sgx_tstdc/src/poison.rs

// Copyright (c) 2017 Baidu, Inc. All Rights Reserved.
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use super::thread;
use core::fmt;
use core::convert;
use core::sync::atomic::{AtomicBool, Ordering};

pub struct Flag { failed: AtomicBool }

// Note that the Ordering uses to access the `failed` field of `Flag` below is
// always `Relaxed`, and that's because this isn't actually protecting any data,
// it's just a flag whether we've panicked or not.
//
// The actual location that this matters is when a mutex is **locked** which is
// where we have external synchronization ensuring that we see memory
// reads/writes to this flag.
//
// As a result, if it matters, we should see the correct value for `failed` in
// all cases.
impl Flag {
    pub const fn new() -> Flag {
        Flag { failed: AtomicBool::new(false) }
    }

    #[inline]
    pub fn borrow(&self) -> LockResult<Guard> {
        let ret = Guard { panicking: thread::panicking() };
        if self.get() {
            Err(PoisonError::new(ret))
        } else {
            Ok(ret)
        }
    }

    #[inline]
    pub fn done(&self, guard: &Guard) {
        if !guard.panicking && thread::panicking() {
            self.failed.store(true, Ordering::Relaxed);
        }
    }

    #[inline]
    pub fn get(&self) -> bool {
        self.failed.load(Ordering::Relaxed)
    }
}

pub struct Guard {
    panicking: bool,
}

/// A type of error which can be returned whenever a lock is acquired.
///
/// Both [`SgxMutex`]es and [`SgxRwLock`]s are poisoned whenever a thread fails while the lock
/// is held. The precise semantics for when a lock is poisoned is documented on
/// each lock, but once a lock is poisoned then all future acquisitions will
/// return this error.
pub struct PoisonError<T> {
    guard: T,
}

/// An enumeration of possible errors which can occur while calling the
/// [`try_lock`] method.
pub enum TryLockError<T> {
    /// The lock could not be acquired because another thread failed while holding
    /// the lock.
    Poisoned(PoisonError<T>),
    /// The lock could not be acquired at this time because the operation would
    /// otherwise block.
    WouldBlock,
}

/// A type alias for the result of a lock method which can be poisoned.
///
/// The [`Ok`] variant of this result indicates that the primitive was not
/// poisoned, and the `Guard` is contained within. The [`Err`] variant indicates
/// that the primitive was poisoned. Note that the [`Err`] variant *also* carries
/// the associated guard, and it can be acquired through the [`into_inner`]
/// method.
pub type LockResult<Guard> = Result<Guard, PoisonError<Guard>>;

/// A type alias for the result of a nonblocking locking method.
///
/// For more information, see [`LockResult`]. A `TryLockResult` doesn't
/// necessarily hold the associated guard in the [`Err`] type as the lock may not
/// have been acquired for other reasons.
pub type TryLockResult<Guard> = Result<Guard, TryLockError<Guard>>;

impl<T> fmt::Debug for PoisonError<T> {
    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
        "PoisonError { inner: .. }".fmt(f)
    }
}

impl<T> fmt::Display for PoisonError<T> {
    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
        "poisoned lock: another task failed inside".fmt(f)
    }
}

impl<T> PoisonError<T> {

    /// Creates a `PoisonError`.
    ///
    /// This is generally created by methods like [`SgxMutex::lock`] or [`SgxRwLock::read`].
    ///
    pub fn new(guard: T) -> PoisonError<T> {
        PoisonError { guard: guard }
    }

    /// Consumes this error indicating that a lock is poisoned, returning the
    /// underlying guard to allow access regardless.
    pub fn into_inner(self) -> T { self.guard }

    /// Reaches into this error indicating that a lock is poisoned, returning a
    /// reference to the underlying guard to allow access regardless.
    pub fn get_ref(&self) -> &T { &self.guard }

    /// Reaches into this error indicating that a lock is poisoned, returning a
    /// mutable reference to the underlying guard to allow access regardless.
    pub fn get_mut(&mut self) -> &mut T { &mut self.guard }
}

impl<T> convert::From<PoisonError<T>> for TryLockError<T> {
    fn from(err: PoisonError<T>) -> TryLockError<T> {
        TryLockError::Poisoned(err)
    }
}

impl<T> fmt::Debug for TryLockError<T> {
    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
        match *self {
            TryLockError::Poisoned(..) => "Poisoned(..)".fmt(f),
            TryLockError::WouldBlock => "WouldBlock".fmt(f)
        }
    }
}

impl<T> fmt::Display for TryLockError<T> {
    fn fmt(&self, f: &mut fmt::Formatter) -> fmt::Result {
        match *self {
            TryLockError::Poisoned(..) => "poisoned lock: another task failed inside",
            TryLockError::WouldBlock => "try_lock failed because the operation would block"
        }.fmt(f)
    }
}

pub fn map_result<T, U, F>(result: LockResult<T>, f: F)
                           -> LockResult<U>
                           where F: FnOnce(T) -> U {
    match result {
        Ok(t) => Ok(f(t)),
        Err(PoisonError { guard }) => Err(PoisonError::new(f(guard)))
    }
}