| use crate::cell::Cell; |
| use crate::ptr; |
| use crate::sync::Arc; |
| use crate::sys_common; |
| use crate::sys_common::mutex::Mutex; |
| |
| pub struct Lazy<T> { |
| // We never call `lock.init()`, so it is UB to attempt to acquire this mutex reentrantly! |
| lock: Mutex, |
| ptr: Cell<*mut Arc<T>>, |
| } |
| |
| #[inline] |
| const fn done<T>() -> *mut Arc<T> { 1_usize as *mut _ } |
| |
| unsafe impl<T> Sync for Lazy<T> {} |
| |
| impl<T> Lazy<T> { |
| pub const fn new() -> Lazy<T> { |
| Lazy { |
| lock: Mutex::new(), |
| ptr: Cell::new(ptr::null_mut()), |
| } |
| } |
| } |
| |
| impl<T: Send + Sync + 'static> Lazy<T> { |
| /// Safety: `init` must not call `get` on the variable that is being |
| /// initialized. |
| pub unsafe fn get(&'static self, init: fn() -> Arc<T>) -> Option<Arc<T>> { |
| let _guard = self.lock.lock(); |
| let ptr = self.ptr.get(); |
| if ptr.is_null() { |
| Some(self.init(init)) |
| } else if ptr == done() { |
| None |
| } else { |
| Some((*ptr).clone()) |
| } |
| } |
| |
| // Must only be called with `lock` held |
| unsafe fn init(&'static self, init: fn() -> Arc<T>) -> Arc<T> { |
| // If we successfully register an at exit handler, then we cache the |
| // `Arc` allocation in our own internal box (it will get deallocated by |
| // the at exit handler). Otherwise we just return the freshly allocated |
| // `Arc`. |
| let registered = sys_common::at_exit(move || { |
| let ptr = { |
| let _guard = self.lock.lock(); |
| self.ptr.replace(done()) |
| }; |
| drop(Box::from_raw(ptr)) |
| }); |
| // This could reentrantly call `init` again, which is a problem |
| // because our `lock` allows reentrancy! |
| // That's why `get` is unsafe and requires the caller to ensure no reentrancy happens. |
| let ret = init(); |
| if registered.is_ok() { |
| self.ptr.set(Box::into_raw(Box::new(ret.clone()))); |
| } |
| ret |
| } |
| } |