8005873: JRuby test_respond_to.rb asserts with: MT-unsafe modification of inline cache

Reviewed-by: vlivanov, jrose, psandoz
Contributed-by: tobias.hartmann@oracle.com

src/share/classes/java/lang/invoke/LambdaForm.java

@@ -482,7 +482,7 @@ class LambdaForm {
                 assert(m.getName().equals("interpret" + sig.substring(sig.indexOf('_'))));
                 LambdaForm form = new LambdaForm(sig);
                 form.vmentry = m;
-                mt.form().setCachedLambdaForm(MethodTypeForm.LF_COUNTER, form);
+                form = mt.form().setCachedLambdaForm(MethodTypeForm.LF_COUNTER, form);
                 // FIXME: get rid of PREPARED_FORMS; use MethodTypeForm cache only
                 forms.put(sig, form);
             }

src/share/classes/java/lang/invoke/MethodHandleImpl.java

@@ -692,8 +692,7 @@ import static java.lang.invoke.MethodHandles.Lookup.IMPL_LOOKUP;
 
         lform = new LambdaForm("guardWithCatch", lambdaType.parameterCount(), names);
 
-        basicType.form().setCachedLambdaForm(MethodTypeForm.LF_GWC, lform);
-        return lform;
+        return basicType.form().setCachedLambdaForm(MethodTypeForm.LF_GWC, lform);
     }
 
     static

src/share/classes/java/lang/invoke/MethodTypeForm.java

@@ -91,8 +91,10 @@ final class MethodTypeForm {
         return lambdaForms[which];
     }
 
-    public LambdaForm setCachedLambdaForm(int which, LambdaForm form) {
-        // Should we perform some sort of CAS, to avoid racy duplication?
+    synchronized public LambdaForm setCachedLambdaForm(int which, LambdaForm form) {
+        // Simulate a CAS, to avoid racy duplication of results.
+        LambdaForm prev = lambdaForms[which];
+        if (prev != null) return prev;
         return lambdaForms[which] = form;
     }