Fix Numpy 1.20.0 by initializing PyArray_Descr portably

Description

According to the Numpy documentation:

To ensure compatibility:

• Never declare a non-pointer instance of the struct
• Never perform pointer arithmatic
• Never use sizof(PyArray_Descr)

The SWIG bindings broke the first of these rules. Improper order of initialization led to a null pointer that was causing a segfault.

Fixes #414 (closed). (We shall see!)

API Changes and Justification

Backwards Compatible Changes

• This change introduces no API changes
• This change adds new API calls

Backwards Incompatible Changes

• This change modifies an existing API
• This change removes an existing API

If any of the Backwards Incompatible check boxes are ticked please provide a justification why this change is necessary and why it needs to be done in a backwards incompatible way.

Review Status

N/A

lal/swig/SWIGPython.i

@@ -655,24 +655,6 @@ SWIGINTERN bool swiglal_release_parent(void *ptr) {
     (PyArray_FastTakeFunc*)NULL,   // fasttake
   };
 
-  // NumPy array descriptor function for type ACFTYPE.
-  static PyArray_Descr swiglal_py_array_objview_##ACFTYPE##_arrdescr = {
-    PyObject_HEAD_INIT(NULL)
-    (PyTypeObject*) NULL,   // typeobj
-    NPY_VOIDLTR,   // kind
-    NPY_VOIDLTR,   // type
-    '=',   // byteorder
-    NPY_LIST_PICKLE | NPY_USE_GETITEM | NPY_USE_SETITEM
-    | NPY_ITEM_IS_POINTER | NPY_NEEDS_INIT | NPY_NEEDS_PYAPI,   // hasobject
-    0,   // type_num
-    0,   // elsize
-    0,   // alignment
-    (PyArray_ArrayDescr*)NULL,   // subarray
-    (PyObject*)NULL,   // fields
-    (PyObject*)NULL,   // names
-    &swiglal_py_array_objview_##ACFTYPE##_arrfuncs,   // f
-  };
-
   // This function returns the NumPy array descriptor appropriate for the supplied SWIG type
   // descriptor. If no array descriptor exists, it creates one from the array descriptor for type
   // ACFTYPE.
@@ -683,22 +665,26 @@ SWIGINTERN bool swiglal_release_parent(void *ptr) {
 
     // Create NumPy array descriptor if none yet exists.
     if (*pdescr == NULL) {
-      *pdescr = PyArray_DescrNew(&swiglal_py_array_objview_##ACFTYPE##_arrdescr);
+      *pdescr = PyArray_DescrNewFromType(NPY_OBJECT);
       if (*pdescr == NULL) {
         return NULL;
       }
       (*pdescr)->typeobj = SwigPyObject_type();
       (*pdescr)->elsize = esize;
       (*pdescr)->alignment = 1;
+      (*pdescr)->f = &swiglal_py_array_objview_##ACFTYPE##_arrfuncs;
+
+      // Numpy workaround for registering user-defined object types. See
+      // https://github.com/numpy/numpy/pull/18303,
+      // https://github.com/numpy/numpy/pull/17320
+      (*pdescr)->names = Py_BuildValue("(s)", "swigobj");
+      (*pdescr)->fields = Py_BuildValue("{s:(s,i)}", "swigobj", "O", 0);
+
       if (PyArray_RegisterDataType(*pdescr) < 0) {
         return NULL;
       }
     }
 
-    // PyArray_NewFromDescr appears to steal a reference to the descriptor passed to it, so a
-    // reference count increment is needed here.
-    Py_INCREF(*pdescr);
-
     return *pdescr;
 
   }