util - with_shared_mut_ref

vm/src/lib.rs

@@ -7,11 +7,14 @@ pub mod types;
 pub mod vm_err_msg;
 pub mod vm_hooks;
 pub mod world_mock;
+mod with_shared;
 
 pub use world_mock::BlockchainMock;
 
 // Re-exporting the executor, for convenience.
 pub use multiversx_chain_vm_executor as executor;
 
+use with_shared::with_shared_mut_ref;
+
 #[macro_use]
 extern crate alloc;

vm/src/with_shared.rs

@@ -0,0 +1,77 @@
+use std::rc::Rc;
+
+/// Temporarily converts a mutable reference into a reference-counted smart pointer (`Rc`).
+/// 
+/// This only takes as long as the closure `f` is executed.
+/// 
+/// All subsequent Rc clones must be dropped before `f` terminates, otherwise the function will panic.
+/// 
+/// The `Clone` of the argument is not used, except to preserve memory consistency in case of failure.
+pub fn with_shared_mut_ref<T, F, R>(t: &mut T, f: F) -> R
+where
+    T: Clone,
+    F: FnOnce(Rc<T>) -> R,
+{
+    unsafe {
+        // forcefully extract the owned object from the mut ref (unsafe)
+        let obj = std::ptr::read(t);
+
+        // wrap the owned object
+        let obj_rc = Rc::new(obj);
+
+        // the main action
+        let result = f(obj_rc.clone());
+
+        // unwrapping the owned object
+        match Rc::try_unwrap(obj_rc) {
+            Ok(obj) => {
+                // Rc unwrapped successfully
+                // no need to write the owned object back to the location given by the pointer t,
+                // because it could not have changed in the mean time, it is already there
+
+                // though readonly, the object might have changed via cells,
+                // so it needs to be copied back
+                std::ptr::write(t, obj);
+            },
+            Err(obj_rc) => {
+                // could not unwrap, this means there are still references to obj elsewhere
+                // to avoid memory corruption, we perform a clone of the contents
+                let obj = (*obj_rc).clone();
+                std::ptr::write(t, obj);
+                panic!("failed to recover owned object from Rc")
+            },
+        }
+
+        result
+    }
+}
+
+#[cfg(test)]
+mod test {
+    use std::cell::RefCell;
+
+    use super::with_shared_mut_ref;
+
+    #[test]
+    fn test_with_shared_1() {
+        let mut s = "test string".to_string();
+        let l = with_shared_mut_ref(&mut s, |s_rc| s_rc.len());
+        assert_eq!(s.len(), l);
+    }
+
+    #[test]
+    fn test_with_shared_2() {
+        let mut s = RefCell::new("test string".to_string());
+        with_shared_mut_ref(&mut s, |s_rc| {
+            s_rc.borrow_mut().push_str(" ... changed");
+        });
+        assert_eq!(s.borrow().as_str(), "test string ... changed");
+    }
+
+    #[test]
+    #[should_panic = "failed to recover owned object from Rc"]
+    fn test_with_shared_fail() {
+        let mut s = "test string".to_string();
+        let _illegal_clone = with_shared_mut_ref(&mut s, |s_rc| s_rc.clone());
+    }
+}