From af7a12dfd69d903dc47c79b8b5ef2197c73d2286 Mon Sep 17 00:00:00 2001
From: Eric Wieser <wieser.eric@gmail.com>
Date: Fri, 23 Dec 2022 08:23:19 +0000
Subject: [PATCH] gh-76963: PEP3118 itemsize of an empty ctypes array should
 not be 0 (GH-5576)

The itemsize returned in a memoryview of a ctypes array is now computed from the item type, instead of dividing the total size by the length and assuming that the length is not zero.
(cherry picked from commit 84bc6a4f25fcf467813ee12b74118f7b1b54e285)

Co-authored-by: Eric Wieser <wieser.eric@gmail.com>
---
 Lib/ctypes/test/test_pep3118.py               |  2 ++
 .../2018-02-06-23-21-13.bpo-32782.EJVSfR.rst  |  3 ++
 Modules/_ctypes/_ctypes.c                     | 33 ++++++++++++++-----
 3 files changed, 30 insertions(+), 8 deletions(-)
 create mode 100644 Misc/NEWS.d/next/Core and Builtins/2018-02-06-23-21-13.bpo-32782.EJVSfR.rst

diff --git a/Lib/ctypes/test/test_pep3118.py b/Lib/ctypes/test/test_pep3118.py
index 81e8ca7638fdeb..efffc80a66fcb8 100644
--- a/Lib/ctypes/test/test_pep3118.py
+++ b/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)),
diff --git a/Misc/NEWS.d/next/Core and Builtins/2018-02-06-23-21-13.bpo-32782.EJVSfR.rst b/Misc/NEWS.d/next/Core and Builtins/2018-02-06-23-21-13.bpo-32782.EJVSfR.rst
new file mode 100644
index 00000000000000..841740130bd141
--- /dev/null
+++ b/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.
diff --git a/Modules/_ctypes/_ctypes.c b/Modules/_ctypes/_ctypes.c
index d6fa11d3481305..b5eafe49e4ee08 100644
--- a/Modules/_ctypes/_ctypes.c
+++ b/Modules/_ctypes/_ctypes.c
@@ -2775,11 +2775,33 @@ static PyMemberDef PyCData_members[] = {
     { NULL },
 };
 
-static int PyCData_NewGetBuffer(PyObject *myself, Py_buffer *view, int flags)
+/* Find the innermost type of an array type, returning a borrowed reference */
+static PyObject *
+PyCData_item_type(PyObject *type)
+{
+    if (PyCArrayTypeObject_Check(type)) {
+        StgDictObject *stg_dict;
+        PyObject *elem_type;
+
+        /* asserts used here as these are all guaranteed by construction */
+        stg_dict = PyType_stgdict(type);
+        assert(stg_dict);
+        elem_type = stg_dict->proto;
+        assert(elem_type);
+        return PyCData_item_type(elem_type);
+    }
+    else {
+        return type;
+    }
+}
+
+static int
+PyCData_NewGetBuffer(PyObject *myself, Py_buffer *view, int flags)
 {
     CDataObject *self = (CDataObject *)myself;
     StgDictObject *dict = PyObject_stgdict(myself);
-    Py_ssize_t i;
+    PyObject *item_type = PyCData_item_type((PyObject*)Py_TYPE(myself));
+    StgDictObject *item_dict = PyType_stgdict(item_type);
 
     if (view == NULL) return 0;
 
@@ -2792,12 +2814,7 @@ 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];
-        }
-    }
+    view->itemsize = item_dict->size;
     view->strides = NULL;
     view->suboffsets = NULL;
     view->internal = NULL;