1 files changed, 63 insertions, 0 deletions
diff --git a/library/std/src/io/lazy.rs b/library/std/src/io/lazy.rs
new file mode 100644
index 00000000000..1968d498bbe
--- /dev/null
+++ b/library/std/src/io/lazy.rs
@@ -0,0 +1,63 @@
+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
+    }
+}