avoid busy waiting in HPy reference cleanup thread

Author: lukasstadler

graalpython/com.oracle.graal.python/src/com/oracle/graal/python/builtins/objects/cext/hpy/GraalHPyContext.java

@@ -927,14 +927,15 @@ public void run() {
                      * thread).
                      */
                     GraalHPyHandleReference refList;
+                    int retries = 0;
                     do {
                         /*
                          * If 'refList' is null then the main is currently updating it. So, we need
                          * to repeat until we get something. The written empty list will just be
                          * lost.
                          */
                         refList = hPyContext.references.getAndSet(null);
-                    } while (refList == null);
+                    } while (refList == null && retries++ < 3);
 
                     if (!refs.isEmpty()) {
                         try {