From dc0db358abe19481e475e10c32149b53370f1a1c Mon Sep 17 00:00:00 2001
From: Daniel Baumann <daniel.baumann@progress-linux.org>
Date: Thu, 30 May 2024 05:57:31 +0200
Subject: Merging upstream version 1.72.1+dfsg1.

Signed-off-by: Daniel Baumann <daniel.baumann@progress-linux.org>
---
 vendor/tokio/src/util/once_cell.rs | 70 ++++++++++++++++++++++++++++++++++++++
 1 file changed, 70 insertions(+)
 create mode 100644 vendor/tokio/src/util/once_cell.rs

(limited to 'vendor/tokio/src/util/once_cell.rs')

diff --git a/vendor/tokio/src/util/once_cell.rs b/vendor/tokio/src/util/once_cell.rs
new file mode 100644
index 000000000..71fc00758
--- /dev/null
+++ b/vendor/tokio/src/util/once_cell.rs
@@ -0,0 +1,70 @@
+#![allow(dead_code)]
+use std::cell::UnsafeCell;
+use std::mem::MaybeUninit;
+use std::sync::Once;
+
+pub(crate) struct OnceCell<T> {
+    once: Once,
+    value: UnsafeCell<MaybeUninit<T>>,
+}
+
+unsafe impl<T: Send + Sync> Send for OnceCell<T> {}
+unsafe impl<T: Send + Sync> Sync for OnceCell<T> {}
+
+impl<T> OnceCell<T> {
+    pub(crate) const fn new() -> Self {
+        Self {
+            once: Once::new(),
+            value: UnsafeCell::new(MaybeUninit::uninit()),
+        }
+    }
+
+    /// Get the value inside this cell, initializing it using the provided
+    /// function if necessary.
+    ///
+    /// If the `init` closure panics, then the `OnceCell` is poisoned and all
+    /// future calls to `get` will panic.
+    #[inline]
+    pub(crate) fn get(&self, init: impl FnOnce() -> T) -> &T {
+        if !self.once.is_completed() {
+            self.do_init(init);
+        }
+
+        // Safety: The `std::sync::Once` guarantees that we can only reach this
+        // line if a `call_once` closure has been run exactly once and without
+        // panicking. Thus, the value is not uninitialized.
+        //
+        // There is also no race because the only `&self` method that modifies
+        // `value` is `do_init`, but if the `call_once` closure is still
+        // running, then no thread has gotten past the `call_once`.
+        unsafe { &*(self.value.get() as *const T) }
+    }
+
+    #[cold]
+    fn do_init(&self, init: impl FnOnce() -> T) {
+        let value_ptr = self.value.get() as *mut T;
+
+        self.once.call_once(|| {
+            let set_to = init();
+
+            // Safety: The `std::sync::Once` guarantees that this initialization
+            // will run at most once, and that no thread can get past the
+            // `call_once` until it has run exactly once. Thus, we have
+            // exclusive access to `value`.
+            unsafe {
+                std::ptr::write(value_ptr, set_to);
+            }
+        });
+    }
+}
+
+impl<T> Drop for OnceCell<T> {
+    fn drop(&mut self) {
+        if self.once.is_completed() {
+            let value_ptr = self.value.get() as *mut T;
+            unsafe {
+                std::ptr::drop_in_place(value_ptr);
+            }
+        }
+    }
+}
-- 
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