synchronize on module entry

Author: javeleon

espresso/src/com.oracle.truffle.espresso/src/com/oracle/truffle/espresso/runtime/JavaAgents.java

@@ -412,12 +412,18 @@ public boolean shouldEnableRedefinition() {
         return false;
     }
 
-    public synchronized void grantReadAccessToUnnamedModules(ModuleTable.ModuleEntry module) {
+    public void grantReadAccessToUnnamedModules(ModuleTable.ModuleEntry module) {
         if (module != null && module.isNamed() && !module.hasDefaultReads()) {
-            getContext().getMeta().jdk_internal_module_Modules_transformedByAgent.invokeDirect(module.module());
-            // no reason to call into guest for a module more than once,
-            // so flip the hasDefaultReads flag
-            module.setHasDefaultReads();
+            synchronized (module) {
+                if (module.hasDefaultReads()) {
+                    // we lost the race, so nothing to do
+                    return;
+                }
+                getContext().getMeta().jdk_internal_module_Modules_transformedByAgent.invokeDirect(module.module());
+                // no reason to call into guest for a module more than once,
+                // so flip the hasDefaultReads flag
+                module.setHasDefaultReads();
+            }
         }
     }