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add support for platforms without atomic ptr #248

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add support for platforms without atomic ptr #248

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137 changes: 137 additions & 0 deletions src/race.rs
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Expand Up @@ -54,6 +54,7 @@ impl OnceNonZeroUsize {
///
/// Returns `Ok(())` if the cell was empty and `Err(())` if it was
/// full.
#[cfg(target_has_atomic = "ptr")]
#[inline]
pub fn set(&self, value: NonZeroUsize) -> Result<(), ()> {
let exchange =
Expand All @@ -64,6 +65,21 @@ impl OnceNonZeroUsize {
}
}

/// Sets the contents of this cell to `value`.
///
/// Returns `Ok(())` if the cell was empty and `Err(())` if it was
/// full.
#[cfg(not(target_has_atomic = "ptr"))]
#[inline]
pub fn set(&self, value: NonZeroUsize) -> Result<(), ()> {
if self.inner.load(Ordering::Acquire) == 0 {
self.inner.store(value.get(), Ordering::Release);
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Hm, this feels incorrect to me.

Consider threads A, B, C and the following sequence of events:

   A   |   B     |   C
       |         |
load 0 |         |
       | load 0  |
       | store 1 |
       |         | load 1
store 2|         |
       |         | load 2

Here, A and B race when initializing the inner, so C observes two different values of once cell, which violates the invariant that makes the whole construct a safe abstraction.

Ok(())
} else {
Err(())
}
}

/// Gets the contents of the cell, initializing it with `f` if the cell was
/// empty.
///
Expand All @@ -88,6 +104,7 @@ impl OnceNonZeroUsize {
/// If several threads concurrently run `get_or_init`, more than one `f` can
/// be called. However, all threads will return the same value, produced by
/// some `f`.
#[cfg(target_has_atomic = "ptr")]
pub fn get_or_try_init<F, E>(&self, f: F) -> Result<NonZeroUsize, E>
where
F: FnOnce() -> Result<NonZeroUsize, E>,
Expand All @@ -107,6 +124,35 @@ impl OnceNonZeroUsize {
};
Ok(res)
}

/// Gets the contents of the cell, initializing it with `f` if
/// the cell was empty. If the cell was empty and `f` failed, an
/// error is returned.
///
/// If several threads concurrently run `get_or_init`, more than one `f` can
/// be called. However, all threads will return the same value, produced by
/// some `f`.
#[cfg(not(target_has_atomic = "ptr"))]
pub fn get_or_try_init<F, E>(&self, f: F) -> Result<NonZeroUsize, E>
where
F: FnOnce() -> Result<NonZeroUsize, E>,
{
let val = self.inner.load(Ordering::Acquire);
let res = match NonZeroUsize::new(val) {
Some(it) => it,
None => {
let mut val = f()?.get();
let old = self.inner.load(Ordering::Acquire);
if old == 0 {
self.inner.store(val, Ordering::Release);
} else {
val = old;
}
unsafe { NonZeroUsize::new_unchecked(val) }
}
};
Ok(res)
}
}

/// A thread-safe cell which can be written to only once.
Expand Down Expand Up @@ -212,6 +258,7 @@ impl<'a, T> OnceRef<'a, T> {
///
/// Returns `Ok(())` if the cell was empty and `Err(value)` if it was
/// full.
#[cfg(target_has_atomic = "ptr")]
pub fn set(&self, value: &'a T) -> Result<(), ()> {
let ptr = value as *const T as *mut T;
let exchange =
Expand All @@ -222,6 +269,21 @@ impl<'a, T> OnceRef<'a, T> {
}
}

/// Sets the contents of this cell to `value`.
///
/// Returns `Ok(())` if the cell was empty and `Err(value)` if it was
/// full.
#[cfg(not(target_has_atomic = "ptr"))]
pub fn set(&self, value: &'a T) -> Result<(), ()> {
let ptr = value as *const T as *mut T;
if self.inner.load(Ordering::Acquire) == ptr::null_mut() {
self.inner.store(ptr, Ordering::Release);
Ok(())
} else {
Err(())
}
}

/// Gets the contents of the cell, initializing it with `f` if the cell was
/// empty.
///
Expand All @@ -246,6 +308,7 @@ impl<'a, T> OnceRef<'a, T> {
/// If several threads concurrently run `get_or_init`, more than one `f` can
/// be called. However, all threads will return the same value, produced by
/// some `f`.
#[cfg(target_has_atomic = "ptr")]
pub fn get_or_try_init<F, E>(&self, f: F) -> Result<&'a T, E>
where
F: FnOnce() -> Result<&'a T, E>,
Expand All @@ -269,6 +332,34 @@ impl<'a, T> OnceRef<'a, T> {
Ok(unsafe { &*ptr })
}

/// Gets the contents of the cell, initializing it with `f` if
/// the cell was empty. If the cell was empty and `f` failed, an
/// error is returned.
///
/// If several threads concurrently run `get_or_init`, more than one `f` can
/// be called. However, all threads will return the same value, produced by
/// some `f`.
#[cfg(not(target_has_atomic = "ptr"))]
pub fn get_or_try_init<F, E>(&self, f: F) -> Result<&'a T, E>
where
F: FnOnce() -> Result<&'a T, E>,
{
let mut ptr = self.inner.load(Ordering::Acquire);

if ptr.is_null() {
// TODO replace with `cast_mut` when MSRV reaches 1.65.0 (also in `set`)
ptr = f()? as *const T as *mut T;
let old = self.inner.load(Ordering::Acquire);
if old == ptr::null_mut() {
self.inner.store(ptr, Ordering::Release);
} else {
ptr = old;
}
}

Ok(unsafe { &*ptr })
}

/// ```compile_fail
/// use once_cell::race::OnceRef;
///
Expand Down Expand Up @@ -343,6 +434,7 @@ mod once_box {
///
/// Returns `Ok(())` if the cell was empty and `Err(value)` if it was
/// full.
#[cfg(target_has_atomic = "ptr")]
pub fn set(&self, value: Box<T>) -> Result<(), Box<T>> {
let ptr = Box::into_raw(value);
let exchange = self.inner.compare_exchange(
Expand All @@ -358,6 +450,22 @@ mod once_box {
Ok(())
}

/// Sets the contents of this cell to `value`.
///
/// Returns `Ok(())` if the cell was empty and `Err(value)` if it was
/// full.
#[cfg(not(target_has_atomic = "ptr"))]
pub fn set(&self, value: Box<T>) -> Result<(), Box<T>> {
let ptr = Box::into_raw(value);
if self.inner.load(Ordering::Acquire) == ptr::null_mut() {
self.inner.store(ptr, Ordering::Release);
Ok(())
} else {
let value = unsafe { Box::from_raw(ptr) };
Err(value)
}
}

/// Gets the contents of the cell, initializing it with `f` if the cell was
/// empty.
///
Expand All @@ -382,6 +490,7 @@ mod once_box {
/// If several threads concurrently run `get_or_init`, more than one `f` can
/// be called. However, all threads will return the same value, produced by
/// some `f`.
#[cfg(target_has_atomic = "ptr")]
pub fn get_or_try_init<F, E>(&self, f: F) -> Result<&T, E>
where
F: FnOnce() -> Result<Box<T>, E>,
Expand All @@ -404,6 +513,34 @@ mod once_box {
};
Ok(unsafe { &*ptr })
}

/// Gets the contents of the cell, initializing it with `f` if
/// the cell was empty. If the cell was empty and `f` failed, an
/// error is returned.
///
/// If several threads concurrently run `get_or_init`, more than one `f` can
/// be called. However, all threads will return the same value, produced by
/// some `f`.
#[cfg(not(target_has_atomic = "ptr"))]
pub fn get_or_try_init<F, E>(&self, f: F) -> Result<&T, E>
where
F: FnOnce() -> Result<Box<T>, E>,
{
let mut ptr = self.inner.load(Ordering::Acquire);

if ptr.is_null() {
let val = f()?;
ptr = Box::into_raw(val);
let old = self.inner.load(Ordering::Acquire);
if old == ptr::null_mut() {
self.inner.store(ptr, Ordering::Release);
} else {
drop(unsafe { Box::from_raw(ptr) });
ptr = old;
}
};
Ok(unsafe { &*ptr })
}
}

unsafe impl<T: Sync + Send> Sync for OnceBox<T> {}
Expand Down