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// Take a look at the license at the top of the repository in the LICENSE file.
use std::cell::RefCell;
use std::ops;
/// Like `Send` but only if we have the unique reference to the object
///
/// Note that implementing this trait has to be done especially careful.
/// It must only be implemented on types where the uniqueness of a reference
/// can be determined, i.e. the reference count field is accessible, and it
/// must only have references itself to other types that are `Send`.
/// `SendUnique` is *not* enough for the other types unless uniqueness of
/// all of them can be guaranteed, which is e.g. not the case if there's a
/// getter for them.
pub unsafe trait SendUnique: 'static {
fn is_unique(&self) -> bool;
}
/// Allows sending reference counted objects that don't implement `Send` to other threads
/// as long as only a single reference to the object exists.
#[derive(Debug)]
pub struct SendUniqueCell<T: SendUnique> {
obj: T,
// Thread id and refcount
thread: RefCell<Option<(usize, usize)>>,
}
unsafe impl<T: SendUnique> Send for SendUniqueCell<T> {}
#[derive(Debug)]
pub struct BorrowError;
impl<T: SendUnique> SendUniqueCell<T> {
/// Create a new `SendUniqueCell` out of `obj`
///
/// Fails if `obj` is not unique at this time
pub fn new(obj: T) -> Result<Self, T> {
if !obj.is_unique() {
return Err(obj);
}
Ok(SendUniqueCell {
obj,
thread: RefCell::new(None),
})
}
/// Borrow the contained object or panic if borrowing
/// is not possible at this time
pub fn borrow(&self) -> Ref<T> {
#[allow(clippy::match_wild_err_arm)]
match self.try_borrow() {
Err(_) => panic!("Can't borrow"),
Ok(r) => r,
}
}
/// Try borrowing the contained object
///
/// Borrowing is possible as long as only a single reference
/// to the object exists, or it is borrowed from the same
/// thread currently
pub fn try_borrow(&self) -> Result<Ref<T>, BorrowError> {
let mut thread = self.thread.borrow_mut();
// If the object is unique, we can borrow it from
// any thread we want and just have to keep track
// how often we borrowed it
if self.obj.is_unique() {
if *thread == None {
*thread = Some((crate::thread_id(), 1));
} else {
thread.as_mut().unwrap().1 += 1;
}
return Ok(Ref(self));
}
// If we don't even know from which thread it is borrowed, this
// means it somehow got borrowed from outside the SendUniqueCell
if *thread == None {
return Err(BorrowError);
}
// If the object is not unique, we can only borrow it
// from the thread that currently has it borrowed
if thread.as_ref().unwrap().0 != crate::thread_id() {
return Err(BorrowError);
}
thread.as_mut().unwrap().1 += 1;
Ok(Ref(self))
}
/// Extract the contained object or panic if it is not possible
/// at this time
pub fn into_inner(self) -> T {
#[allow(clippy::match_wild_err_arm)]
match self.try_into_inner() {
Err(_) => panic!("Can't convert into inner type"),
Ok(obj) => obj,
}
}
/// Try extracting the contained object
///
/// Borrowing is possible as long as only a single reference
/// to the object exists, or it is borrowed from the same
/// thread currently
pub fn try_into_inner(self) -> Result<T, Self> {
if self.try_borrow().is_err() {
Err(self)
} else {
Ok(self.obj)
}
}
}
pub struct Ref<'a, T: SendUnique>(&'a SendUniqueCell<T>);
impl<'a, T: SendUnique> AsRef<T> for Ref<'a, T> {
fn as_ref(&self) -> &T {
&self.0.obj
}
}
impl<'a, T: SendUnique> ops::Deref for Ref<'a, T> {
type Target = T;
fn deref(&self) -> &T {
&self.0.obj
}
}
impl<'a, T: SendUnique> Drop for Ref<'a, T> {
fn drop(&mut self) {
let is_unique = self.0.obj.is_unique();
let mut thread = self.0.thread.borrow_mut();
if is_unique && thread.as_ref().unwrap().1 == 1 {
*thread = None;
} else {
thread.as_mut().unwrap().1 -= 1;
}
}
}