Writing an IronPython Debugger: Dynamic Stack Trace

Now that I can interact with my debugger, it’s time to add a command. I decided to start with something simple – or at least something I thought would be simple – printing a stack trace.

In the unmanaged debugger API, threads have the concept of both stack chains and stack frames. A stack chain represents a segment of the physical stack. In a typical managed app, you’ll have at least two stack chains: the unmanaged stack chain and the managed stack chain. You can interate through the stack chains for a given thread via the Chains property. However, ipydbg is a managed only debugger, so I can ignore the unmanaged stack chain. Instead, I just retrieve the current (managed) chain via the thread’s ActiveChain property.

Within a managed stack chain, there is a collection of stack frames. This is the call stack that managed developers are typically used to working with. It turns out that printing a raw stack trace is very easy to do. Here was my first stab at it:

elif k.Key == ConsoleKey.T:
  print "nManaged Stack Trace"
  for f in active_thread.ActiveChain.Frames:
    offset, sp = get_location(f)
    metadata_import = CorMetadataImport(f.Function.Module)
    method_info = metadata_import.GetMethodInfo(f.FunctionToken)
    print "  ",
      "%s::%s --" % (method_info.DeclaringType.Name, method_info.Name),
      sp if sp != None else "(offset %d)" % offset

This elif block is part of the input method I showed last time. It loops thru the frames in the Active Chain of the active thread and prints some data to the console. As I said, pretty easy. Of course, the devil is in the details.

First detail I should call out is that active_thread variable. As per Mike Stall, “there is no notion of “active thread” in the underlying debug APIs. It’s purely a construct in a debugger UI to make it easier for end-users.” My console based UI may be rudimentary, but it’s still a UI. Events like OnBreakpoint include the active thread as a event argument, so I stash that away in a variable so it’ll be available to the input loop.

Second detail is the call to get_location. When we last saw get_location, it was returning a formatted string. Since my last post, I’ve refactored the code so it returns the raw location data – a tuple of the raw IP offset and the associated sequence point, if available. I’ve also added a __str__ method to my sequence point object, so when I print it to the console, I get the filename and line nicely formatted.

Finally, there’s all CorMetadataImport code. In addition to wrapping the unmanaged debugger API, CorDebug also wraps the unmanaged metadata API. This code lets me get MethodInfo compatible view of the function metadata for a given stack frame. I use it here to get the type and function name for each frame on the stack.

The end result looks something like this. Note, I’ve replaced “Microsoft.Scripting” with “MS.Scripting” to avoid word wrapping.

OnBreakpoint Initialize Location: simpletest.py:1 (offset: 84)
» t
Managed Stack Trace
   S$2::Initialize simpletest.py:1 (offset: 84)
   MS.Scripting.Runtime.OptimizedScriptCode::InvokeTarget (offset 72)
   MS.Scripting.ScriptCode::Run (offset 0)
   IronPython.Hosting.PythonCommandLine::RunFileWorker (offset 77)
   IronPython.Hosting.PythonCommandLine::RunFile (offset 15)
   MS.Scripting.Hosting.Shell.CommandLine::Run (offset 46)
   IronPython.Hosting.PythonCommandLine::Run (offset 240)
   MS.Scripting.Hosting.Shell.CommandLine::Run (offset 74)
   MS.Scripting.Hosting.Shell.ConsoleHost::RunCommandLine (offset 158)
   MS.Scripting.Hosting.Shell.ConsoleHost::ExecuteInternal (offset 32)
   MS.Scripting.Hosting.Shell.ConsoleHost::Execute (offset 63)
   MS.Scripting.Hosting.Shell.ConsoleHost::Run (offset 390)
   PythonConsoleHost::Main — (offset 125)

As we can see, we may be on the first line of the python script, but we’ve got a pretty deep stack trace already. Everything but the top-most frame are from the underlying IronPython implementation. Those extra frames obscure the stack frames I actually care about, so it would be nice to hide any stack frames from IronPython or the DLR. It’s easy enough to write a python generator function that filters out frames that from the DLR or IronPython namespaces. In order to get the type name, we need the method_info like we did above. I’ve factored that code into a separate function in order to avoid code duplication.

def get_method_info_for_frame(frame):
    if frame.FrameType != CorFrameType.ILFrame:
      return None
    metadata_import = CorMetadataImport(frame.Function.Module)
    return metadata_import.GetMethodInfo(frame.FunctionToken)

def get_dynamic_frames(chain):
  for f in chain.Frames:
    method_info = get_method_info_for_frame(f)
    if method_info == None:
    typename = method_info.DeclaringType.Name
    if typename.startswith("Microsoft.Scripting.")
      or typename.startswith("IronPython.")
      or typename == "PythonConsoleHost":
    yield f

You’ll notice I’ve added a guard to get_method_info_for_frame in order to ensure that the frame argument is an IL Frame. There are three types of stack frames in the debugger API: IL, native and internal. Most of the frames we’re dealing with are IL frames, but you do run into the occasional lightweight function (i.e. DynamicMethod) frame when debugging IronPython code. Typically, IronPython generates DynamicMethods for all python code except for a few cases related to .NET interop. However, you can’t debug DynamicMethods, so when you run with –D, we generate normal non-dynamic methods instead. However, even when running with –D, we still use DynamicMethods for call site dispatch. Since they’re an implementation detail, we want to filter those out in get_dynamic_frames too.

This gives us a much more manageable stack trace:

OnBreakpoint Initialize Location: simpletest.py:1 (offset: 84)
» t
Stack Trace
   S$2::Initialize -- simpletest.py:1 (offset: 84)

As usual, the latest ipydbg source is up on GitHub.