add assert that we own the gil when entering a python callee context

Author: timfel

graalpython/com.oracle.graal.python/src/com/oracle/graal/python/runtime/ExecutionContext.java

@@ -217,6 +217,7 @@ public Node copy() {
          * Wrap the execution of a Python callee called from a Python frame.
          */
         public void enter(VirtualFrame frame) {
+            assert PythonLanguage.getContext().ownsGil() : PythonContext.dumpStackOnAssertionHelper("callee w/o GIL");
             // tfel: Create our frame reference here and store it so that
             // there's no reference to it from the caller side.
             PFrame.Reference thisFrameRef = new PFrame.Reference(PArguments.getCallerFrameInfo(frame));

graalpython/com.oracle.graal.python/src/com/oracle/graal/python/runtime/PythonContext.java

@@ -1166,7 +1166,7 @@ void acquireGil() throws InterruptedException {
         }
     }
 
-    private static final String dumpStackOnAssertionHelper(String msg) {
+    static final String dumpStackOnAssertionHelper(String msg) {
         Thread.dumpStack();
         return msg;
     }