Incompatible with python 3.11

[mkoeppe]

  gcc -Wsign-compare -Wunreachable-code -DNDEBUG -g -fwrapv -O3 -Wall -Wno-unused -g -O2 -g -O2 -I./cypari2 -I/Users/mkoeppe/s/sage/sage-rebasing/worktree-py311/local/var/lib/sage/venv-python3.11.0b1/lib/python3.11/site-packages/cysignals -I/usr/local/include -I/Users/mkoeppe/s/sage/sage-rebasing/worktree-py311/local/var/lib/sage/venv-python3.11.0b1/include/python3.11 -c cypari2/convert.c -o build/temp.macosx-12.4-x86_64-3.11/cypari2/convert.o
  cypari2/convert.c:3347:21: error: cannot take the address of an rvalue of type 'Py_ssize_t' (aka 'long')
    __pyx_v_sizeptr = &Py_SIZE(((PyObject *)__pyx_v_x));
                      ^~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~~
  1 error generated.

https://trac.sagemath.org/ticket/33842

[kliem]

The offending code is the following:

x = _PyLong_New(sizedigits)
cdef digit* D = (<PyLongObject*>x).ob_digit

cdef digit d
cdef ulong w
cdef size_t i, j, bit 
for i in range(sizedigits):
    # The least significant bit of digit number i of the output
    # integer is bit number "bit" of word "j".
    bit = i * PyLong_SHIFT
    j = bit // BITS_IN_LONG
    bit = bit % BITS_IN_LONG

    w = int_W(g, j)[0]
    d = w >> bit 

    # Do we need bits from the next word too?
    if BITS_IN_LONG - bit < PyLong_SHIFT and j+1 < sizewords:
        w = int_W(g, j+1)[0]
        d += w << (BITS_IN_LONG - bit)

    d = d & PyLong_MASK
    D[i] = d 

    # Keep track of last non-zero digit
    if d:
        sizedigits_final = i+1 

# Set correct size (use a pointer to hack around Cython's
# non-support for lvalues).
cdef Py_ssize_t* sizeptr = Py_SIZE_PTR(x)
if signe(g) > 0:
    sizeptr[0] = sizedigits_final
else:
    sizeptr[0] = -sizedigits_final

It assumes that x is of type Py_ssize_t, but this is no longer the case. It is now of type py_long, which is

ctypedef class __builtin__.py_long [object PyLongObject]:
    cdef digit* ob_digit

(according to https://github.com/cython/cython/blob/0.29.x/Cython/Includes/cpython/longintrepr.pxd).

[kliem]

Given how bad this looks, the solution is apparently surprisingly easy:

Setting the size of a python long was done with Py_SIZE (https://docs.python.org/3.8/c-api/structures.html#c.Py_SIZE). Python 3.9 introduces Py_SET_SIZE and python 3.10 disables setting the size with Py_SIZE (https://docs.python.org/3.10/c-api/structures.html#c.Py_SIZE). So what we do is already illegal with python 3.10, but maybe with inlining or so it still works.

Python 3.11 now makes setting the size using Py_SIZE an error: https://docs.python.org/3.11/c-api/structures.html#c.Py_SIZE.

(The actual error message here is a bit cryptic, but basically Py_SIZE is no longer a reference, but a new value. Saving the address of an rvalue doesn't work.)

Tests are currently running here: https://github.com/kliem/cypari2/runs/6523399435?check_suite_focus=true