Writing an IronPython Debugger: Debugging Just My Code

As I wrote last time, in order to make debug stepping actually useful in ipydbg I need to avoid stepping into frames that are part of the IronPython infrastructure. I did something similar when I hide infrastructure frames in the stack trace. Originally, I had planned to automatically stepping again if we ended up on a frame that didn’t correspond to a python file. However, Mike Stall showed me a much cleaner and better performing solution: Just My Code. As I mentioned at the start of this series, support for JMC is one of the main reasons I wanted to build my own debugger rather than use MDbg.

Enabling JMC in the stepper object is trivial:

def create_stepper(thread, JMC = True):
  stepper = thread.ActiveFrame.CreateStepper()
  stepper.SetUnmappedStopMask(CorDebugUnmappedStop.STOP_NONE)
  stepper.SetJmcStatus(JMC)  
  return stepper

If I make that single change and run ipydbg, any step effectively turns into a full continue since none of the code has been marked as “My Code” yet. As you see, the tricky part of JMC isn’t enabling it on the stepper, it’s “painting” the parts of the code where you want JMC stepping to work. You can set JMC status at the module, class or the method level. In the case of ipdbg, it’s easiest to work at the class level:

infrastructure_methods =  ['TryGetExtraValue',
    'TrySetExtraValue',
    '.cctor',
    '.ctor',
    'CustomSymbolDictionary.GetExtraKeys',
    'IModuleDictionaryInitialization.InitializeModuleDictionary']

def OnClassLoad(self, sender, e):
    cmi = CorMetadataImport(e.Class.Module)
    mt = cmi.GetType(e.Class.Token)
    print "OnClassLoad", mt.Name

    if not e.Class.Module.IsDynamic:
      e.Class.JMCStatus = False
    elif mt.Name.startswith('IronPython.NewTypes'):
      e.Class.JMCStatus = False
    else:
      e.Class.JMCStatus = True
      for mmi in mt.GetMethods():
        if mmi.Name in infrastructure_methods:
          f = e.Class.Module.GetFunctionFromToken(mmi.MetadataToken)
          f.JMCStatus = False

OnClassLoad is where the action is. This event handler is responsible for enabling JMC for all class methods that map to python code. To understand how the logic in OnClassLoad works, you need to understand a little about the .NET types and code that IronPython generates. Note, the following description is for the IronPython 2.0 branch. Code generation evolves from release to release and I know for a fact there are changes in the upcoming 2.6 version. I assume that I’ll eventually have to sniff the IronPython version in order to set JMC correctly.

Today, IronPython generates all code into dynamic modules and methods. Since I want to limit stepping to python code only, I automatically disable JMC for non-dynamic modules. I can imagine a scenario where I want to step into non-dynamically generated code, but I think the best way to handle that would be to disable JMC at the stepper rather than widening the amount of code marked as JMC enabled.

For every module that gets loaded, IronPython generates a type. At a minimum you’re going to load two modules: site.py and whatever python script you ran. If you have the python standard library installed, site.py loads a bunch of other modules as well. Each of these module types have a bunch of standard methods that always get generated. For example, the global scope code in the module is placed in a static method on the module type called Initialize. Any python functions you define get generated static methods with mangled names on the module type ¹. All these methods have corresponding python code and should be JMC enabled. The other standard methods on a module type should not be JMC enabled. So in my debugger, I mark the class as JMC enabled but then iterate over the list of methods and mark any in the list of standard methods (except for Initialize) as JMC disabled.

Of course, you can also create classes in Python. As you might expect, classes in Python are generated as .NET types. However, the semantics of Python classes are very different than .NET types. For example, you can change the inheritance hierarchy of python classes at runtime. That’s obviously not allowed for .NET types. So the .NET types we generate have all the logic to implement Python class semantics. As it turns out, these .NET types only have the logic to implement Python class semantics, which is to say they have none of Python class methods code. This makes sense when you think about it – since Python can add and remove methods from a class at runtime, IronPython can’t put the method code in the .NET type itself. Instead, Python class methods are generated as static methods on the module type, just like top-level functions are. Since Python class types only contain Python class semantics logic, we never want to enable JMC for Python class types. Python class types get generated in the IronPython.NewTypes namespace, so it’s fairly easy to check the class name in OnClassLoad and automatically disable JMC for classes any in that namespace.

Adding JMC support makes ipydbg significantly more usable. It’s almost like a real tool now, isn’t it? Latest bits are up on GitHub.