Implementing a stepping debugger with method handles

[Mark Roos]

Along the line of doing unusual things with method handles ( and because I 
get good suggestions )
here is an implementation of a stepping debugger.  It can step over or 
into and resume.

In Smalltalk the IDE and the application are collocated in the same 
process.  The debugger is one
thread which monitors the application threads.  I looked at using jvmti to 
handle the stepping but
its outside looking in and does slow down the process.  Since I want to 
step at a granularity of
my callsites anyway  I tried a mh based implementation.

Not much code and it works.  So another nice use of the tools.  Again not 
much impact on execution time.

mark

At the entry to each callsite target I add

      MethodType mt=MethodType.methodType(void.class, RtObject.class); 
      debugEntry = lookup.findStatic(RtDebugger.class, "debugEntry", mt); 
      invoker = MethodHandles.foldArguments(invoker , debugEntry);  // 
does nothing except suspend if necessary

the check is this for thread and level with a notification to the debugger 
of the step

    if(_debugThread == Thread.currentThread()){
      if(getStackSize(_debugThread) <= _debugDepth){  // jni call to jvmti 
to get the stack depth
      // send debugger update notice async
      notifyDebugStep();
      suspendThreadPrim(_debugThread);    //_jni version of 
debugThread.suspend();
      }
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