[event_trace] Add a way to pass a Python object to an ETW trace's

context

It may be useful to pass a python object (e.g. class) to the ETW
processing callback. The ETW tracing API allows the user to fill out a
"Context" field with a pointer value that will be passed to the event
being sent, the rest is ctypes magic for wrapping the python object
into a native pointer value.

windows/winobject/event_trace.py

@@ -48,6 +48,10 @@ class EventRecord(gdef.EVENT_RECORD):
     def level(self):
         return self.EventHeader.EventDescriptor.Level
 
+    @property
+    def context(self):
+        return ctypes.py_object.from_address(self.UserContext).value
+
     @property
     def user_data(self):
         """Event specific data
@@ -217,7 +221,7 @@ class EtwTrace(object):
         return windows.winproxy.EnableTraceEx2(self.handle, guid, EVENT_CONTROL_CODE_ENABLE_PROVIDER, level , any_keyword, all_keyword, 0, None) 
 
 
-    def process(self, callback, begin=None, end=None):
+    def process(self, callback, begin=None, end=None, context = None):
         if end == "now":
             end = gdef.FILETIME()
             windows.winproxy.GetSystemTimeAsFileTime(end)
@@ -233,6 +237,10 @@ class EtwTrace(object):
             # logfile.ProcessTraceMode |= gdef.PROCESS_TRACE_MODE_REAL_TIME
             logfile.LogFileName = self.logfile
 
+        if context:
+            context_ptr = ctypes.pointer(ctypes.py_object(context))
+            logfile.Context = ctypes.cast(context_ptr, ctypes.c_void_p)
+
         @ctypes.WINFUNCTYPE(gdef.PVOID, PEventRecord)
         def real_callback(record_ptr):
             try: