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use std::cell::UnsafeCell;
use std::mem::ManuallyDrop;
use std::ops::Deref;
use std::ops::DerefMut;
use crate::storage::GlobalTermPoolGuard;
use crate::storage::THREAD_TERM_POOL;
/// Global mutex that prevents garbage collection.
pub struct GcMutex<T> {
inner: UnsafeCell<T>,
}
unsafe impl<T: Send> Send for GcMutex<T> {}
unsafe impl<T: Send> Sync for GcMutex<T> {}
impl<T> GcMutex<T> {
pub fn new(value: T) -> GcMutex<T> {
GcMutex {
inner: UnsafeCell::new(value),
/// Provides mutable access to the underlying value.
pub fn write(&self) -> GcMutexGuard<'_, T> {
GcMutexGuard {
mutex: self,
guard: ManuallyDrop::new(THREAD_TERM_POOL.with_borrow(|tp| unsafe {
std::mem::transmute(tp.term_pool().read_recursive().expect("Lock poisoned!"))
})),
/// Provides immutable access to the underlying value.
pub fn read(&self) -> GcMutexGuard<'_, T> {
pub struct GcMutexGuard<'a, T> {
mutex: &'a GcMutex<T>,
/// Only used to avoid garbage collection, will be released on drop.
guard: ManuallyDrop<GlobalTermPoolGuard<'a>>,
impl<T> Deref for GcMutexGuard<'_, T> {
type Target = T;
fn deref(&self) -> &Self::Target {
unsafe { &*self.mutex.inner.get() }
impl<T> DerefMut for GcMutexGuard<'_, T> {
fn deref_mut(&mut self) -> &mut Self::Target {
// We are the only guard after `write()`, so we can provide mutable access to the underlying object.
unsafe { &mut *self.mutex.inner.get() }
impl<T> Drop for GcMutexGuard<'_, T> {
fn drop(&mut self) {
if self.guard.read_depth() == 1 {
// If this is the last guard, we can trigger garbage collection when it was delayed earlier.
THREAD_TERM_POOL.with_borrow(|tp| unsafe { tp.trigger_delayed_garbage_collection(&mut self.guard) })
} else {
// Just drop the guard
unsafe { ManuallyDrop::drop(&mut self.guard) };