1 files changed, 116 insertions, 0 deletions
diff --git a/vendor/gimli/src/read/lazy.rs b/vendor/gimli/src/read/lazy.rs
new file mode 100644
index 000000000..6138735c8
--- /dev/null
+++ b/vendor/gimli/src/read/lazy.rs
@@ -0,0 +1,116 @@
+pub(crate) use imp::*;
+
+#[cfg(not(feature = "std"))]
+mod imp {
+    use alloc::sync::Arc;
+    use core::sync::atomic::{AtomicPtr, Ordering};
+    use core::{mem, ptr};
+
+    #[derive(Debug, Default)]
+    pub(crate) struct LazyArc<T> {
+        // Only written once with a value obtained from `Arc<T>::into_raw`.
+        // This holds a ref count for the `Arc`, so it is always safe to
+        // clone the `Arc` given a reference to the `LazyArc`.
+        value: AtomicPtr<T>,
+    }
+
+    impl<T> Drop for LazyArc<T> {
+        fn drop(&mut self) {
+            let value_ptr = self.value.load(Ordering::Acquire);
+            if !value_ptr.is_null() {
+                // SAFETY: all writes to `self.value` are pointers obtained from `Arc::into_raw`.
+                drop(unsafe { Arc::from_raw(value_ptr) });
+            }
+        }
+    }
+
+    impl<T> LazyArc<T> {
+        pub(crate) fn get<E, F: FnOnce() -> Result<T, E>>(&self, f: F) -> Result<Arc<T>, E> {
+            // Clone an `Arc` given a pointer obtained from `Arc::into_raw`.
+            // SAFETY: `value_ptr` must be a valid pointer obtained from `Arc<T>::into_raw`.
+            unsafe fn clone_arc_ptr<T>(value_ptr: *const T) -> Arc<T> {
+                let value = Arc::from_raw(value_ptr);
+                let clone = Arc::clone(&value);
+                mem::forget(value);
+                clone
+            }
+
+            // Return the existing value if already computed.
+            // `Ordering::Acquire` is needed so that the content of the loaded `Arc` is
+            // visible to this thread.
+            let value_ptr = self.value.load(Ordering::Acquire);
+            if !value_ptr.is_null() {
+                // SAFETY: all writes to `self.value` are pointers obtained from `Arc::into_raw`.
+                return Ok(unsafe { clone_arc_ptr(value_ptr) });
+            }
+
+            // Race to compute and set the value.
+            let value = f().map(Arc::new)?;
+            let value_ptr = Arc::into_raw(value);
+            match self.value.compare_exchange(
+                ptr::null_mut(),
+                value_ptr as *mut T,
+                // Success: `Ordering::Release` is needed so that the content of the stored `Arc`
+                // is visible to other threads. No ordering is required for the null ptr that is
+                // loaded, but older rust versions (< 1.64) require that its ordering must not
+                // be weaker than the failure ordering, so we use `Ordering::AcqRel`.
+                Ordering::AcqRel,
+                // Failure: `Ordering::Acquire` is needed so that the content of the loaded `Arc`
+                // is visible to this thread.
+                Ordering::Acquire,
+            ) {
+                Ok(_) => {
+                    // Return the value we computed.
+                    // SAFETY: `value_ptr` was obtained from `Arc::into_raw`.
+                    Ok(unsafe { clone_arc_ptr(value_ptr) })
+                }
+                Err(existing_value_ptr) => {
+                    // We lost the race, drop unneeded `value_ptr`.
+                    // SAFETY: `value_ptr` was obtained from `Arc::into_raw`.
+                    drop(unsafe { Arc::from_raw(value_ptr) });
+                    // Return the existing value.
+                    // SAFETY: all writes to `self.value` are pointers obtained from `Arc::into_raw`.
+                    Ok(unsafe { clone_arc_ptr(existing_value_ptr) })
+                }
+            }
+        }
+    }
+}
+
+#[cfg(feature = "std")]
+mod imp {
+    use std::sync::{Arc, Mutex};
+
+    #[derive(Debug, Default)]
+    pub(crate) struct LazyArc<T> {
+        value: Mutex<Option<Arc<T>>>,
+    }
+
+    impl<T> LazyArc<T> {
+        pub(crate) fn get<E, F: FnOnce() -> Result<T, E>>(&self, f: F) -> Result<Arc<T>, E> {
+            let mut lock = self.value.lock().unwrap();
+            if let Some(value) = &*lock {
+                return Ok(value.clone());
+            }
+            let value = f().map(Arc::new)?;
+            *lock = Some(value.clone());
+            Ok(value)
+        }
+    }
+}
+
+#[cfg(test)]
+mod tests {
+    use super::*;
+
+    #[test]
+    fn lazy_arc() {
+        let lazy = LazyArc::default();
+        let value = lazy.get(|| Err(()));
+        assert_eq!(value, Err(()));
+        let value = lazy.get(|| Ok::<i32, ()>(3)).unwrap();
+        assert_eq!(*value, 3);
+        let value = lazy.get(|| Err(())).unwrap();
+        assert_eq!(*value, 3);
+    }
+}