shadow_shim_helper_rs/rootedcell/rc.rs
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use std::{
cell::{Cell, UnsafeCell},
ptr::NonNull,
};
use crate::explicit_drop::ExplicitDrop;
use super::{Root, Tag};
struct RootedRcInternal<T> {
val: UnsafeCell<Option<T>>,
strong_count: Cell<u32>,
weak_count: Cell<u32>,
}
impl<T> RootedRcInternal<T> {
pub fn new(val: T) -> Self {
Self {
val: UnsafeCell::new(Some(val)),
strong_count: Cell::new(1),
weak_count: Cell::new(0),
}
}
pub fn inc_strong(&self) {
self.strong_count.set(self.strong_count.get() + 1)
}
pub fn dec_strong(&self) {
self.strong_count.set(self.strong_count.get() - 1)
}
pub fn inc_weak(&self) {
self.weak_count.set(self.weak_count.get() + 1)
}
pub fn dec_weak(&self) {
self.weak_count.set(self.weak_count.get() - 1)
}
}
enum RefType {
Weak,
Strong,
}
// Shared implementation for strong and weak references
struct RootedRcCommon<T> {
tag: Tag,
internal: Option<NonNull<RootedRcInternal<T>>>,
}
impl<T> RootedRcCommon<T> {
pub fn new(root: &Root, val: T) -> Self {
Self {
tag: root.tag(),
internal: Some(
NonNull::new(Box::into_raw(Box::new(RootedRcInternal::new(val)))).unwrap(),
),
}
}
// Validates that no other thread currently has access to self.internal, and
// return a reference to it.
pub fn borrow_internal(&self, root: &Root) -> &RootedRcInternal<T> {
assert_eq!(
root.tag, self.tag,
"Tried using root {:?} instead of {:?}",
root.tag, self.tag
);
// SAFETY:
// * Holding a reference to `root` proves no other threads can currently
// access `self.internal`.
// * `self.internal` is accessible since we hold a strong reference.
unsafe { self.internal.unwrap().as_ref() }
}
/// Decrement the reference. If this was the last strong reference, return the value.
pub fn safely_drop(mut self, root: &Root, t: RefType) -> Option<T> {
let internal: &RootedRcInternal<T> = self.borrow_internal(root);
match t {
RefType::Weak => internal.dec_weak(),
RefType::Strong => internal.dec_strong(),
};
let strong_count = internal.strong_count.get();
let weak_count = internal.weak_count.get();
// If there are no more strong references, prepare to drop the value.
// If the value was already dropped (e.g. because we're now dropping a
// weak reference after all the strong refs were already dropped), this
// is a no-op.
let val: Option<T> = if strong_count == 0 {
// SAFETY: Since no strong references remain, nothing else can be
// referencing the internal value.
unsafe { internal.val.get().as_mut().unwrap().take() }
} else {
None
};
// Clear `self.internal`, so that `drop` knows that `safely_drop` ran.
let internal: NonNull<RootedRcInternal<T>> = self.internal.take().unwrap();
// If there are neither strong nor weak references, drop `internal` itself.
if strong_count == 0 && weak_count == 0 {
// SAFETY: We know the pointer is still valid since we had the last
// reference, and since the counts are now zero, there can be no
// other references.
drop(unsafe { Box::from_raw(internal.as_ptr()) });
}
val
}
pub fn clone(&self, root: &Root, t: RefType) -> Self {
let internal: &RootedRcInternal<T> = self.borrow_internal(root);
match t {
RefType::Weak => internal.inc_weak(),
RefType::Strong => internal.inc_strong(),
};
Self {
tag: self.tag,
internal: self.internal,
}
}
}
impl<T> Drop for RootedRcCommon<T> {
#[inline]
fn drop(&mut self) {
if self.internal.is_some() {
// `explicit_drop` is the public interface for the internal `safely_drop`
log::error!("Dropped without calling `explicit_drop`");
// We *can* continue without violating Rust safety properties; the
// underlying object will just be leaked, since the ref count will
// never reach zero.
//
// If we're not already panicking, it's useful to panic here to make
// the leak more visible.
//
// If we are already panicking though, that may already explain how
// a call to `safely_drop` got skipped, and panicking again would
// just obscure the original panic.
#[cfg(debug_assertions)]
if !std::thread::panicking() {
panic!("Dropped without calling `explicit_drop`");
}
}
}
}
// SAFETY: RootedRcCommon ensures that its internals can only be accessed when
// the Root is held by the current thread, effectively synchronizing the
// reference count.
unsafe impl<T: Sync + Send> Send for RootedRcCommon<T> {}
unsafe impl<T: Sync + Send> Sync for RootedRcCommon<T> {}
/// Analagous to [std::rc::Rc]. In particular like [std::rc::Rc] and unlike
/// [std::sync::Arc], it doesn't perform any atomic operations internally,
/// making it relatively inexpensive
///
/// Unlike [std::rc::Rc], this type [Send] and [Sync] if `T` is. This is safe because
/// the owner is required to prove ownership of the associated [Root]
/// to perform any sensitive operations.
///
/// Instances must be destroyed explicitly, using [`RootedRc::explicit_drop`],
/// [`RootedRc::explicit_drop_recursive`], or [`RootedRc::into_inner`]. These
/// validate that the [Root] is held before manipulating reference counts, etc.
///
/// Dropping `RootedRc` without calling one of these methods results in a
/// `panic` in debug builds, or leaking the object in release builds.
pub struct RootedRc<T> {
common: RootedRcCommon<T>,
}
impl<T> RootedRc<T> {
/// Creates a new object associated with `root`.
#[inline]
pub fn new(root: &Root, val: T) -> Self {
Self {
common: RootedRcCommon::new(root, val),
}
}
/// Create a weak reference.
///
/// We use fully qualified syntax here for consistency with Rc and Arc and
/// to avoid name conflicts with `T`'s methods.
#[inline]
pub fn downgrade(this: &Self, root: &Root) -> RootedRcWeak<T> {
RootedRcWeak {
common: this.common.clone(root, RefType::Weak),
}
}
/// Like [Clone::clone], but requires that the corresponding Root is held.
///
/// Intentionally named clone to shadow Self::deref()::clone().
///
/// Panics if `root` is not the associated [Root].
#[inline]
pub fn clone(&self, root: &Root) -> Self {
Self {
common: self.common.clone(root, RefType::Strong),
}
}
/// Drop the `RootedRc`, and return the inner value if this was the last
/// strong reference.
#[inline]
pub fn into_inner(this: Self, root: &Root) -> Option<T> {
this.common.safely_drop(root, RefType::Strong)
}
/// Drops `self`, and if `self` was the last strong reference, call
/// `ExplicitDrop::explicit_drop` on the internal value.
pub fn explicit_drop_recursive(
self,
root: &Root,
param: &T::ExplicitDropParam,
) -> Option<T::ExplicitDropResult>
where
T: ExplicitDrop,
{
Self::into_inner(self, root).map(|val| val.explicit_drop(param))
}
}
impl<T> ExplicitDrop for RootedRc<T> {
type ExplicitDropParam = Root;
type ExplicitDropResult = ();
/// If T itself implements `ExplicitDrop`, consider
/// `RootedRc::explicit_drop_recursive` instead to call it when dropping the
/// last strong reference.
fn explicit_drop(self, root: &Self::ExplicitDropParam) -> Self::ExplicitDropResult {
self.common.safely_drop(root, RefType::Strong);
}
}
impl<T> std::ops::Deref for RootedRc<T> {
type Target = T;
#[inline]
fn deref(&self) -> &Self::Target {
// No need to require a reference to `Root` here since we're not
// touching the counts, only the value itself, which we already required
// to be Sync and Send for RootedRc<T> to be Sync and Send.
// SAFETY: Pointer to `internal` is valid, since we hold a strong ref.
let internal = unsafe { self.common.internal.unwrap().as_ref() };
// SAFETY: Since we hold a strong ref, we know that `val` is valid, and
// that there are no mutable references to it. (The only time we create
// a mutable reference is to drop the T value when the strong ref count
// reaches zero)
let val = unsafe { &*internal.val.get() };
val.as_ref().unwrap()
}
}
#[cfg(test)]
mod test_rooted_rc {
use std::{sync::Arc, thread};
use super::*;
#[test]
fn construct_and_drop() {
let root = Root::new();
let rc = RootedRc::new(&root, 0);
rc.explicit_drop(&root)
}
#[test]
#[cfg(debug_assertions)]
#[should_panic]
fn drop_without_lock_panics_with_debug_assertions() {
let root = Root::new();
drop(RootedRc::new(&root, 0));
}
#[test]
#[cfg(not(debug_assertions))]
fn drop_without_lock_leaks_without_debug_assertions() {
let root = Root::new();
let rc = std::rc::Rc::new(());
let rrc = RootedRc::new(&root, rc.clone());
drop(rrc);
// Because we didn't call `explicit_drop`, RootedRc can't safely call the
// inner rc's Drop. Instead of panicking, we just leak it.
assert_eq!(std::rc::Rc::strong_count(&rc), 2);
}
#[test]
fn send_to_worker_thread() {
let root = Root::new();
let rc = RootedRc::new(&root, 0);
thread::spawn(move || {
// Can access immutably
let _ = *rc + 2;
// Need to explicitly drop, since it mutates refcount.
rc.explicit_drop(&root)
})
.join()
.unwrap();
}
#[test]
fn send_to_worker_thread_and_retrieve() {
let root = Root::new();
let root = thread::spawn(move || {
let rc = RootedRc::new(&root, 0);
rc.explicit_drop(&root);
root
})
.join()
.unwrap();
let rc = RootedRc::new(&root, 0);
rc.explicit_drop(&root)
}
#[test]
fn clone_to_worker_thread() {
let root = Root::new();
let rc = RootedRc::new(&root, 0);
// Create a clone of rc that we'll pass to worker thread.
let rc_thread = rc.clone(&root);
// Worker takes ownership of rc_thread and root;
// Returns ownership of root.
let root = thread::spawn(move || {
let _ = *rc_thread;
rc_thread.explicit_drop(&root);
root
})
.join()
.unwrap();
// Take the lock to drop rc
rc.explicit_drop(&root);
}
#[test]
fn threads_contend_over_lock() {
let root = Arc::new(std::sync::Mutex::new(Root::new()));
let rc = RootedRc::new(&root.lock().unwrap(), 0);
let threads: Vec<_> = (0..100)
.map(|_| {
// Create a clone of rc that we'll pass to worker thread.
let rc = rc.clone(&root.lock().unwrap());
let root = root.clone();
thread::spawn(move || {
let rootlock = root.lock().unwrap();
let rc2 = rc.clone(&rootlock);
rc.explicit_drop(&rootlock);
rc2.explicit_drop(&rootlock);
})
})
.collect();
for handle in threads {
handle.join().unwrap();
}
rc.explicit_drop(&root.lock().unwrap());
}
#[test]
fn into_inner_recursive() {
let root = Root::new();
let inner = RootedRc::new(&root, ());
let outer1 = RootedRc::new(&root, inner);
let outer2 = outer1.clone(&root);
// Dropping the first outer returns None, since there is still another strong ref.
assert!(RootedRc::into_inner(outer1, &root).is_none());
// Dropping the second outer returns the inner ref.
let inner = RootedRc::into_inner(outer2, &root).unwrap();
// Now we can safely drop the inner ref.
inner.explicit_drop(&root);
}
#[test]
fn explicit_drop() {
let root = Root::new();
let rc = RootedRc::new(&root, ());
rc.explicit_drop(&root);
}
#[test]
fn explicit_drop_recursive() {
// Defining `ExplicitDrop` for `MyOuter` lets us use `RootedRc::explicit_drop_recursive`
// to safely drop the inner `RootedRc` when dropping a `RootedRc<MyOuter>`.
struct MyOuter(RootedRc<()>);
impl ExplicitDrop for MyOuter {
type ExplicitDropParam = Root;
type ExplicitDropResult = ();
fn explicit_drop(self, root: &Self::ExplicitDropParam) -> Self::ExplicitDropResult {
self.0.explicit_drop(root);
}
}
let root = Root::new();
let inner = RootedRc::new(&root, ());
let outer1 = RootedRc::new(&root, MyOuter(inner));
let outer2 = RootedRc::new(&root, MyOuter(outer1.0.clone(&root)));
outer1.explicit_drop_recursive(&root, &root);
outer2.explicit_drop_recursive(&root, &root);
}
}
pub struct RootedRcWeak<T> {
common: RootedRcCommon<T>,
}
impl<T> RootedRcWeak<T> {
#[inline]
pub fn upgrade(&self, root: &Root) -> Option<RootedRc<T>> {
let internal = self.common.borrow_internal(root);
if internal.strong_count.get() == 0 {
return None;
}
Some(RootedRc {
common: self.common.clone(root, RefType::Strong),
})
}
/// Like [Clone::clone], but requires that the corresponding Root is held.
///
/// Intentionally named clone to shadow Self::deref()::clone().
///
/// Panics if `root` is not the associated [Root].
#[inline]
pub fn clone(&self, root: &Root) -> Self {
Self {
common: self.common.clone(root, RefType::Weak),
}
}
}
impl<T> ExplicitDrop for RootedRcWeak<T> {
type ExplicitDropParam = Root;
type ExplicitDropResult = ();
#[inline]
fn explicit_drop(self, root: &Self::ExplicitDropParam) -> Self::ExplicitDropResult {
let val = self.common.safely_drop(root, RefType::Weak);
// Since this isn't a strong reference, this can't be the *last* strong
// reference, so the value should never be returned.
debug_assert!(val.is_none());
}
}
// SAFETY: RootedRc ensures that its internals can only be accessed when the
// Root is held by the current thread, effectively synchronizing the reference
// count.
unsafe impl<T: Sync + Send> Send for RootedRcWeak<T> {}
unsafe impl<T: Sync + Send> Sync for RootedRcWeak<T> {}
#[cfg(test)]
mod test_rooted_rc_weak {
use super::*;
#[test]
fn successful_upgrade() {
let root = Root::new();
let strong = RootedRc::new(&root, 42);
let weak = RootedRc::downgrade(&strong, &root);
let upgraded = weak.upgrade(&root).unwrap();
assert_eq!(*upgraded, *strong);
upgraded.explicit_drop(&root);
weak.explicit_drop(&root);
strong.explicit_drop(&root);
}
#[test]
fn failed_upgrade() {
let root = Root::new();
let strong = RootedRc::new(&root, 42);
let weak = RootedRc::downgrade(&strong, &root);
strong.explicit_drop(&root);
assert!(weak.upgrade(&root).is_none());
weak.explicit_drop(&root);
}
#[test]
#[cfg(debug_assertions)]
#[should_panic]
fn drop_without_lock_panics_with_debug_assertions() {
let root = Root::new();
let strong = RootedRc::new(&root, 42);
drop(RootedRc::downgrade(&strong, &root));
strong.explicit_drop(&root);
}
// Validate that circular references are cleaned up correctly.
#[test]
fn circular_reference() {
std::thread_local! {
static THREAD_ROOT: Root = Root::new();
}
struct MyStruct {
// Circular reference
weak_self: Cell<Option<RootedRcWeak<Self>>>,
}
impl MyStruct {
fn new() -> RootedRc<Self> {
THREAD_ROOT.with(|root| {
let rv = RootedRc::new(
root,
MyStruct {
weak_self: Cell::new(None),
},
);
let weak = RootedRc::downgrade(&rv, root);
rv.weak_self.set(Some(weak));
rv
})
}
}
impl Drop for MyStruct {
fn drop(&mut self) {
let weak = self.weak_self.replace(None).unwrap();
THREAD_ROOT.with(|root| {
weak.explicit_drop(root);
});
}
}
let val = MyStruct::new();
THREAD_ROOT.with(|root| {
val.explicit_drop(root);
})
}
#[test]
#[cfg(not(debug_assertions))]
fn drop_without_lock_doesnt_leak_value() {
let root = Root::new();
let rc = std::rc::Rc::new(());
let strong = RootedRc::new(&root, rc.clone());
drop(RootedRc::downgrade(&strong, &root));
strong.explicit_drop(&root);
// Because we safely dropped all of the strong references,
// the internal std::rc::Rc value should still have been dropped.
// The `internal` field itself will be leaked since the weak count
// never reaches 0.
assert_eq!(std::rc::Rc::strong_count(&rc), 1);
}
}