I've been trying to track down an issue with some Python code compiled from C++ (using swig).
The following information was retrieved by executing gdb using python3-dbg (python version 3.8).
Python crashes with this error:
Fatal Python error: deallocating None
Python runtime state: initialized
The offending line of python code on the call stack is:
value = self.get(name)
Note that the value of name should be a valid string. There is nothing here that seems like it could cause a problem, but it usually happens while processing the same set of data. This line of code is executed hundreds of times before the fault.
The first call to none_dealloc in the stack trace is here:
#26 0x000000000046ef6e in none_dealloc (ignore=<optimized out>) at ../Objects/object.c:1585
#27 0x00000000004706da in _Py_Dealloc (op=<optimized out>) at ../Objects/object.c:2215
#28 0x00000000004470e8 in _Py_DECREF (op=<optimized out>, lineno=430, filename=0x699244 "../Objects/frameobject.c") at ../Include/object.h:478
#29 frame_dealloc (f=0x25938d0) at ../Objects/frameobject.c:430
#30 0x00000000004706da in _Py_Dealloc (op=op@entry=0x25938d0) at ../Objects/object.c:2215
#31 0x00000000004e02e6 in _Py_DECREF (op=0x25938d0, lineno=4314, filename=0x6e3343 "../Python/ceval.c") at ../Include/object.h:478
#32 _PyEval_EvalCodeWithName (_co=0x7fdc678c0520, globals=<optimized out>, locals=locals@entry=0x0, args=<optimized out>, argcount=2, kwnames=0x0, kwargs=0x7fdc6468cf90, kwcount=<optimized out>, kwstep=1, defs=0x7fdc678bf6f8, defcount=1, kwdefs=0x0, closure=0x0, name=0x7fdc67d0c270, qualname=0x7fdc678c2280) at ../Python/ceval.c:4314
This is confusing to me since since the Python code initiating the dealloc at frameobject.c 430 in Python 3.8 is:
/* Kill all local variables */
valuestack = f->f_valuestack;
for (p = f->f_localsplus; p < valuestack; p++)
Py_CLEAR(*p);
The definition of Py_CLEAR checks for NULL before trying to decrease the reference count and potentially deallocate the pointer. How could this cause any sort of fault? When I look at the value of op or _py_tmp in the call stack, it reads as "optimized out".
#define Py_CLEAR(op) \
do { \
PyObject *_py_tmp = _PyObject_CAST(op); \
if (_py_tmp != NULL) { \
(op) = NULL; \
Py_DECREF(_py_tmp); \
} \
} while (0)
What does this error mean? What should I look for?
None is a special, global object in Python. What the "deallocating None" error means is that the reference count on "None" has reached 0, and None is to be deallocated (AKA deleted). This is a bad thing!
It likely means that a function within your imported Python module written in C/C++ is returning Py_None without first calling Py_INCREF(Py_None). The error then shows itself elsewhere in a seemingly random, innocuous line of code when someone throws away its reference to None because the ref count on None is now 1 too few. Refer to this question for more information: Why should Py_INCREF(Py_None) be required before returning Py_None in C?