this code path,
 407     // but then again we're not really ready for panic on OOM anyway. If
 408     // we do start doing this, then we should propagate this allocation to
 409     // be performed in the parent of this thread instead of the thread that's
 410     // panicking.
 411
 412     rust_panic_with_hook(&mut PanicPayload::new(msg), None, file_line_col);
 413
 414     struct PanicPayload<A> {
 415         inner: Option<A>,
 416     }
 417
 418     impl<A: Send + 'static> PanicPayload<A> {
 419         fn new(inner: A) -> PanicPayload<A> {
 420             PanicPayload { inner: Some(inner) }
 421         }
 422     }
 423
 424     unsafe impl<A: Send + 'static> BoxMeUp for PanicPayload<A> {
 425         fn box_me_up(&mut self) -> *mut (dyn Any + Send) {
 426             let data = match self.inner.take() {
 427                 Some(a) => Box::new(a) as Box<dyn Any + Send>,
 428                 None => Box::new(()),
 429             };
 430             Box::into_raw(data)
 431         }
 432
 433         fn get(&mut self) -> &(dyn Any + Send) {
 434             match self.inner {
 435                 Some(ref a) => a,
 436                 None => &(),
 437             }
 438         }
 439     }
 440 }
 441
 442 /// Central point for dispatching panics.
 443 ///
 444 /// Executes the primary logic for a panic, including checking for recursive
 445 /// panics, panic hooks, and finally dispatching to the panic runtime to either
 446 /// abort or unwind.
 447 fn rust_panic_with_hook(payload: &mut dyn BoxMeUp,
 448                         message: Option<&fmt::Arguments>,
 449                         file_line_col: &(&str, u32, u32)) -> ! {
 450     let (file, line, col) = *file_line_col;
 451
 452     let panics = update_panic_count(1);
 453
 454     // If this is the third nested call (e.g., panics == 2, this is 0-indexed),
 455     // the panic hook probably triggered the last panic, otherwise the
 456     // double-panic check would have aborted the process. In this case abort the
 457     // process real quickly as we don't want to try calling it again as it'll
 458     // probably just panic again.
 459     if panics > 2 {
 460         util::dumb_print(format_args!("thread panicked while processing \
 461                                        panic. aborting.\n"));
 462         unsafe { intrinsics::abort() }
 463     }
 464
 465     unsafe {
 466         let mut info = PanicInfo::internal_constructor(
 467             message,
 468             Location::internal_constructor(file, line, col),
 469         );
 470         HOOK_LOCK.read();
 471         match HOOK {
 472             // Some platforms know that printing to stderr won't ever actually
 473             // print anything, and if that's the case we can skip the default
 474             // hook.
 475             Hook::Default if panic_output().is_none() => {}
 476             Hook::Default => {
 477                 info.set_payload(payload.get());
 478                 default_hook(&info);
 479             }
 480             Hook::Custom(ptr) => {
 481                 info.set_payload(payload.get());
 482                 (*ptr)(&info);
 483             }
 484         };
 485         HOOK_LOCK.read_unlock();
 486     }
 487
 488     if panics > 1 {
 489         // If a thread panics while it's already unwinding then we
 490         // have limited options. Currently our preference is to
 491         // just abort. In the future we may consider resuming
 492         // unwinding or otherwise exiting the thread cleanly.
 493         util::dumb_print(format_args!("thread panicked while panicking. \
 494                                        aborting.\n"));
 495         unsafe { intrinsics::abort() }
 496     }
 497
 498     rust_panic(payload)
 499 }
 500
 501 /// Shim around rust_panic. Called by resume_unwind.
 502 pub fn update_count_then_panic(msg: Box<dyn Any + Send>) -> ! {
 503     update_panic_count(1);
 504
 505     struct RewrapBox(Box<dyn Any + Send>);
 506
 507     unsafe impl BoxMeUp for RewrapBox {
 508         fn box_me_up(&mut self) -> *mut (dyn Any + Send) {
 509             Box::into_raw(mem::replace(&mut self.0, Box::new(())))
 510         }
 511
 512         fn get(&mut self) -> &(dyn Any + Send) {
 513             &*self.0
 514         }
 515     }
 516
 517     rust_panic(&mut RewrapBox(msg))
 518 }
 519
 520 /// An unmangled function (through `rustc_std_internal_symbol`) on which to slap
 521 /// yer breakpoints.
 522 #[inline(never)]
 523 #[cfg_attr(not(test), rustc_std_internal_symbol)]
 524 fn rust_panic(mut msg: &mut dyn BoxMeUp) -> ! {
 525     let code = unsafe {
 526         let obj = &mut msg as *mut &mut dyn BoxMeUp;
 527         __rust_start_panic(obj as usize)
 528     };
 529     rtabort!("failed to initiate panic, error {}", code)
 530 }