Account for possible extension type struct padding when calculating the itemsize for the "size changed" check (GH-4894) (GH-5289)

Backported from https://github.com/cython/cython/pull/4894

2 files changed, 40 insertions, 6 deletions

diff --git a/Cython/Compiler/ModuleNode.py b/Cython/Compiler/ModuleNode.py
index 56845330d..e81b63d0a 100644
--- a/Cython/Compiler/ModuleNode.py
+++ b/Cython/Compiler/ModuleNode.py
@@ -3078,12 +3078,12 @@ class ModuleNode(Nodes.Node, Nodes.BlockNode):
             if not condition:
                 code.putln("")  # start in new line
             code.putln("#if defined(PYPY_VERSION_NUM) && PYPY_VERSION_NUM < 0x050B0000")
-            code.putln('sizeof(%s),' % objstruct)
+            code.putln('sizeof(%s), __PYX_GET_STRUCT_ALIGNMENT(%s),' % (objstruct, objstruct))
             code.putln("#else")
-            code.putln('sizeof(%s),' % sizeof_objstruct)
+            code.putln('sizeof(%s), __PYX_GET_STRUCT_ALIGNMENT(%s),' % (sizeof_objstruct, sizeof_objstruct))
             code.putln("#endif")
         else:
-            code.put('sizeof(%s), ' % objstruct)
+            code.putln('sizeof(%s), __PYX_GET_STRUCT_ALIGNMENT(%s),' % (objstruct, objstruct))
 
         # check_size
         if type.check_size and type.check_size in ('error', 'warn', 'ignore'):
diff --git a/Cython/Utility/ImportExport.c b/Cython/Utility/ImportExport.c
index 1689c1138..e1458fc8f 100644
--- a/Cython/Utility/ImportExport.c
+++ b/Cython/Utility/ImportExport.c
@@ -301,13 +301,24 @@ set_path:
 #ifndef __PYX_HAVE_RT_ImportType_proto
 #define __PYX_HAVE_RT_ImportType_proto
 
+#if __STDC_VERSION__ >= 201112L
+#include <stdalign.h>
+#endif
+
+#if __STDC_VERSION__ >= 201112L || __cplusplus >= 201103L
+#define __PYX_GET_STRUCT_ALIGNMENT(s) alignof(s)
+#else
+// best guess at what the alignment could be since we can't measure it
+#define __PYX_GET_STRUCT_ALIGNMENT(s) sizeof(void*)
+#endif
+
 enum __Pyx_ImportType_CheckSize {
    __Pyx_ImportType_CheckSize_Error = 0,
    __Pyx_ImportType_CheckSize_Warn = 1,
    __Pyx_ImportType_CheckSize_Ignore = 2
 };
 
-static PyTypeObject *__Pyx_ImportType(PyObject* module, const char *module_name, const char *class_name, size_t size, enum __Pyx_ImportType_CheckSize check_size);  /*proto*/
+static PyTypeObject *__Pyx_ImportType(PyObject* module, const char *module_name, const char *class_name, size_t size, size_t alignment, enum __Pyx_ImportType_CheckSize check_size);  /*proto*/
 
 #endif
 
@@ -316,13 +327,15 @@ static PyTypeObject *__Pyx_ImportType(PyObject* module, const char *module_name,
 #ifndef __PYX_HAVE_RT_ImportType
 #define __PYX_HAVE_RT_ImportType
 static PyTypeObject *__Pyx_ImportType(PyObject *module, const char *module_name, const char *class_name,
-    size_t size, enum __Pyx_ImportType_CheckSize check_size)
+    size_t size, size_t alignment, enum __Pyx_ImportType_CheckSize check_size)
 {
     PyObject *result = 0;
     char warning[200];
     Py_ssize_t basicsize;
+    Py_ssize_t itemsize;
 #ifdef Py_LIMITED_API
     PyObject *py_basicsize;
+    PyObject *py_itemsize;
 #endif
 
     result = PyObject_GetAttrString(module, class_name);
@@ -336,6 +349,7 @@ static PyTypeObject *__Pyx_ImportType(PyObject *module, const char *module_name,
     }
 #ifndef Py_LIMITED_API
     basicsize = ((PyTypeObject *)result)->tp_basicsize;
+    itemsize = ((PyTypeObject *)result)->tp_itemsize;
 #else
     py_basicsize = PyObject_GetAttrString(result, "__basicsize__");
     if (!py_basicsize)
@@ -345,8 +359,28 @@ static PyTypeObject *__Pyx_ImportType(PyObject *module, const char *module_name,
     py_basicsize = 0;
     if (basicsize == (Py_ssize_t)-1 && PyErr_Occurred())
         goto bad;
+    py_itemsize = PyObject_GetAttrString(result, "__itemsize__");
+    if (!py_itemsize)
+        goto bad;
+    itemsize = PyLong_AsSsize_t(py_itemsize);
+    Py_DECREF(py_itemsize);
+    py_itemsize = 0;
+    if (itemsize == (Py_ssize_t)-1 && PyErr_Occurred())
+        goto bad;
 #endif
-    if ((size_t)basicsize < size) {
+    if (itemsize) {
+        // If itemsize is smaller than the alignment the struct can end up with some extra
+        // padding at the end. In this case we need to work out the maximum size that
+        // the padding could be when calculating the range of valid struct sizes.
+        if (size % alignment) {
+            // if this is true we've probably calculated the alignment wrongly
+            // (most likely because alignof isn't available)
+            alignment = size % alignment;
+        }
+        if (itemsize < (Py_ssize_t)alignment)
+            itemsize = (Py_ssize_t)alignment;
+    }
+    if ((size_t)(basicsize + itemsize) < size) {
         PyErr_Format(PyExc_ValueError,
             "%.200s.%.200s size changed, may indicate binary incompatibility. "
             "Expected %zd from C header, got %zd from PyObject",