atomics: Add a ref_get() variant returning non-zero on overflows

This is useful for users using ref_get() for unique identifier allocation,
but the zero value has special meaning.

src/libstrongswan/utils/utils/atomics.h

@@ -124,6 +124,33 @@ bool cas_ptr(void **ptr, void *oldval, void *newval);
 
 #endif /* HAVE_GCC_ATOMIC_OPERATIONS */
 
+/**
+ * Get a new reference, but skip zero on overflow.
+ *
+ * If a reference counter is used to allocate unique identifiers, the
+ * refcount value may overflow if it is never decremented. The 0 identifier
+ * may have special semantics, hence returning can be problematic for some
+ * users.
+ *
+ * This call does an additional ref_get() if ref_get() overflows and returns
+ * zero. This ensures that zero is never returned, in the assumption that it
+ * has special meaning.
+ *
+ * @param ref	pointer to ref counter
+ * @return		new value of ref
+ */
+static inline refcount_t ref_get_nonzero(refcount_t *ref)
+{
+	refcount_t v;
+
+	v = ref_get(ref);
+	if (v == 0)
+	{
+		v = ref_get(ref);
+	}
+	return v;
+}
+
 /**
  * Initialize atomics utility functions
  */