Prevent edge case race condition, hopefully

Author: coolreader18

Cargo.lock

@@ -20,7 +20,7 @@ once_cell = "1.4.1"
 siphasher = "0.3"
 rand = "0.8"
 volatile = "0.3"
-radium = "0.6"
+radium = "0.7"
 libc = "0.2.101"
 ascii = "1.0"
 unic-ucd-category = "0.9"

common/Cargo.toml

@@ -38,6 +38,14 @@ impl RefCount {
         }
     }
 
+    /// Returns true if successful
+    #[inline]
+    pub fn safe_inc(&self) -> bool {
+        self.strong
+            .fetch_update(AcqRel, Acquire, |prev| (prev != 0).then(|| prev + 1))
+            .is_ok()
+    }
+
     /// Decrement the reference count. Returns true when the refcount drops to 0.
     #[inline]
     pub fn dec(&self) -> bool {

common/src/refcount.rs

@@ -314,11 +314,12 @@ cfg_if::cfg_if! {
 
 impl PyWeak {
     pub(crate) fn upgrade(&self) -> Option<PyObjectRef> {
-        // TODO: figure out orderings for here, in drop, and the store in WeakRefList::clear
         let guard = unsafe { self.parent.as_ref().lock() };
         let obj_ptr = guard.obj?;
         unsafe {
-            obj_ptr.as_ref().0.refcount.inc();
+            if !obj_ptr.as_ref().0.refcount.safe_inc() {
+                return None;
+            }
             Some(PyObjectRef::from_raw(obj_ptr.as_ptr()))
         }
     }