Fix unprotected access in refcount_context_num_refs

include/refcount/config.h.in

@@ -3,10 +3,4 @@
 
 #cmakedefine REFCOUNT_HAS_THREADS
 
-#if defined(REFCOUNT_HAS_THREADS) \
-    && (!defined(__STDC_VERSION__) || __STDC_VERSION__ < 201112L)
-#    error \
-        "RefCount needs to be compiled with at least C11 for thread support to work."
-#endif
-
 #endif

include/refcount/refcount.h

@@ -162,8 +162,21 @@ static inline bool refcount_context_is_static(const RefcountContext *ctx,
  */
 static inline uint64_t refcount_context_num_refs(const RefcountContext *ctx,
                                                  void *obj) {
+#ifdef REFCOUNT_HAS_THREADS
+    if (!mtx_lock(&REFCOUNT_OBJECT_ENTRY(ctx, obj)->weak_ref->mtx)) {
+        // just try our best to estimate, if we are on a platform where 64bit
+        // stores are not atomic (or this is not aligned properly), this might
+        // race and give a bogus number
         return REFCOUNT_OBJECT_ENTRY(ctx, obj)->impl.counted.ref_count;
     }
+#endif
+    const uint64_t count =
+        REFCOUNT_OBJECT_ENTRY(ctx, obj)->impl.counted.ref_count;
+#ifdef REFCOUNT_HAS_THREADS
+    mtx_unlock(&REFCOUNT_OBJECT_ENTRY(ctx, obj)->weak_ref->mtx);
+#endif
+    return count;
+}
 
 bool refcount_context_init_obj(const RefcountContext *ctx, void *obj);