library/std/src/sync/poison.rs

   1 use crate::error::Error;
   2 use crate::fmt;
   3 use crate::sync::atomic::{AtomicBool, Ordering};
   4 use crate::thread;
   5
   6 pub struct Flag {
   7     failed: AtomicBool,
   8 }
   9
  10 // Note that the Ordering uses to access the `failed` field of `Flag` below is
  11 // always `Relaxed`, and that's because this isn't actually protecting any data,
  12 // it's just a flag whether we've panicked or not.
  13 //
  14 // The actual location that this matters is when a mutex is **locked** which is
  15 // where we have external synchronization ensuring that we see memory
  16 // reads/writes to this flag.
  17 //
  18 // As a result, if it matters, we should see the correct value for `failed` in
  19 // all cases.
  20
  21 impl Flag {
  22     pub const fn new() -> Flag {
  23         Flag { failed: AtomicBool::new(false) }
  24     }
  25
  26     #[inline]
  27     pub fn borrow(&self) -> LockResult<Guard> {
  28         let ret = Guard { panicking: thread::panicking() };
  29         if self.get() { Err(PoisonError::new(ret)) } else { Ok(ret) }
  30     }
  31
  32     #[inline]
  33     pub fn done(&self, guard: &Guard) {
  34         if !guard.panicking && thread::panicking() {
  35             self.failed.store(true, Ordering::Relaxed);
  36         }
  37     }
  38
  39     #[inline]
  40     pub fn get(&self) -> bool {
  41         self.failed.load(Ordering::Relaxed)
  42     }
  43 }
  44
  45 pub struct Guard {
  46     panicking: bool,
  47 }
  48
  49 /// A type of error which can be returned whenever a lock is acquired.
  50 ///
  51 /// Both [`Mutex`]es and [`RwLock`]s are poisoned whenever a thread fails while the lock
  52 /// is held. The precise semantics for when a lock is poisoned is documented on
  53 /// each lock, but once a lock is poisoned then all future acquisitions will
  54 /// return this error.
  55 ///
  56 /// # Examples
  57 ///
  58 /// ```
  59 /// use std::sync::{Arc, Mutex};
  60 /// use std::thread;
  61 ///
  62 /// let mutex = Arc::new(Mutex::new(1));
  63 ///
  64 /// // poison the mutex
  65 /// let c_mutex = Arc::clone(&mutex);
  66 /// let _ = thread::spawn(move || {
  67 ///     let mut data = c_mutex.lock().unwrap();
  68 ///     *data = 2;
  69 ///     panic!();
  70 /// }).join();
  71 ///
  72 /// match mutex.lock() {
  73 ///     Ok(_) => unreachable!(),
  74 ///     Err(p_err) => {
  75 ///         let data = p_err.get_ref();
  76 ///         println!("recovered: {}", data);
  77 ///     }
  78 /// };
  79 /// ```
  80 /// [`Mutex`]: crate::sync::Mutex
  81 /// [`RwLock`]: crate::sync::RwLock
  82 #[stable(feature = "rust1", since = "1.0.0")]
  83 pub struct PoisonError<T> {
  84     guard: T,
  85 }
  86
  87 /// An enumeration of possible errors associated with a [`TryLockResult`] which
  88 /// can occur while trying to acquire a lock, from the [`try_lock`] method on a
  89 /// [`Mutex`] or the [`try_read`] and [`try_write`] methods on an [`RwLock`].
  90 ///
  91 /// [`try_lock`]: crate::sync::Mutex::try_lock
  92 /// [`try_read`]: crate::sync::RwLock::try_read
  93 /// [`try_write`]: crate::sync::RwLock::try_write
  94 /// [`Mutex`]: crate::sync::Mutex
  95 /// [`RwLock`]: crate::sync::RwLock
  96 #[stable(feature = "rust1", since = "1.0.0")]
  97 pub enum TryLockError<T> {
  98     /// The lock could not be acquired because another thread failed while holding
  99     /// the lock.
 100     #[stable(feature = "rust1", since = "1.0.0")]
 101     Poisoned(#[stable(feature = "rust1", since = "1.0.0")] PoisonError<T>),
 102     /// The lock could not be acquired at this time because the operation would
 103     /// otherwise block.
 104     #[stable(feature = "rust1", since = "1.0.0")]
 105     WouldBlock,
 106 }
 107
 108 /// A type alias for the result of a lock method which can be poisoned.
 109 ///
 110 /// The [`Ok`] variant of this result indicates that the primitive was not
 111 /// poisoned, and the `Guard` is contained within. The [`Err`] variant indicates
 112 /// that the primitive was poisoned. Note that the [`Err`] variant *also* carries
 113 /// the associated guard, and it can be acquired through the [`into_inner`]
 114 /// method.
 115 ///
 116 /// [`into_inner`]: PoisonError::into_inner
 117 #[stable(feature = "rust1", since = "1.0.0")]
 118 pub type LockResult<Guard> = Result<Guard, PoisonError<Guard>>;
 119
 120 /// A type alias for the result of a nonblocking locking method.
 121 ///
 122 /// For more information, see [`LockResult`]. A `TryLockResult` doesn't
 123 /// necessarily hold the associated guard in the [`Err`] type as the lock might not
 124 /// have been acquired for other reasons.
 125 #[stable(feature = "rust1", since = "1.0.0")]
 126 pub type TryLockResult<Guard> = Result<Guard, TryLockError<Guard>>;
 127
 128 #[stable(feature = "rust1", since = "1.0.0")]
 129 impl<T> fmt::Debug for PoisonError<T> {
 130     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
 131         f.debug_struct("PoisonError").finish_non_exhaustive()
 132     }
 133 }
 134
 135 #[stable(feature = "rust1", since = "1.0.0")]
 136 impl<T> fmt::Display for PoisonError<T> {
 137     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
 138         "poisoned lock: another task failed inside".fmt(f)
 139     }
 140 }
 141
 142 #[stable(feature = "rust1", since = "1.0.0")]
 143 impl<T> Error for PoisonError<T> {
 144     #[allow(deprecated)]
 145     fn description(&self) -> &str {
 146         "poisoned lock: another task failed inside"
 147     }
 148 }
 149
 150 impl<T> PoisonError<T> {
 151     /// Creates a `PoisonError`.
 152     ///
 153     /// This is generally created by methods like [`Mutex::lock`](crate::sync::Mutex::lock)
 154     /// or [`RwLock::read`](crate::sync::RwLock::read).
 155     #[stable(feature = "sync_poison", since = "1.2.0")]
 156     pub fn new(guard: T) -> PoisonError<T> {
 157         PoisonError { guard }
 158     }
 159
 160     /// Consumes this error indicating that a lock is poisoned, returning the
 161     /// underlying guard to allow access regardless.
 162     ///
 163     /// # Examples
 164     ///
 165     /// ```
 166     /// use std::collections::HashSet;
 167     /// use std::sync::{Arc, Mutex};
 168     /// use std::thread;
 169     ///
 170     /// let mutex = Arc::new(Mutex::new(HashSet::new()));
 171     ///
 172     /// // poison the mutex
 173     /// let c_mutex = Arc::clone(&mutex);
 174     /// let _ = thread::spawn(move || {
 175     ///     let mut data = c_mutex.lock().unwrap();
 176     ///     data.insert(10);
 177     ///     panic!();
 178     /// }).join();
 179     ///
 180     /// let p_err = mutex.lock().unwrap_err();
 181     /// let data = p_err.into_inner();
 182     /// println!("recovered {} items", data.len());
 183     /// ```
 184     #[stable(feature = "sync_poison", since = "1.2.0")]
 185     pub fn into_inner(self) -> T {
 186         self.guard
 187     }
 188
 189     /// Reaches into this error indicating that a lock is poisoned, returning a
 190     /// reference to the underlying guard to allow access regardless.
 191     #[stable(feature = "sync_poison", since = "1.2.0")]
 192     pub fn get_ref(&self) -> &T {
 193         &self.guard
 194     }
 195
 196     /// Reaches into this error indicating that a lock is poisoned, returning a
 197     /// mutable reference to the underlying guard to allow access regardless.
 198     #[stable(feature = "sync_poison", since = "1.2.0")]
 199     pub fn get_mut(&mut self) -> &mut T {
 200         &mut self.guard
 201     }
 202 }
 203
 204 #[stable(feature = "rust1", since = "1.0.0")]
 205 impl<T> From<PoisonError<T>> for TryLockError<T> {
 206     fn from(err: PoisonError<T>) -> TryLockError<T> {
 207         TryLockError::Poisoned(err)
 208     }
 209 }
 210
 211 #[stable(feature = "rust1", since = "1.0.0")]
 212 impl<T> fmt::Debug for TryLockError<T> {
 213     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
 214         match *self {
 215             TryLockError::Poisoned(..) => "Poisoned(..)".fmt(f),
 216             TryLockError::WouldBlock => "WouldBlock".fmt(f),
 217         }
 218     }
 219 }
 220
 221 #[stable(feature = "rust1", since = "1.0.0")]
 222 impl<T> fmt::Display for TryLockError<T> {
 223     fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
 224         match *self {
 225             TryLockError::Poisoned(..) => "poisoned lock: another task failed inside",
 226             TryLockError::WouldBlock => "try_lock failed because the operation would block",
 227         }
 228         .fmt(f)
 229     }
 230 }
 231
 232 #[stable(feature = "rust1", since = "1.0.0")]
 233 impl<T> Error for TryLockError<T> {
 234     #[allow(deprecated, deprecated_in_future)]
 235     fn description(&self) -> &str {
 236         match *self {
 237             TryLockError::Poisoned(ref p) => p.description(),
 238             TryLockError::WouldBlock => "try_lock failed because the operation would block",
 239         }
 240     }
 241
 242     #[allow(deprecated)]
 243     fn cause(&self) -> Option<&dyn Error> {
 244         match *self {
 245             TryLockError::Poisoned(ref p) => Some(p),
 246             _ => None,
 247         }
 248     }
 249 }
 250
 251 pub fn map_result<T, U, F>(result: LockResult<T>, f: F) -> LockResult<U>
 252 where
 253     F: FnOnce(T) -> U,
 254 {
 255     match result {
 256         Ok(t) => Ok(f(t)),
 257         Err(PoisonError { guard }) => Err(PoisonError::new(f(guard))),
 258     }
 259 }