Crash when bounds of prange() are Python objects

[scoder]

The following construct crashes:

cdef unsigned long i
cdef double[:] data = ...
for i in prange(len(data), nogil=True):
    ...

The generated C code is as follows:

      #ifdef WITH_THREAD
      PyThreadState *_save;
      Py_UNBLOCK_THREADS
      __Pyx_FastGIL_Remember();
      #endif
      /*try:*/ {
        if (unlikely(!__pyx_v_data.memview)) { __Pyx_RaiseUnboundMemoryviewSliceNogil("data"); __PYX_ERR(0, 24, __pyx_L4_error) }
        __pyx_t_1 = __pyx_memoryview_fromslice(__pyx_v_data, 1, (PyObject *(*)(char *)) __pyx_memview_get_double, (int (*)(char *, PyObject *)) __pyx_memview_set_double, 0);; if (unlikely(!__pyx_t_1)) __PYX_ERR(0, 24, __pyx_L4_error)
        __Pyx_GOTREF(__pyx_t_1);
        __pyx_t_2 = PyObject_Length(__pyx_t_1); if (unlikely(__pyx_t_2 == ((Py_ssize_t)-1))) __PYX_ERR(0, 24, __pyx_L4_error)
        __Pyx_DECREF(__pyx_t_1); __pyx_t_1 = 0;

len() is evaluated after releasing the GIL. The loop bounds should be evaluated before, and the GIL check applied to all parts of the loop.

Also, actually evaluating Python's len() after first converting the memory view to a Python object is stupid. Might also be part of the problem here.

[pshashank8]

Hi @scoder

I would like to work on this issue. I am trying to reproduce this, could you please help me with this

Thanks

[scoder]

@pshashank8, sorry, I think I missed your comment somehow. If you're still interested, see the HackerGuide on how to write a test, then look at the existing prange tests in tests/memoryview/memslice.pyx.

Regarding the implementation, I think it might be enough to call .coerce_to_simple() on the non-trivial prange() arguments during type analysis. That should trigger their ahead-of-time evaluation into a temporary variable, if necessary.