0.3.0: converge relibc to upstream 0.6.0 + Red Bear patches

src/sync/waitval.rs

@@ -0,0 +1,42 @@
+use core::{
+    cell::UnsafeCell,
+    mem::MaybeUninit,
+    sync::atomic::{AtomicU32 as AtomicUint, Ordering},
+};
+
+/// An unsafe "one thread to one thread" synchronization primitive. Used for and modeled after
+/// pthread_join only, at the moment.
+#[derive(Debug)]
+pub struct Waitval<T> {
+    state: AtomicUint,
+    value: UnsafeCell<MaybeUninit<T>>,
+}
+
+unsafe impl<T: Send + Sync> Send for Waitval<T> {}
+unsafe impl<T: Send + Sync> Sync for Waitval<T> {}
+
+impl<T> Waitval<T> {
+    #[allow(clippy::new_without_default)]
+    pub const fn new() -> Self {
+        Self {
+            state: AtomicUint::new(0),
+            value: UnsafeCell::new(MaybeUninit::uninit()),
+        }
+    }
+
+    // SAFETY: Caller must ensure both (1) that the value has not yet been initialized, and (2)
+    // that this is never run by more than one thread simultaneously.
+    pub unsafe fn post(&self, value: T) {
+        unsafe { self.value.get().write(MaybeUninit::new(value)) };
+        self.state.store(1, Ordering::Release);
+        crate::sync::futex_wake(&self.state, i32::MAX);
+    }
+
+    pub fn wait(&self) -> &T {
+        while self.state.load(Ordering::Acquire) == 0 {
+            crate::sync::futex_wait(&self.state, 0, None);
+        }
+
+        unsafe { (*self.value.get()).assume_init_ref() }
+    }
+}