[capi-sig] intermittent failure of threading

Paul Grunau paul at anilabsys.com
Thu Jul 1 23:36:17 CEST 2010


I have a problem with threading using the Python/C API. I have an extension that 
implements a timer, and the C++ timer callback function calls a Python function. 
The relevant code looks like this:
staticPyObject *timer_setmodname( PyObject *pSelf, PyObject *pArgs )
{
 
      char *b;
      PyArg_ParseTuple( pArgs, "s", &b );
      mod = PyImport_ImportModule(b);
      if( mod == NULL )
      {
            printf("Could not import %s\n",b);
            return Py_None;
      }
      modsetFlag = TRUE;
      return Py_None;
}
 
staticPyObject *timer_setprocname( PyObject *pSelf, PyObject *pArgs )
{
      char *b;
      if( !modsetFlag )return Py_None;
      PyArg_ParseTuple( pArgs, "s", &b );
      attr = PyObject_GetAttrString(mod,b);
      if( attr == NULL )
      {
            printf("Could not import %s\n",b);
            return Py_None;
      }
      attrsetFlag = TRUE;
      return Py_None;
}
      
staticvoid CALLBACK PeriodicTimer(UINT wTimerID, UINT msg, 
    DWORD dwUser, DWORD dw1, DWORD dw2) 
{ 
      PyGILState_STATE pgs;
 
      pgs = PyGILState_Ensure();
      if(attrsetFlag)
      {
            pres = PyObject_CallFunction(attr,NULL);
            if( pres == NULL )printf("CallFunction failed!\n");
      }
      PyGILState_Release( pgs );
 
} 
 
The Python code that sets this up looks like this:
 
fetimer.setmodname("Timeslice3")
fetimer.setprocname("Timetester")
print "\n Program Waiting for Time Slice"
while True:
    time.sleep(0.010)
 
 
and the module Timeslice3.py looks like this:
 
#Timeslice3.py
def Timetester():
      pass 
 
When I run this stuff, it works fine for hundreds, often even thousands, of 
timer ticks (I’ve been testing with about thirty ticks per second, but it 
doesn’t matter – it still crashes at ten or fewer ticks per second). Sometimes 
it runs for only a few seconds, sometimes for ten minutes or so. But it always 
eventually crashes Python. Usually it gives no error message. Sometimes, though, 
it does give an error message, but not always the same one. I’ve noted three 
that it has given in my testing so far:
 
Fatal Python Error: This thread state must be current when releasing
 
Fatal Python Error: PyThreadState_DeleteCurrent: no current tstate
 
Fatal Python Error: PyEval_SaveThread: NULL tstate
 
Can anybody help me make this code stable, so that it works all the time? I’m 
using Python 2.6.5 under Windows Vista, but it crashes under Windows XP as well.


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