bpo-32745: Fix a regression in the handling of ctypes' c_wchar_p type (#8721)

Embedded nulls would cause a ValueError to be raised. Thanks go to
Eryk Sun for their analysis.

Co-authored-by: Łukasz Langa <lukasz@langa.pl>

Lib/ctypes/test/test_unicode.py

@@ -26,6 +26,14 @@ class UnicodeTestCase(unittest.TestCase):
         self.assertEqual(buf[::2], 'a\xe4\xfc')
         self.assertEqual(buf[6:5:-1], "")
 
+    def test_embedded_null(self):
+        class TestStruct(ctypes.Structure):
+            _fields_ = [("unicode", ctypes.c_wchar_p)]
+        t = TestStruct()
+        # This would raise a ValueError:
+        t.unicode = "foo\0bar\0\0"
+
+
 func = ctypes.CDLL(_ctypes_test.__file__)._testfunc_p_p
 
 class StringTestCase(UnicodeTestCase):

Misc/NEWS.d/next/Library/2018-08-09-23-47-10.bpo-32745.iQi9hI.rst

@@ -0,0 +1,3 @@
+Fix a regression in the handling of ctypes' :data:`ctypes.c_wchar_p` type:
+embedded null characters would cause a :exc:`ValueError` to be raised. Patch
+by Zackery Spytz.

Modules/_ctypes/cfield.c

@@ -1341,6 +1341,7 @@ Z_set(void *ptr, PyObject *value, Py_ssize_t size)
 {
     PyObject *keep;
     wchar_t *buffer;
+    Py_ssize_t bsize;
 
     if (value == Py_None) {
         *(wchar_t **)ptr = NULL;
@@ -1364,7 +1365,7 @@ Z_set(void *ptr, PyObject *value, Py_ssize_t size)
 
     /* We must create a wchar_t* buffer from the unicode object,
        and keep it alive */
-    buffer = PyUnicode_AsWideCharString(value, NULL);
+    buffer = PyUnicode_AsWideCharString(value, &bsize);
     if (!buffer)
         return NULL;
     keep = PyCapsule_New(buffer, CTYPES_CFIELD_CAPSULE_NAME_PYMEM, pymem_destructor);