1 files changed, 90 insertions, 0 deletions
diff --git a/library/std/src/sys_common/lazy_box.rs b/library/std/src/sys_common/lazy_box.rs
new file mode 100644
index 000000000..63c3316bd
--- /dev/null
+++ b/library/std/src/sys_common/lazy_box.rs
@@ -0,0 +1,90 @@
+#![allow(dead_code)] // Only used on some platforms.
+
+// This is used to wrap pthread {Mutex, Condvar, RwLock} in.
+
+use crate::marker::PhantomData;
+use crate::ops::{Deref, DerefMut};
+use crate::ptr::null_mut;
+use crate::sync::atomic::{
+    AtomicPtr,
+    Ordering::{AcqRel, Acquire},
+};
+
+pub(crate) struct LazyBox<T: LazyInit> {
+    ptr: AtomicPtr<T>,
+    _phantom: PhantomData<T>,
+}
+
+pub(crate) trait LazyInit {
+    /// This is called before the box is allocated, to provide the value to
+    /// move into the new box.
+    ///
+    /// It might be called more than once per LazyBox, as multiple threads
+    /// might race to initialize it concurrently, each constructing and initializing
+    /// their own box. All but one of them will be passed to `cancel_init` right after.
+    fn init() -> Box<Self>;
+
+    /// Any surplus boxes from `init()` that lost the initialization race
+    /// are passed to this function for disposal.
+    ///
+    /// The default implementation calls destroy().
+    fn cancel_init(x: Box<Self>) {
+        Self::destroy(x);
+    }
+
+    /// This is called to destroy a used box.
+    ///
+    /// The default implementation just drops it.
+    fn destroy(_: Box<Self>) {}
+}
+
+impl<T: LazyInit> LazyBox<T> {
+    #[inline]
+    pub const fn new() -> Self {
+        Self { ptr: AtomicPtr::new(null_mut()), _phantom: PhantomData }
+    }
+
+    #[inline]
+    fn get_pointer(&self) -> *mut T {
+        let ptr = self.ptr.load(Acquire);
+        if ptr.is_null() { self.initialize() } else { ptr }
+    }
+
+    #[cold]
+    fn initialize(&self) -> *mut T {
+        let new_ptr = Box::into_raw(T::init());
+        match self.ptr.compare_exchange(null_mut(), new_ptr, AcqRel, Acquire) {
+            Ok(_) => new_ptr,
+            Err(ptr) => {
+                // Lost the race to another thread.
+                // Drop the box we created, and use the one from the other thread instead.
+                T::cancel_init(unsafe { Box::from_raw(new_ptr) });
+                ptr
+            }
+        }
+    }
+}
+
+impl<T: LazyInit> Deref for LazyBox<T> {
+    type Target = T;
+    #[inline]
+    fn deref(&self) -> &T {
+        unsafe { &*self.get_pointer() }
+    }
+}
+
+impl<T: LazyInit> DerefMut for LazyBox<T> {
+    #[inline]
+    fn deref_mut(&mut self) -> &mut T {
+        unsafe { &mut *self.get_pointer() }
+    }
+}
+
+impl<T: LazyInit> Drop for LazyBox<T> {
+    fn drop(&mut self) {
+        let ptr = *self.ptr.get_mut();
+        if !ptr.is_null() {
+            T::destroy(unsafe { Box::from_raw(ptr) });
+        }
+    }
+}