From 6bf0a5cb5034a7e684dcc3500e841785237ce2dd Mon Sep 17 00:00:00 2001
From: Daniel Baumann <daniel.baumann@progress-linux.org>
Date: Sun, 7 Apr 2024 19:32:43 +0200
Subject: Adding upstream version 1:115.7.0.

Signed-off-by: Daniel Baumann <daniel.baumann@progress-linux.org>
---
 .../rust/tokio-util/src/sync/reusable_box.rs       | 148 +++++++++++++++++++++
 1 file changed, 148 insertions(+)
 create mode 100644 third_party/rust/tokio-util/src/sync/reusable_box.rs

(limited to 'third_party/rust/tokio-util/src/sync/reusable_box.rs')

diff --git a/third_party/rust/tokio-util/src/sync/reusable_box.rs b/third_party/rust/tokio-util/src/sync/reusable_box.rs
new file mode 100644
index 0000000000..3204207db7
--- /dev/null
+++ b/third_party/rust/tokio-util/src/sync/reusable_box.rs
@@ -0,0 +1,148 @@
+use std::alloc::Layout;
+use std::future::Future;
+use std::panic::AssertUnwindSafe;
+use std::pin::Pin;
+use std::ptr::{self, NonNull};
+use std::task::{Context, Poll};
+use std::{fmt, panic};
+
+/// A reusable `Pin<Box<dyn Future<Output = T> + Send + 'a>>`.
+///
+/// This type lets you replace the future stored in the box without
+/// reallocating when the size and alignment permits this.
+pub struct ReusableBoxFuture<'a, T> {
+    boxed: NonNull<dyn Future<Output = T> + Send + 'a>,
+}
+
+impl<'a, T> ReusableBoxFuture<'a, T> {
+    /// Create a new `ReusableBoxFuture<T>` containing the provided future.
+    pub fn new<F>(future: F) -> Self
+    where
+        F: Future<Output = T> + Send + 'a,
+    {
+        let boxed: Box<dyn Future<Output = T> + Send + 'a> = Box::new(future);
+
+        let boxed = NonNull::from(Box::leak(boxed));
+
+        Self { boxed }
+    }
+
+    /// Replace the future currently stored in this box.
+    ///
+    /// This reallocates if and only if the layout of the provided future is
+    /// different from the layout of the currently stored future.
+    pub fn set<F>(&mut self, future: F)
+    where
+        F: Future<Output = T> + Send + 'a,
+    {
+        if let Err(future) = self.try_set(future) {
+            *self = Self::new(future);
+        }
+    }
+
+    /// Replace the future currently stored in this box.
+    ///
+    /// This function never reallocates, but returns an error if the provided
+    /// future has a different size or alignment from the currently stored
+    /// future.
+    pub fn try_set<F>(&mut self, future: F) -> Result<(), F>
+    where
+        F: Future<Output = T> + Send + 'a,
+    {
+        // SAFETY: The pointer is not dangling.
+        let self_layout = {
+            let dyn_future: &(dyn Future<Output = T> + Send) = unsafe { self.boxed.as_ref() };
+            Layout::for_value(dyn_future)
+        };
+
+        if Layout::new::<F>() == self_layout {
+            // SAFETY: We just checked that the layout of F is correct.
+            unsafe {
+                self.set_same_layout(future);
+            }
+
+            Ok(())
+        } else {
+            Err(future)
+        }
+    }
+
+    /// Set the current future.
+    ///
+    /// # Safety
+    ///
+    /// This function requires that the layout of the provided future is the
+    /// same as `self.layout`.
+    unsafe fn set_same_layout<F>(&mut self, future: F)
+    where
+        F: Future<Output = T> + Send + 'a,
+    {
+        // Drop the existing future, catching any panics.
+        let result = panic::catch_unwind(AssertUnwindSafe(|| {
+            ptr::drop_in_place(self.boxed.as_ptr());
+        }));
+
+        // Overwrite the future behind the pointer. This is safe because the
+        // allocation was allocated with the same size and alignment as the type F.
+        let self_ptr: *mut F = self.boxed.as_ptr() as *mut F;
+        ptr::write(self_ptr, future);
+
+        // Update the vtable of self.boxed. The pointer is not null because we
+        // just got it from self.boxed, which is not null.
+        self.boxed = NonNull::new_unchecked(self_ptr);
+
+        // If the old future's destructor panicked, resume unwinding.
+        match result {
+            Ok(()) => {}
+            Err(payload) => {
+                panic::resume_unwind(payload);
+            }
+        }
+    }
+
+    /// Get a pinned reference to the underlying future.
+    pub fn get_pin(&mut self) -> Pin<&mut (dyn Future<Output = T> + Send)> {
+        // SAFETY: The user of this box cannot move the box, and we do not move it
+        // either.
+        unsafe { Pin::new_unchecked(self.boxed.as_mut()) }
+    }
+
+    /// Poll the future stored inside this box.
+    pub fn poll(&mut self, cx: &mut Context<'_>) -> Poll<T> {
+        self.get_pin().poll(cx)
+    }
+}
+
+impl<T> Future for ReusableBoxFuture<'_, T> {
+    type Output = T;
+
+    /// Poll the future stored inside this box.
+    fn poll(self: Pin<&mut Self>, cx: &mut Context<'_>) -> Poll<T> {
+        Pin::into_inner(self).get_pin().poll(cx)
+    }
+}
+
+// The future stored inside ReusableBoxFuture<'_, T> must be Send.
+unsafe impl<T> Send for ReusableBoxFuture<'_, T> {}
+
+// The only method called on self.boxed is poll, which takes &mut self, so this
+// struct being Sync does not permit any invalid access to the Future, even if
+// the future is not Sync.
+unsafe impl<T> Sync for ReusableBoxFuture<'_, T> {}
+
+// Just like a Pin<Box<dyn Future>> is always Unpin, so is this type.
+impl<T> Unpin for ReusableBoxFuture<'_, T> {}
+
+impl<T> Drop for ReusableBoxFuture<'_, T> {
+    fn drop(&mut self) {
+        unsafe {
+            drop(Box::from_raw(self.boxed.as_ptr()));
+        }
+    }
+}
+
+impl<T> fmt::Debug for ReusableBoxFuture<'_, T> {
+    fn fmt(&self, f: &mut fmt::Formatter<'_>) -> fmt::Result {
+        f.debug_struct("ReusableBoxFuture").finish()
+    }
+}
-- 
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