bpo-35714: Reject null characters in struct format strings (GH-16928)

struct.error is now raised if there is a null character in a struct
format string.

Lib/test/test_struct.py

@@ -671,6 +671,14 @@ class StructTest(unittest.TestCase):
         self.assertIn(b"Exception ignored in:", stderr)
         self.assertIn(b"C.__del__", stderr)
 
+    def test_issue35714(self):
+        # Embedded null characters should not be allowed in format strings.
+        for s in '\0', '2\0i', b'\0':
+            with self.assertRaisesRegex(struct.error,
+                                        'embedded null character'):
+                struct.calcsize(s)
+
+
 class UnpackIteratorTest(unittest.TestCase):
     """
     Tests for iterative unpacking (struct.Struct.iter_unpack).

Misc/NEWS.d/next/Library/2019-10-25-23-45-49.bpo-35714.fw3xb7.rst

@@ -0,0 +1,2 @@
+:exc:`struct.error` is now raised if there is a null character in a
+:mod:`struct` format string.

Modules/_struct.c

@@ -1296,6 +1296,11 @@ prepare_s(PyStructObject *self)
     size_t ncodes;
 
     fmt = PyBytes_AS_STRING(self->s_format);
+    if (strlen(fmt) != (size_t)PyBytes_GET_SIZE(self->s_format)) {
+        PyErr_SetString(_structmodulestate_global->StructError,
+                        "embedded null character");
+        return -1;
+    }
 
     f = whichtable(&fmt);