gh-141297: add PyExpat_CheckCompatibility for checking Expat C API compatibility

Include/pyexpat.h

@@ -3,19 +3,29 @@
 
 /* note: you must import expat.h before importing this module! */
 
+#ifndef PyExpat_CAPI_H
+#define PyExpat_CAPI_H
+#ifdef __cplusplus
+extern "C" {
+#endif
+
+/*
+ * Only bump the magic number when PyExpat_CAPI changes but not its size.
+ * If the structure changes because of an additional field, the magic number
+ * should not be bumped (see https://github.com/python/cpython/issues/115398).
+ */
 #define PyExpat_CAPI_MAGIC  "pyexpat.expat_CAPI 1.1"
 #define PyExpat_CAPSULE_NAME "pyexpat.expat_CAPI"
 
-struct PyExpat_CAPI
-{
-    char* magic; /* set to PyExpat_CAPI_MAGIC */
-    int size; /* set to sizeof(struct PyExpat_CAPI) */
+typedef struct PyExpat_CAPI {
+    char *magic; /* set to PyExpat_CAPI_MAGIC */
+    int size; /* set to sizeof(PyExpat_CAPI) */
     int MAJOR_VERSION;
     int MINOR_VERSION;
     int MICRO_VERSION;
     /* pointers to selected expat functions.  add new functions at
        the end, if needed */
-    const XML_LChar * (*ErrorString)(enum XML_Error code);
+    const XML_LChar *(*ErrorString)(enum XML_Error code);
     enum XML_Error (*GetErrorCode)(XML_Parser parser);
     XML_Size (*GetErrorColumnNumber)(XML_Parser parser);
     XML_Size (*GetErrorLineNumber)(XML_Parser parser);
@@ -63,5 +73,22 @@ struct PyExpat_CAPI
     XML_Bool (*SetBillionLaughsAttackProtectionMaximumAmplification)(
         XML_Parser parser, float maxAmplificationFactor);
     /* always add new stuff to the end! */
-};
+} PyExpat_CAPI;
+
+static inline int
+PyExpat_CheckCompatibility(const PyExpat_CAPI *capi)
+{
+    assert(capi != NULL);
+    return (
+        strcmp(capi->magic, PyExpat_CAPI_MAGIC) == 0
+        && (size_t)capi->size >= sizeof(PyExpat_CAPI)
+        && capi->MAJOR_VERSION == XML_MAJOR_VERSION
+        && capi->MINOR_VERSION == XML_MINOR_VERSION
+        && capi->MICRO_VERSION == XML_MICRO_VERSION
+    );
+}
 
+#ifdef __cplusplus
+}
+#endif
+#endif /* !PyExpat_CAPI_H */

Misc/NEWS.d/next/C_API/2025-11-09-17-26-19.gh-issue-141226.1GYBSN.rst

@@ -0,0 +1,2 @@
+Expose :c:func:`!PyExpat_CheckCompatibility` for checking Expat C API
+compatibility. Patch by Bénédikt Tran.

Modules/_elementtree.c

@@ -98,7 +98,6 @@ typedef struct {
     PyTypeObject *TreeBuilder_Type;
     PyTypeObject *XMLParser_Type;
 
-    PyObject *expat_capsule;
     struct PyExpat_CAPI *expat_capi;
 } elementtreestate;
 
@@ -156,7 +155,6 @@ elementtree_clear(PyObject *m)
     Py_CLEAR(st->ElementIter_Type);
     Py_CLEAR(st->TreeBuilder_Type);
     Py_CLEAR(st->XMLParser_Type);
-    Py_CLEAR(st->expat_capsule);
 
     st->expat_capi = NULL;
     return 0;
@@ -177,7 +175,7 @@ elementtree_traverse(PyObject *m, visitproc visit, void *arg)
     Py_VISIT(st->ElementIter_Type);
     Py_VISIT(st->TreeBuilder_Type);
     Py_VISIT(st->XMLParser_Type);
-    Py_VISIT(st->expat_capsule);
+
     return 0;
 }
 
@@ -4430,28 +4428,18 @@ module_exec(PyObject *m)
     if (st->deepcopy_obj == NULL) {
         goto error;
     }
-
-    assert(!PyErr_Occurred());
-    if (!(st->elementpath_obj = PyImport_ImportModule("xml.etree.ElementPath")))
+    st->elementpath_obj = PyImport_ImportModule("xml.etree.ElementPath");
+    if (st->elementpath_obj == NULL) {
         goto error;
+    }
 
     /* link against pyexpat */
-    if (!(st->expat_capsule = PyImport_ImportModuleAttrString("pyexpat", "expat_CAPI")))
-        goto error;
-    if (!(st->expat_capi = PyCapsule_GetPointer(st->expat_capsule, PyExpat_CAPSULE_NAME)))
+    st->expat_capi = PyCapsule_Import(PyExpat_CAPSULE_NAME, 0);
+    if (st->expat_capi == NULL) {
         goto error;
-    if (st->expat_capi) {
-        /* check that it's usable */
-        if (strcmp(st->expat_capi->magic, PyExpat_CAPI_MAGIC) != 0 ||
-            (size_t)st->expat_capi->size < sizeof(struct PyExpat_CAPI) ||
-            st->expat_capi->MAJOR_VERSION != XML_MAJOR_VERSION ||
-            st->expat_capi->MINOR_VERSION != XML_MINOR_VERSION ||
-            st->expat_capi->MICRO_VERSION != XML_MICRO_VERSION) {
-            PyErr_SetString(PyExc_ImportError,
-                            "pyexpat version is incompatible");
-            goto error;
-        }
-    } else {
+    }
+    if (!PyExpat_CheckCompatibility(st->expat_capi)) {
+        PyErr_SetString(PyExc_ImportError, "pyexpat version is incompatible");
         goto error;
     }