Fix #191: Synchronize WeakHashMap

Author: kariya-mitsuru

graalpython/com.oracle.graal.python/src/com/oracle/graal/python/builtins/modules/PythonCextBuiltins.java

@@ -61,7 +61,9 @@
 import java.text.ParsePosition;
 import java.util.ArrayList;
 import java.util.Arrays;
+import java.util.Collections;
 import java.util.List;
+import java.util.Map;
 import java.util.WeakHashMap;
 import java.util.logging.Level;
 
@@ -2425,7 +2427,7 @@ static Object doDirect(VirtualFrame frame, @SuppressWarnings("unused") PythonMod
     @Builtin(name = "make_may_raise_wrapper", minNumOfPositionalArgs = 1, maxNumOfPositionalArgs = 2)
     @GenerateNodeFactory
     abstract static class MakeMayRaiseWrapperNode extends PythonBuiltinNode {
-        private static final WeakHashMap<RootCallTarget, WeakReference<RootCallTarget>> weakCallTargetMap = new WeakHashMap<>();
+        private static final Map<RootCallTarget, WeakReference<RootCallTarget>> weakCallTargetMap = Collections.synchronizedMap(new WeakHashMap<>());
 
         @Specialization
         @TruffleBoundary