bpo-32782: PEP3118 itemsize of an empty ctypes array should not be 0

The itemsize returned in a memoryview of a ctypes array should be computed from the item type, not by dividing the total size by the length and assuming that the length is not zero.

Lib/ctypes/test/test_pep3118.py

@@ -176,7 +176,9 @@ class Complete(Structure):
     ## arrays and pointers
 
     (c_double * 4,              "<d",                   (4,),           c_double),
+    (c_double * 0,              "<d",                   (0,),           c_double),
     (c_float * 4 * 3 * 2,       "<f",                   (2,3,4),        c_float),
+    (c_float * 4 * 0 * 2,       "<f",                   (2,0,4),        c_float),
     (POINTER(c_short) * 2,      "&<" + s_short,         (2,),           POINTER(c_short)),
     (POINTER(c_short) * 2 * 3,  "&<" + s_short,         (3,2,),         POINTER(c_short)),
     (POINTER(c_short * 2),      "&(2)<" + s_short,      (),             POINTER(c_short)),

Misc/NEWS.d/next/Core and Builtins/2018-02-06-23-21-13.bpo-32782.EJVSfR.rst

@@ -0,0 +1,3 @@
+``ctypes`` arrays of length 0 now report a correct itemsize when a
+``memoryview`` is constructed from them, rather than always giving a value
+of 0.

Modules/_ctypes/_ctypes.c

@@ -2596,10 +2596,45 @@ static PyMemberDef PyCData_members[] = {
     { NULL },
 };
 
+/*
+  Get the StgDictObject corresponding to a single item of a multidimensional
+  array.
+  Takes and returns a borrowed reference.
+*/
+static StgDictObject *PyCData_item_stgdict(StgDictObject *dict)
+{
+    if (dict->ndim == 0) {
+        /* scalar is its own item */
+        return dict;
+    }
+    else {
+        /* follow _type_, eliminating a dimension */
+        PyObject *type_attr;
+        StgDictObject *item_dict;
+
+        type_attr = PyDict_GetItemString((PyObject *)dict, "_type_");
+        if (!type_attr) {
+            PyErr_SetString(PyExc_AttributeError,
+                            "class must define a '_type_' attribute");
+            return NULL;
+        }
+
+        item_dict = PyType_stgdict(type_attr);
+        if (!item_dict) {
+            PyErr_SetString(PyExc_TypeError,
+                            "_type_ must have storage info");
+            return NULL;
+        }
+
+        return PyCData_item_stgdict(item_dict);
+    }
+}
+
 static int PyCData_NewGetBuffer(PyObject *myself, Py_buffer *view, int flags)
 {
     CDataObject *self = (CDataObject *)myself;
     StgDictObject *dict = PyObject_stgdict(myself);
+    StgDictObject *item_dict;
     Py_ssize_t i;
 
     if (view == NULL) return 0;
@@ -2613,12 +2648,11 @@ static int PyCData_NewGetBuffer(PyObject *myself, Py_buffer *view, int flags)
     view->format = dict->format ? dict->format : "B";
     view->ndim = dict->ndim;
     view->shape = dict->shape;
-    view->itemsize = self->b_size;
-    if (view->itemsize) {
-        for (i = 0; i < view->ndim; ++i) {
-            view->itemsize /= dict->shape[i];
-        }
+    item_dict = PyCData_item_stgdict(dict);
+    if (item_dict == NULL) {
+        return -1;
     }
+    view->itemsize = item_dict->size;
     view->strides = NULL;
     view->suboffsets = NULL;
     view->internal = NULL;